T-034 - Networking.pdf
-
Upload
walter-costa -
Category
Documents
-
view
234 -
download
0
Transcript of T-034 - Networking.pdf
-
8/16/2019 T-034 - Networking.pdf
1/108
ApplicationApplication EngineeringEngineering
T-034: Networking Application Manual
EnglishOriginal Instructions 4-2013 A044E523 (Issue 1
-
8/16/2019 T-034 - Networking.pdf
2/108
-
8/16/2019 T-034 - Networking.pdf
3/108
Table of Contents
1. INTRODUCTION............................................................................................................................ 1
1.1 Overview ................................................................................................................................. 1
1.2 About This Manual.................................................................................................................. 2
1.3 Application Manuals................................................................................................................ 3
1.4 Safety...................................................................................................................................... 3
2. NETWORKING BASICS ................................................................................................................ 5
2.1 Overview ................................................................................................................................. 5
2.2 Simple Network Introduction................................................................................................... 5
2.3 Common Terminology............................................................................................................. 8
2.3.1 Protocols ...................................................................................................................... 8
2.3.2 Topology ...................................................................................................................... 9
2.3.3 Bus ............................................................................................................................... 9
2.3.4 Channel........................................................................................................................ 9
2.3.5 Media ......................................................................................................................... 10
2.3.6 Node........................................................................................................................... 11
2.3.7 Bandwidth .................................................................................................................. 11
2.4 Evolution of Networks ........................................................................................................... 11
2.4.1 Discrete Communication............................................................................................ 11
2.4.2 Serial Communication................................................................................................ 11
2.5 Early Protocols...................................................................................................................... 12
2.5.1 RS-232....................................................................................................................... 12
2.5.2 RS-485....................................................................................................................... 13
2.6 Addressing ............................................................................................................................ 14
2.7 Media Access Controls ......................................................................................................... 14
2.8 Determinism.......................................................................................................................... 15
2.9 Full-Duplex Switched Ethernet.............................................................................................. 15
2.10 Open Systems and Object Oriented Data Structures......................................................... 17
2.11 Multiple Networks in a Single System ................................................................................ 17
3. NETWORK DEVICES .................................................................................................................. 19
3.1 Overview ............................................................................................................................... 19
3.2 Input/Output Devices ........................................................................................................... 19
3.2.1 Digital Inputs .............................................................................................................. 20
3.2.2 Digital Outputs............................................................................................................ 20
3.2.3 Analog Inputs ............................................................................................................. 21
3.2.4 Analog Outputs .......................................................................................................... 23
3.3 Intelligent Devices................................................................................................................. 23
3.3.1 Network Interface Card .............................................................................................. 23
3.4 Network Management Devices............................................................................................. 24
3.4.1 Hub/Repeater............................................................................................................. 24
3.4.2 Switch......................................................................................................................... 24
A044E523 (Issue 1) i
-
8/16/2019 T-034 - Networking.pdf
4/108
Table of Contents 4-2013
3.4.3 Bridge......................................................................................................................... 24
3.4.4 Routers....................................................................................................................... 25
3.4.5 Gateways ................................................................................................................... 25
3.5 Web Servers ......................................................................................................................... 26
4. NETWORK CHARACTERISTICS................................................................................................ 29
4.1 Overview ............................................................................................................................... 294.2 Network Topology ................................................................................................................. 29
4.2.1 Bus Topology ............................................................................................................. 30
4.2.2 Ring Topology ............................................................................................................ 30
4.2.3 Star Topology............................................................................................................. 31
4.2.4 Other Topologies........................................................................................................ 31
4.3 Bandwidth ............................................................................................................................. 31
4.4 Optimizing Network Speed ................................................................................................... 32
4.5 Distance ................................................................................................................................ 32
5. MEDIA CONSIDERATIONS......................................................................................................... 35
5.1 Overview ............................................................................................................................... 355.2 Cable Concepts .................................................................................................................... 35
5.2.1 Attenuation................................................................................................................. 35
5.2.2 Characteristic Impedance and Network Termination................................................. 35
5.2.3 Electromagnetic Interference ..................................................................................... 36
5.2.4 Crosstalk .................................................................................................................... 36
5.2.5 Shielding .................................................................................................................... 37
5.2.6 Insulating Jacket - Plenum vs. PVC .......................................................................... 38
5.3 Cable Types ......................................................................................................................... 38
5.3.1 Twisted Pair Cable..................................................................................................... 39
5.3.2 Fiber-Optic Cable....................................................................................................... 40
5.3.3 Coaxial Cable............................................................................................................. 405.4 Twisted Pair Cable Standards .............................................................................................. 41
5.4.1 NEMA Level IV Cable ................................................................................................ 41
5.5 Physical Layer Protocols ..................................................................................................... 41
5.5.1 Ethernet...................................................................................................................... 41
5.5.2 Modbus over Serial Line ............................................................................................ 42
5.5.3 PCCNet ...................................................................................................................... 43
5.5.4 LonWorks ................................................................................................................... 43
5.5.5 Wireless Media........................................................................................................... 44
6. PROTOCOLS............................................................................................................................... 47
6.1 Overview ............................................................................................................................... 476.2 Protocols ............................................................................................................................... 47
6.2.1 Proprietary vs. Open Protocols .................................................................................. 47
6.3 The OSI Reference Model .................................................................................................... 48
6.3.1 Layer 1 - Physical Layer ............................................................................................ 48
6.3.2 Layer 2 - Data Link Layer .......................................................................................... 50
6.3.3 Layer 3 - The Network Layer ..................................................................................... 52
6.3.4 Layer 4 - Transport Layer .......................................................................................... 55
ii A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
5/108
4-2013 Table of Contents
6.3.5 Application Layer Protocols ....................................................................................... 57
6.4 Ethernet Frame Construction................................................................................................ 57
6.5 The TCP/IP Model ................................................................................................................ 58
6.6 Common Protocols ............................................................................................................... 59
6.7 Data Link and Physical Layer Protocols ............................................................................... 59
6.7.1 Ethernet...................................................................................................................... 59
6.7.2 RS-232....................................................................................................................... 606.7.3 RS-485....................................................................................................................... 60
6.7.4 Application Layer Protocols ....................................................................................... 60
6.7.5 HyperText Transfer Protocol (HTTP) ......................................................................... 60
6.7.6 Simple Mail Transfer Protocol (SMTP) ...................................................................... 61
6.7.7 Simple Network Management Protocol (SNMP)........................................................ 61
6.7.8 Modbus ...................................................................................................................... 61
6.7.9 LonWorks ................................................................................................................... 61
6.7.10 BACnet..................................................................................................................... 62
6.7.11 Profibus .................................................................................................................... 62
6.7.12 Generic Object Oriented Substation Events (GOOSE) .......................................... 62
6.7.13 CAN (Controller Area Network) .............................................................................. 63
6.7.14 PCCNet .................................................................................................................... 63
6.8 Protocol Conversions............................................................................................................ 63
6.8.1 Ethernet Conversions................................................................................................. 64
6.8.2 Application Layer Protocol Conversions .................................................................... 64
7. MONITORING SYSTEMS............................................................................................................ 65
7.1 Overview ............................................................................................................................... 65
7.2 Purpose of Monitoring........................................................................................................... 65
7.2.1 System Reliability....................................................................................................... 65
7.2.2 Operating Cost........................................................................................................... 65
7.2.3 Agency Requirements................................................................................................ 66
7.3 Monitoring System Functions................................................................................................ 66
7.3.1 Status Display ............................................................................................................ 66
7.3.2 Alarm Notification....................................................................................................... 69
7.3.3 Data Logging.............................................................................................................. 69
7.3.4 Reporting.................................................................................................................... 72
7.4 Monitoring System Architecture............................................................................................ 72
7.4.1 SCADA Systems........................................................................................................ 73
7.5 Web-based Monitoring Systems........................................................................................... 75
7.5.1 Web Server Location ................................................................................................. 77
7.6 Which Monitoring System to Choose.................................................................................... 78
8. COMMUNICATION SYSTEMS IN POWER GENERATION APPLICATIONS ............................ 79
8.1 Overview ............................................................................................................................... 79
8.2 Web-based monitoring – Alarm Notification ......................................................................... 79
8.3 Wireless Web-based Monitoring........................................................................................... 80
8.4 Local Utility Service............................................................................................................... 80
8.5 Large Scada Networks.......................................................................................................... 81
A044E523 (Issue 1) ii i
-
8/16/2019 T-034 - Networking.pdf
6/108
Table of Contents 4-2013
9. APPENDIX ................................................................................................................................... 85
9.1 Glossary................................................................................................................................ 85
9.2 Acronyms .............................................................................................................................. 93
9.3 Codes and Standards ........................................................................................................... 96
9.3.1 Related Product Standards........................................................................................ 96
9.3.2 Modification of Products............................................................................................. 97
iv A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
7/108
WARRANTY
This manual is published solely for information purposes and should not be considered all inclusive. If further information is required, consult Cummins Power Generation. Sale of product shown or described inthis literature is subject to terms and conditions outlined in appropriate Cummins Power Generation sellingpolicies or other contractual agreement between the parties. This literature is not intended to and does notenlarge or add to any such contract. The sole source governing the rights and remedies of any purchaser of this equipment is the contract between the purchaser and Cummins Power Generation.
NO WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING WARRANTIES OF FITNESS FOR APARTICULAR PURPOSE OR MERCHANTABILITY, OR WARRANTIES ARISING FROM COURSE OFDEALING OR USAGE OF TRADE, ARE MADE REGARDING THE INFORMATION,RECOMMENDATIONS AND DESCRIPTIONS CONTAINED HEREIN. Each customer is responsible for thedesign and functioning of its building systems. We cannot ensure that the specifications of Cummins Power Generation products are the proper and sufficient ones for your purposes. You must satisfy yourself on thatpoint.
In no event will Cummins Power Generation be responsible to the purchaser or user in contract, in tort(including negligence), strict liability or otherwise for any special, indirect, incidental or consequentialdamage or loss whatsoever, including but not limited to damage or loss of use of equipment, plant or power
system, cost of capital, loss of power, additional expenses in the use of existing power facilities, or claimsagainst the purchaser or user by its customers resulting from the use of the information, recommendationsand descriptions contained herein.
A044E523 (Issue 1) v
-
8/16/2019 T-034 - Networking.pdf
8/108
WARRANTY 4-2013
This page is intentionally blank.
vi A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
9/108
1 Introduction
1.1 Ov ervi ew
The purpose of this Application Manual is to educate engineers, system integrators, distributors,and interested users in the fundamentals of networks, as they apply and are used in on-site
power generation systems.
Communication networks have long been used to make equipment and processes operate more
reliably and efficiently in the following ways:
• Networks improve system reliability by communicating fault or potential fault conditions to
equipment or personnel that can take appropriate action to prevent failures or equipment
damage.
• Networks improve serviceability of a system as technicians are able to assess a system’s
service requirements remotely and in some cases allow for download of software upgrades
over the internet.
• Networks minimize operating costs by making service more efficient and simplifying status
indication and reporting requirements.
• Networks minimize installation costs by reducing point to point wiring and the associated
wiring errors.
• Networks enable more efficient operation of a system as operating and performance
information enables a system controller to cycle equipment on or off and make other
adjustments of operating parameters.
• Networks allow building management systems to monitor and control equipment. This
includes heating and cooling, lighting, transportation and security systems as well as power
generation and distribution equipment.
• Data provided over a network is often used for trend analysis which can optimize systemperformance over time or be used to diagnose problems.
Networks will play an expanded role in power generation, distribution and transmission systems
moving forward. Demand for power is growing in excess of existing generation capacity at the
same time tolerance for interruptions in our power supply is decreasing. Developing additional
centralized generation capacity is capital intensive and fraught with political difficulties as fuel
sources are perceived as non-renewable, harmful to the environment and pose health and
safety risks. This results in a demand for distributed rather than centralized power generation
sources and for improvements to power transmission systems in terms of supplying energy
needs more efficiently and more effective fault isolation and fault tolerance systems.
As power generation systems migrate from centralized to distributed generation and control, the
communications infrastructure will need to become more comprehensive and standardized sothat equipment from multiple suppliers will be able to communicate with each other seamlessly.
Seamless communication is the enabling technology that allows the potential advantages of
distributed power generation to be realized. Examples of this include
• Switching loads off of the grid when there is a danger of a utility transformer becoming
overloaded or to do maintenance on utility distribution equipment
• Exporting power to the grid from distributed sources to serve loads that have been
interrupted due to a fault in the distribution system.
A044E523 (Issue 1) 1
-
8/16/2019 T-034 - Networking.pdf
10/108
1. Introduction 4-2013
• Supplementing alternative energy sources with diesel gensets to mitigate the intermittent
nature of the alternative source
• Implementing load demand schemes across multiple distributed sources in order to use the
available capacity most efficiently.
Even equipment that is not connected to a grid will be expected to communicate with other
equipment for applications ranging from simple remote alarm notification to full monitoring
control and analysis. Networking capability is becoming an enabling technology for power
generation equipment across the entire range of power generation applications.
1.2 About This Manual
The intent of this application manual is to describe communication networks in general rather
than explain Cummins Power Generation networking equipment. However, there are several
instances where Cummins Power Generation equipment will be used as an example to illustrate
a concept and give guidance to engineers and integrators on the scope of work that may be
involved with implementing certain communication functions within such products.
The manual is divided into 8 chapters.
Chapter 1 discusses objectives and structure of the manual.
Chapter 2 defines purposes of having networks in power generation and describes a brief
history of how networks have evolved in control systems over the years. Chapter 2 also explains
some of the basic concepts of networks which will be used throughout this manual.
Chapter 3 identifies the types of devices that may be found in a network, from simple
input/output (I/O) devices and controllers to devices that manage traffic and communication over
a network, such as routers and switches, to devices that connect local networks to wide area
networks and the internet. This chapter will refer to Cummins Power Generation devices as
examples.
Chapter 4 defines core network characteristics, topology, bandwidth and distance, the
interaction between these characteristics, and provides a baseline for comparison of variousmedia and protocols.
Chapter 5 discusses media over which communication signals travel. General application
considerations such as termination, grounding, and shielding are covered as well as commonly
used media and how and where they are used.
Chapter 6 defines and describes protocols using the open systems interconnection (OSI) model
as a framework for the discussion. Protocols used by Cummins Power Generation products as
well other commonly used protocols are defined in the context of the OSI model and how and
where they are typically used. For protocols that are not used by standard Cummins Power
Generation products, a description of how these products may interact with equipment using
that protocol is included.
Chapter 7 describes monitoring systems. On-site supervisory control and data acquisition(SCADA) systems and web-based monitoring systems are explored. Additionally, typical
applications for each type of monitoring system are explained. Considerations for
communicating between Cummins Power Generation’s equipment and 3rd party monitoring
systems will be explained.
Chapter 8 displays examples of several types of communication systems in power generation
applications.
2 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
11/108
4-2013 1. Introduction
The Appendix contains a glossary of commonly used network and communication
terminologies and a list of acronyms commonly used in the industry and in this manual.
1.3 Application Manuals
Every standby generator set installation will require power transfer equipment, either transfer
switches or paralleling switchgear. The proper system for the job and its proper application arecrucial to reliable and safe operation. The following Cummins Power Generation application
manuals address related aspects of standby and emergency power systems. Because these
manuals cover aspects requiring decisions that must be taken into consideration early in the
design process, they should be reviewed along with this manual.
Application Manual T-011 - Automatic Transfer Switches. Many applications utilize multiple
power sources to enhance electric power system reliability. These often include both utility
(mains) service and generator set service to critical loads. T-011 covers the various types of
power transfer systems available and considerations for their use and application. Careful
consideration of the power switching system at the start of a project will enable a designer to
offer the most economically viable and most reliable service to the facility user.
Application Manual T-016 - Paralleling and Paralleling Switch Gear. Paralleling equipmentmakes two or more generator sets perform as one large set. This can be economically
advantageous, especially when the total load is greater than 1000 kW. The decision whether to
parallel sets must be made in the early stages of design, especially if space and the need for
future expansion are critical factors.
Application Manual T030 – Liquid Cooled Generator Sets. Generator sets may operate as prime
power sources or provide emergency power in the event of utility power failure. They may also
be used to reduce the cost of electricity where the local utility rate structure and policy make
that a viable option. Because of their important role, generator sets must be specified and
applied in such a way as to provide reliable electrical power of the quality and capacity required.
T-030 provides guidance to system and facility designers in the selection of appropriate
equipment for a specific facility, and the design of the facility, so that these common system
needs are fulfilled.
1.4 Safety
Safety should be a primary concern of the facility design engineer. Safety involves two aspects:
safe operation of the generator set itself (and its accessories) and reliable operation of the
system. Reliable operation of the system is related to safety because equipment affecting life
and health is often dependent on the generator set – such as hospital life-support systems,
emergency egress lighting, building ventilators, elevators, fire pumps, security and
communications.
Refer to the Codes and Standards section for information on applicable electrical and fire codes
around the world. Standards, and the codes that reference them, are periodically updated,requiring continual review. Compliance with all applicable codes is the responsibility of the
facility design engineer. For example, some areas may require a certificate-of-need, zoning
permit, building permit or other site-specific certificate. Be sure to check with all local
governmental authorities early in the planning process.
A044E523 (Issue 1) 3
-
8/16/2019 T-034 - Networking.pdf
12/108
1. Introduction 4-2013
NOTE: While the information in this and related manuals is intended to be accurateand useful, there is no substitute for the judgment of a skilled, experiencedfacility design professional. Each end user must determine whether theselected generator set and emergency/standby system is proper for theapplication.
4 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
13/108
2 Networking Basics
2.1 Ov ervi ew
A network can be defined as a collection of devices or “nodes” that communicate with oneanother over a common medium (i.e. wire, fiber, wireless) where information is exchanged via
sets of rules known as protocols. In this chapter we will define some of the key terms and
concepts associated with networks and discuss how networks have evolved historically.
2.2 Simple Network Introduction
The purpose of a network is to efficiently provide control functions or transmit information, either
between multiple pieces of equipment or between equipment and people.
For example, most power generation equipment that is used in critical applications is required to
be provided with remote monitoring equipment so that an operator can be aware of whether or
not a generator set is running and if it has some fault condition present that might cause agenerator failure.
In Figure 1 you can see a control panel on a generator set that provides alarm and status
information to a remote annunciation point. Note that DC power (generally from the generator
starting batteries) lights a specific lamp when a specific switch is closed to indicate a specific
condition that is occurring. For example, the top switch closing might indicate that the generator
set is running by lighting a light. Note that there is a switch for each condition to be remotely
indicated and a lamp directly connected to each switch.
FIGURE 1. CONTROL PANEL WITH ALARM AND STATUS INFORMATION
As you can imagine, this type of system monitoring is pretty limited because it gets expensive
and complex as the number of conditions to be indicated grows and as the distance between
the generator and the remote monitoring point increases, or if the number of monitoring points
increases. While a system of this type could provide simple control commands, such as a
remote start capability, it can not handle analog information (for example indication of a range of
temperatures) but rather just provides discrete yes/no type of information.
A044E523 (Issue 1) 5
-
8/16/2019 T-034 - Networking.pdf
14/108
2. Networking Basics 4-2013
Even a simple generator set application could include some pretty complicated wiring, as
indicated in Figure 2, because every wire would have a specific purpose and would need to be
connected in exactly the correct location.
FIGURE 2. TRADITIONAL COMMUNICATIONS SYSTEM WIRING
Note that even for this installation, there are inputs (a mechanism to tell the system the status of
a specific piece of information), outputs (a mechanism to provide information to a remote person
or device), media (wiring between the devices), and a power supply to operate the system (in
this case DC battery power).
Network-based communication was developed to deal with some of the weaknesses of
traditional communication links between devices. There are a number of similarities between
network-based communications and traditionally designed systems.
Figure 3 shows an annunciator panel, with the same outward functionality as shown in Figure
1, which utilizes a design with network-based communication. In this example there are still
switch inputs (only one is shown for simplicity), and these switch inputs, instead of directly
carrying current to a lamp, provide an input to an input/output (I/O) chip. The I/O chip connects
to a microprocessor (uP) that reads data off the chip and then transfers it through media (in this
case a twisted pair of wires) to a matching microprocessor in the annunciator. This second
microprocessor operates an output switch in the annunciator I/O module, and this output switch
can operate the lamp when required.
6 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
15/108
4-2013 2. Networking Basics
FIGURE 3. ANNUNCIATOR PANEL WITH NETWORK-BASED COMMUNICATION
Note that a power supply is still required to operate the remote annunciator, but it does not
necessarily need to come from the generator set control. There are still inputs, outputs, media,
and with this design, the microprocessors must communicate with each other in the same
language, or protocol. They also must accept the same type of media to transmit messages.
The major advantages seen in the network-based design versus the more traditional designs is
a huge simplification in the installation of the generator set and, because of that, a greatly
reduced installation cost in both dollars and time. A major advantage that may not be clearly
visible is that since all the operation of the equipment is managed by software in the
microprocessors, changes can be made with software manipulation rather than with hard wire
changes, again providing lower cost and also a greater degree of flexibility in design. Changes
and additions are also easy to accomplish with this design.
As can be seen in Figure 4 vs. Figure 2, there are many less connections to make (just control
power to each component and the media that interconnects them—note in this illustration the
twisted pair cable is represented by a dashed line).
A044E523 (Issue 1) 7
-
8/16/2019 T-034 - Networking.pdf
16/108
2. Networking Basics 4-2013
FIGURE 4. NETWORK BASED COMMUNICATIONS WIRING
A network, then, can be loosely defined as a series of devices (in our illustration a transfer
control, generator set control, and annunciator) that communicate with each other over a
common media (twisted pair cable, wireless, fiber optic cable) where information is exchanged
via an established language (set of rules) which is called protocol. The devices in a networkhave inputs and outputs (I/O) that provide the necessary functionality for each system. Points
with network connections are sometimes called nodes.
2.3 Common Terminology
In this section we will define some of the more commonly used networking terms that are used
in this manual. See the Appendix, Chapter 9, for a complete glossary of terms.
2.3.1 Protocols
A protocol can be defined as a set of rules used mutually by two or more devices or software
applications to communicate. Protocols specify all of the characteristics of a communication
network from the physical and electrical qualities to the languages used to communicate.
8 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
17/108
4-2013 2. Networking Basics
2.3.2 Topology
Topology refers to the physical shape and interconnection scheme of the network. The three
most common topologies used in control networks are known as "bus", "ring", and "star". A bus
topology is one in which all of the devices are connected in a line. A ring topolgy is similar to a
bus topology except the devices on the ends are connected forming a closed loop. A star
topology is one in which one device is in the center and connects to all of the other devices like
the hub and spokes of a wheel.
2.3.3 Bus
The terms media, bus, and channel are often used interchangeably to describe the physical
carrier of the communication. While the terms are related they do have separate meanings.
A bus refers to the "backbone" of a communication channel. When a bus topology is used all of
the nodes on the network are physically connected to the bus.
2.3.4 Channel
A channel refers to an uninterrupted physical communication path. Most PowerCommand
networks consist of a single channel. More sophisticated networks will have multiple channels. Asingle channel consists of only one protocol and one type of media. Devices such as routers
and gateways can combine separate channels consisting of different protocols and media into a
single network. Routers and gateways will be discussed in detail in Chapter 3.
Figure 5 illustrates the concept of a channel and a bus. Each of the routers separate the
network into two channels: a Lon channel and an Ethernet channel. Each of the channels is
connected in a bus topology with each device on the channel connected to a common pair of
wires. This pair of wires is referred to as the bus.
A044E523 (Issue 1) 9
-
8/16/2019 T-034 - Networking.pdf
18/108
2. Networking Basics 4-2013
FIGURE 5. CHANNEL BUS CONCEPT
2.3.5 Media
Media is the generic term for the physical carrier of the signal, whether it is copper wire, fiber
optic cable or, as in the case of wireless communication, air.
10 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
19/108
4-2013 2. Networking Basics
2.3.6 Node
The term "node" is a generic term referring to any device connected to a network. A node could
be a generator set control, an annunciator, a router or a web server, or any device on the
network.
2.3.7 BandwidthBandwidth refers to the speed of communication over the network. It is the amount of data that
can be transmitted in a fixed amount of time, usually expressed as bits per second (bps or
bit/s). The term bandwidth is also used to express the finite amount of time in which certain
communication may occur, so phrases like “that’s not an efficient use of bandwidth” refers not
only to the speed of the propagation of the data but also to the total amount of time that a
transmitting device uses to access the communication channel.
2.4 Evolution of Networks
2.4.1 Discrete Communication
Prior to the introduction of microprocessors to control systems, devices communicated over
discrete wires with a separate wire representing each signal. Discrete wiring is not inherently
bad and is necessary when signals need to transmit significant levels of power such as when
activating a relay. Switchgear applications still include very large wiring harnesses. However,
when signals are used just to convey information, communication networks offer materials and
labor cost savings and advantages in scalability over bundles of discrete wire. As
microprocessors became widespread in control devices, network communication became a
viable and economical alternative to communication via discrete wires.
2.4.2 Serial Communication
As personal computers (PCs) became common, a demand arose for PCs to be able to
communicate with other devices, such as printers and modems. Simple serial communicationprotocols were developed to support communications between two devices. As communication
equipment became available PCs and Programmable Logic Controllers (PLCs) began to use
serial communication to monitor and control industrial networks.
Serial communication can be defined as the process of sequentially sending data one bit at a
time over a common communication channel. This allows a single wire or pair of wires to
transmit many different pieces of information. Data is transmitted by a series of electrical pulses
and translated into a series of binary digits or bits, each of which can have one of two values,
commonly represented as “0” or “1”.
Establishing timing between sending and receiving devices is necessary for the receiving device
to recognize where one character (meaning a number or a letter) ends and the next begins
within a string of pulses. There are two methods for achieving this, known as synchronous serialcommunication and asynchronous serial communication. With synchronous communication a
separate clock signal is transmitted between the two devices and a new message begins based
on a pulse of the clock signal.
A044E523 (Issue 1) 11
-
8/16/2019 T-034 - Networking.pdf
20/108
2. Networking Basics 4-2013
Asynchronous communication is the more commonly used method of communication. With
asynchronous communication there is no separate clock signal. Messages are broken into
packets of 7-8 data bits with a start bit at the beginning of the packet, followed by the data bits,
and then possibly a parity bit and up to 2 stop bits (see Figure 6). Devices need to be
configured to transmit at the same speed (known as baud rate) the same number of data bits
and stop bits and the same parity test. As implied by the term “asynchronous” a device may
initiate a message at any time. As the start bit is transmitted the receiving device knows how
many data bits will be transmitted in the packet at what speed and will recognize when the
transmission is complete.
FIGURE 6. AN ASYNCHRONOUS MESSAGE PACKET
A parity bit is often used as a simple form of error checking. When a parity bit is used the
sending device will set that bit so that it is transmitting either an even or odd number of 1s in the
packet based on a predefined setting in the devices for even or odd parity. For example, if the
packet consists of 7 data bits and even parity is selected, then if the 7 data bits are “1010001”
(3 “1s”) then the parity bit should be a 1 so that there are an even number of 1s. If the parity bit
is 0, the receiving device would recognize that there has been an error in communication.
2.5 Early Protocols
One of the earliest forms of serial communication is Morse code transmitted over telegraph
wires. The signals were simple electrical pulses transmitted over the wires, a short pulse
represented by a “dot” and a longer pulse represented by a “dash”. Each letter of the alphabet
was represented by a series of dots and dashes so a message could be broken down to this
serial data, transmitted, received, and re-assembled by the receiver, thus Morse code can be
considered one of the first communication protocols.
2.5.1 RS-232
As communication equipment became more widely used, there was a need to develop a
standard communication protocol so that equipment from several different manufacturers could
communicate with each other. The standard protocol was developed by the Electronic Industries
Association (EIA) and was known as RS-232. The RS-232 standard defines communication
between two specific types of equipment known as Data Terminal Equipment (DTE, typically a
PC) and Data Communications Equipment (DCE, typically a modem) over a distance of nomore than 50 feet (15 meters). RS-232 also specified the pin designations on the 9- and 25-pin
connectors that were most common at the time. RS-232 defines separate transmit and receive
wiring paths with the transmit pin on one device physically connected to the receive pin on the
other device. This means that both devices can transmit at the same time. This is known as full-
12 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
21/108
4-2013 2. Networking Basics
duplex communication (see Figure 7). Although it was designed for a very narrow scope of
applications (communication between only two devices no more than 50 feet (15 meters) apart)
RS-232 is still commonly used in those applications today due to its simplicity and relatively low
cost to develop and deploy. Cummins Power Generation’s proprietary Mon protocol, which is
used by the InPower service tool to connect to controls, is built on the RS-232 physical layer.
FIGURE 7. TYPICAL RS-232 NETWORK
In the 1990s, control devices with microprocessors and networking capability were becoming
more popular. Networks consisted of several devices spread over large distances. A
communication channel that is shared by multiple devices presents several challenges beyond
what is required for communication between only two devices. It is necessary for signals to
travel over greater distances without distortion. The speed of communication becomes more
important. There is a need for identifying which device a message is intended for. When there
are many devices connected to a common communication channel it is not practical to have
individual transmit and receive lines for each device. This means that multiple devices cannot
transmit messages at the same time so there needs to be some method to determine which
device can access the channel. Protocols that support multiple devices on a network need to
address these concerns of distance, bandwidth, addressing, and media access.
2.5.2 RS-485
RS-485 was developed as the standard for connecting more than two devices on a network
over longer distances. Unlike RS-232, which has separate circuits for transmitting and receiving
signals, RS-485 specifies a single circuit for both transmitting and receiving signals, known as a
transceiver. The RS-485 transceiver is designed in such a way that multiple transceivers can beconnected to the network without affecting the one transceiver that is transmitting a signal (see
Figure 8). RS-485 was designed so that signals can travel long distances without being
attenuated by resistance and inductance in the wires and have a high level of noise immunity.
FIGURE 8. TYPICAL RS-485 NETWORK
RS-232 allowed two devices to send messages at the same time by having separate send and
receive paths between the two devices. Since RS-485 uses only a single transceiver for both
transmitting and receiving data, this is not possible. At any given time a device may either
transmit or receive data but the device cannot perform both operations. This is known as half-
duplex communication. There is also a 4 wire implementation available on the market which
supports full-duplex communication. These devices consist of two separate RS-485
transceivers, so that one can transmit and the other can receive at the same time.
A044E523 (Issue 1) 13
-
8/16/2019 T-034 - Networking.pdf
22/108
2. Networking Basics 4-2013
Cummins Power Generation’s PCCNet protocol, used to communicate between a generator set
and its accessories, such as human machine interfaces (HMIs) and annunciators, is built on the
RS-485 physical layer. As the RS-485 standard defines only the physical layer of
communication, it defines distance and bandwidth. Addressing and controlling access to the
communication channel is outside of the scope of the RS-485 standard and is defined by higher
level protocols.
2.6 Addres sing
Each device on a network must have a unique identifier often known as an address. Many
protocols will define both a logical and a physical address. The logical address is assigned
when the network is commissioned; if the same device is used in a different network in the
future, it will likely be assigned a different logical address. Also, if a device fails and needs to be
replaced, the new device will most likely be assigned the same logical address as the device
that it replaced.
Some protocols, such as Ethernet and LonWorks, will also specify a physical address. The
physical address is assigned by a standards body, is unique in the world, and is permanently
assigned to a specific device.
2.7 Media Access Controls
There are several methods for determining which device can access a channel to send a
message. A few methods are outlined below.
1. Master/Slave: One device is the master of the bus, initiates all communication, and controls
access to the bus. The master communicates to one device at a time. The addressed slave
device responds to the master per its request. Most early communication networks used
master/slave mode of communication and it is still common when there is a programmable
logic control (PLC) controlling several devices in a system. Modbus RTU uses master/slave
communication. This type of access control is not appropriate when peer-to-peer or
broadcast communication is required, meaning that any device will be able to initiatecommunication.
2. Token Passing: This is similar to master/slave, but a signal called a token is passed around
the bus from device to device giving each device its own chance to act as the master and
initiate communication for a short time until it has to pass the token to the next
device. Cummins Power Generation's PCCNet uses a token passing scheme.
3. Carrier Sense Multiple Access with Collision Detection (CSMA/CD): Each device “listens” to
the bus to see if anyone is “talking”. If not, then the device may transmit its message. If two
or more devices transmit at the same time, each transmitting device detects this, stops
transmitting, and then waits a random amount of time before attempting to transmit
again. LonWorks and CAN both use CSMA/CD for bus access. Earlier versions of Ethernet
also used CSMA/CD, although with the fully switched Ethernet networks that are commontoday collisions are avoided. See the discussion on switched ethernet in Section 2.9.
14 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
23/108
4-2013 2. Networking Basics
2.8 Determinism
When networks are used to transmit time critical data for controllers, there is a need for the
communication to be deterministic, meaning that the amount of time it takes for a certain
message to travel between devices must be predetermined and consistent. In this case,
protocols will include some priority arbitration scheme so that critical messages will pre-empt
noncritical messages and devices transmitting these messages will get priority in accessing thebus.
2.9 Full-Duplex Switched Ethernet
Ethernet is a protocol defined by the IEEE 802.3 standard which specifies physical and data link
layer aspects of communication such as bit encoding, physical addressing, and media access. It
is currently used for approximately 85% of the world's LAN-connected PCs and workstations. Its
prevalence created a demand for control networks to communicate over Ethernet.
When Ethernet was standardized in 1985, communication was half-duplex, meaning a device
could not transmit and receive data simultaneously. All devices on the network communicated to
a central hub which would broadcast messages to all of the devices on the network. Ethernetused CSMA/CD for media access and collision detections. Due to the possibility of collisions,
the communication was not deterministic so it was not suitable for real time control applications.
Full duplex communication was standardized for Ethernet in 1998, which essentially doubled the
available bandwidth; however, it still did not make the protocol deterministic because collisions
still occurred. For a truly deterministic network, it is not sufficient to detect collisions. Collisions
must be avoided. The most common collision avoidance method is to divide the network into
segments consisting of only two devices communicating over a full-duplex connection. This is
accomplished by using Ethernet switches (see Figure 9).
A044E523 (Issue 1) 15
-
8/16/2019 T-034 - Networking.pdf
24/108
2. Networking Basics 4-2013
FIGURE 9. SWITCHED ETHERNET
In a switched Ethernet network the device will send messages to an Ethernet switch. The
Ethernet switch will check the message for errors, and then forward it to the intended recipient.
The message will also include a prioritization code so in the event that multiple devices are
attempting to communicate with the same recipient, the higher priority message will be
transmitted first.
A full duplex switched ethernet allows devices to transmit and receive data at the same time and
eliminates collisions by having only one device connected to each port of the switch. These two
features enable deterministic communication which makes Ethernet suitable for industrial control
networks.
Notice that Figure 9 shows two small networks with Ethernet switches connected by routers.
Routers are similar to switches in that they both manage network traffic. The difference is that
switches direct traffic within a local network while routers direct traffic between networks.
Routers enable networks using different types of media or different protocols to interact with
each other. The most common example of this is a control network using routers to connect to a
remote monitoring system over the internet. Switches and routers will be discussed in detail in
Chapter 3.
16 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
25/108
4-2013 2. Networking Basics
2.10 Open Systems and Object Oriented Data Structures
As the communication infrastructure has evolved, from communicating between two devices that
are physically next to each other to communicating with devices anywhere in the world, the
nature of the data that is communicated has evolved as well. In both cases, the trend is toward
open communication to allow equipment from multiple suppliers to integrate seamlessly.
Traditional data, communicated by a piece of equipment, has no structure. It is simply a list of numbers that has no meaning by itself. It is up to the system integrators who are programming
or configuring the network devices to provide the context, to tell the equipment what the data
actually means. This is known as a flat data structure. For example, a generator may have the
value 2773 stored in its Modbus register number 40026, but it will provide no information on
what that number represents. In order for this to be useful, the generator manufacturer must
publish a register map or some other document explaining that register 40026 contains the line
1 to neutral voltage and it is scaled at 0.1 volts per count. A system integrator will need to take
that information and program their device so that when the value for the line 1 to neutral voltage
is required, the device polls the generator for the value stored in register 40026 and then divides
that value by 10. In general, there is no standardization between manufacturers on this.
In an object oriented data structure, related data is bundled into an object. A generator would be
defined as an object with certain properties such as Line 1 to neutral voltage and functions it
can perform, such as starting and stopping. If all generator manufacturers define the generator
object the same way, it would make it very easy for a system integrator to design systems with
equipment from many different manufacturers.
The trend in networks is toward open systems and interoperability. The expectation is emerging
that all kinds of equipment will be interconnected and power generation and distribution
equipment will be no exception. With the trend toward distributed generation and control, and as
SmartGrid technologies are applied and matured, the demand for interoperability will only get
stronger.
2.11 Multiple Networks in a Single System
With the prevalence of communications networks in all types of control systems it is not
uncommon to have several networks within a single system. Some Cummins generator sets, for
example, simultaneously support three networks, each using a different protocol (each of the
protocols mentioned will be discussed in more detail in Section 6.2).
1. A Modbus network for communication with monitoring and control systems. Many of these
systems are designed and commissioned by 3rd parties who use Modbus protocol in their
system. Because Modbus is commonly used open (meaning available to everyone)
protocol it is well suited for communicating with monitoring and control systems.
2. A PCCNet network for communicating between a generator set and its peripherals such asa display (commonly known as an HMI or Human Machine Interface). PCCNet is a
Cummins proprietary protocol designed specifically for use with Cummins equipment. It is
very inexpensive and simple to maintain and requires no configuration. The fact that it is
only used by Cummins means that we don't have to worry about unknown equipment trying
to communicate which simplifies ongoing support of devices that use the protocol and
enables "plug and play" configuration.
A044E523 (Issue 1) 17
-
8/16/2019 T-034 - Networking.pdf
26/108
2. Networking Basics 4-2013
3. A CAN network for communicating between the genset controller and the Engine Control
Module (ECM). Can is a commonly used protocol in the automotive industry used to
communicate between the engines and other control systems in a vehicle. It is the standard
protocol used by Cummins ECM's in all kinds of applications and in a genset is used only
for communicating between the genset control and the ECM.
18 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
27/108
3 Network Devices
3.1 Ov ervi ew
In this chapter we will identify the types of devices that are commonly found in a power generation system network. We will refer to Cummins Power Generation’s products as
examples.
We will discuss five types of devices: I/O devices, intelligent controllers, network interface
cards, network management devices (hubs, switches, routers, gateways), and web servers.
3.2 Input/Output Devices
Input/Output Devices allow controllers to get information from the physical world and to control
physical components. I/O devices convert data between physical states or values and logical
signals communicated over a network. For example, the position of a switch, a physical state, is
converted to a logical value and communicated to a controller, or a controller will send a logical
command to activate a relay, or change its physical state. Defining input and output
requirements for system components is a key requirement in the overall system design.
An input device will convert some physical parameter, such as a position of a switch or a
voltage level, to a logical value, which will be communicated over a network. An output device
will receive a logical signal over a network and change the state of an output, which will result in
some physical change to a piece of equipment, such as grounding a relay coil, which will cause
the relay contacts to close. An output from one device can be an input to another device.
Examples of Cummins Power Generation’s network I/O devices include the LonWorks DIM (see
Figure 10), CCM modules, and the Aux101.
FIGURE 10. CUMMINS POWER GENERATION'S AUX101 I/O MODULE
I/O can be either digital (discrete) or analog. Digital refers to signals that can be in one of two
states, for example on or off, or open or closed. Analog I/O refers to a value that is continuously
variable. Voltage, amperage, and temperature are examples of analog signals.
A044E523 (Issue 1) 19
-
8/16/2019 T-034 - Networking.pdf
28/108
3. Network Devices 4-2013
3.2.1 Digital Inputs
Digital inputs, also known as discrete or switch inputs, are used to indicate switch positions in a
power system. Oil pressure switches, temperature switches, status of generator run/off/auto
switches, and auxiliary position indicating switches on circuit breakers and transfer switches are
the most common uses for digital inputs. Digital inputs are inexpensive and simple to implement
compared to analog inputs so it is also common to use digital switches for analog parameters
such as oil pressure or coolant temperature, which will activate when the parameter hasbecome higher or lower than some threshold value. This method does have a disadvantage with
respect to using an analog input in that with a switch a control is not able to recognize a failed
switch. With an analog signal the control is able to differentiate a failed sensor from a parameter
which has passed some fault threshold because a failed sensor will yield an "out of range"
value.
Digital inputs are typically activated by a contact closure between the input line and either
ground or a control voltage reference provided by the control device. Inputs that are activated by
a closure to ground are called “active low”; those that are activated by closure to a control
voltage are called “active high”.
In some cases, a “pull up” or “pull down” resistor (see Figure 11) is required on a digital input to
positively connect the input when it is in its passive state. For example, if a customer activatesan input by switching it to the control voltage (active high), a pull down resistor is required
between the input and ground so that the I/O device will read the input as a “low” value when
the customer’s switch is open. This is the case with the digital inputs on Cummins Power
Generation’s CCM boards when they are used as “Active high” inputs. This is not a requirement
with the Aux101, DIM devices, or CCM inputs when they are used in an active low
configuration. In these cases, the pull up or pull down function is implemented on the control
boards.
FIGURE 11. PULL DOWN RESISTOR
3.2.2 Digital OutputsDigital outputs are used to start or stop generators, operate equipment, such as fans or louvers,
close motor starting contacts, and indicate conditions based on some event that occurs in a
control system. For example, with the Aux 101, generator events are mapped to relays using
the service tool. With some generator set controls the Human Machine Interface (HMI) is also
capable of mapping events to output relays. With the CCM and DIM, the signal is initiated by
some controller and mapped to outputs on the CCM or DIM using configuration software.
20 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
29/108
4-2013 3. Network Devices
A common method for implementing a digital output is to use a form C contact (see Figure
12). A form C contact consists of one normally open and one normally closed contact and a
common line between the two contacts, which ensures that one contact is always open and one
is always closed to the common line and activating the output will change the state of both
contacts. The digital outputs on Cummins CCM, DIM, and Aux 101 are form C contacts.
FIGURE 12. FORM C CONTACTS
The amount of current that a digital output circuit can source or sink is a key consideration in
determining how to apply the device. If the current rating of the digital output is not sufficient to
operate the piece of equipment it is connected to, an external pilot relay will be required.
3.2.3 Analog Inputs
Analog inputs measure continuously variable signals. The parameter to be measured is
converted to a direct current (DC) voltage and read by an input device. The DC voltage is
converted to a numerical value using an analog to digital (A/D) converter and that numerical
value is used by a control or communicated over a network. The sensing circuit will be scaled
so that the maximum current or voltage that the input device can accept will correspond to the
maximum value that the signal can have. For example, 0-5V DC is a common standard for
analog input devices. 0V would correspond to the minimum value and 5V would correspond to
the maximum value that the sensed parameter can have. Analog inputs are used to measure
generator output voltage and current, temperatures of oil, exhaust, ambient air, and alternator
windings, oil pressure, fuel level and voltage, and speed bias lines.
Some sensors, such as temperature and pressure senders, have a resistance that is
proportional to the sensed parameter. A fixed current is passed through the resistive elementwhich will create a voltage for the I/O device to read. With Cummins Power Generation’s Control
Communication Module (CCM) and Aux 101 boards, the current source is provided on the
board. Since oil pressure and different temperature senders all have very different
characteristics, they will require different levels of current to generate voltages in the appropriate
range to be measured by I/O devices. With the CCM there are different current sources for each
analog input and two of the inputs have two different levels of current available, which can be
selected using an on-board switch. With each of the Aux 101 inputs, the operator uses the HMI
to select what parameter is measured and the current source for that input is configured
appropriately.
Some controls can read voltage and speed bias inputs. These are simply customer supplied
potentiometers (variable resistors) through which a fixed current is passed to create a voltage
which can be read by the A/D converter.
Control and communication devices can read phase voltages and load current. Phase voltages
are stepped down through transformers to a level that is suitable for rectification to create a DC
voltage and then read by the A/D converter. Load current is passed through a current
transformer (CT) for sensing and the output current of the CT is run though a burden resistor
that creates a voltage that can be measured. With Cummins Power Generation’s equipment
there are typically two sets of CTs in the sensing circuit, one set right at the generator output or
load terminals of a transfer switch and a second set of smaller CTs mounted on a circuit board.
A044E523 (Issue 1) 21
-
8/16/2019 T-034 - Networking.pdf
30/108
3. Network Devices 4-2013
There are also devices on the market that create a current which is proportional to a measured
parameter. A common standard is 4 – 20 mA devices. To monitor this with a CCM, the current
is passed through a known resistance (off of the CCM board) and the CCM can read the voltage
across the resistor.
To make use of the values created by the A/D circuit, it is necessary to know how the
parameters are scaled. Scaling will depend on the physical properties of the sensing circuit and
the resolution of the A/D converter. For example, a temperature sensing circuit will converttemperature to voltage depending on the resistance/temperature characteristic of the sender
and the current source. The A/D converter will convert that voltage to a number of “counts”
based on its resolution. For instance, a 10 bit A/D converter will convert the measured voltage to
a number of counts between 0 and 1024 (2^10). When the conversions between the measured
parameter and analog voltage, and between analog voltage and A/D counts are known, the
conversion between the measured parameter and counts can be calculated.
In many cases, the I/O device, or the control that gathers input from the I/O device, will make
this calculation or there will be a tool to make this calculation. In other cases, it will be easier to
establish the conversion through a test if it is known that the relationship between the measured
parameter and the analog voltage is linear. Simply reading the A/D output for 2 known inputs
will allow a simple calculation of the conversion factor. For example, consider a temperature
sender. Through test it is determined that at a temperature of 197 °F the sender had a voltageof 3.23V and at 172 °F the voltage was 2.9 V. With these 2 points we can determine the sender
gain and the offset for the sender. The gain defines how much the voltage changes for each
degree of temperature change. It is simply the ratio of the difference in temperature to the
difference in voltage. In this example it is (197-172)/(3.23-2.92) = 80.65 °F/V. This means that
for every 80.65 degrees that the temperature changes, the voltage will change by 1 volt. The
fact that the gain turned out to be a positive number indicates that as the temperature increases
the voltage will increase. Some types of sensors have a negative gain, meaning that as the
sensed parameter increases the corresponding voltage decreases.
Some senders do not have an output of 0 volts when the parameter being sensed is at 0. In
those cases we need to calculate an offset to fully define the sender. To continue the example
above, we will calculate the offset for that sender, which is the temperature that would
correspond to 0 volts. We can calculate this using the sender gain and one point. We'll use the
2.92V, 172 degrees point. The equation is 172 - 2.92*80.65 = -63.5. This means that 0 volts
would correspond to a temperature of -63.5 degrees. A plot of the linear relationship between
temperature and voltage is shown in Figure 13. Note that different analog devices have
different calibration tools and methods. This is just one example.
FIGURE 13. SENDER CHARACTERISTIC
22 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
31/108
4-2013 3. Network Devices
3.2.4 Analog Outputs
There are analog output devices available on the market which are typically controlled by
programmable logic controllers (PLCs). These devices commonly source a 4 – 20 mA current as
their output.
In general, analog outputs are not very common in network products. When a process needs a
controlled, variable analog voltage it typically uses a customized controller rather than an off-the-shelf networked product. While there are standard digital to analog converters (D/A)
available for integration into a circuit board, most analog outputs are generated by using a
pulse-width modulated signal (PWM) which is filtered to yield a steady analog signal. Cummins
Power Generation does not produce any analog output devices for use on a network but uses
them in system control products that interface with 3rd party equipment.
3.3 Intelligent Devices
Intelligent devices have a microprocessor and are capable of making decisions based on
information received over the network and interpreting and processing the information it
receives from its inputs before sending the information over the network. Generator and transfer
switch controls and HMIs are examples of intelligent devices. To illustrate, an HMI will translatea series of button clicks by an operator (digital inputs) and send the information to a controller,
which it will then process and generate a digital output to start the generator. Intelligent devices
are often capable of operating as a standalone piece of equipment; they do not always need the
network communications to function.
3.3.1 Network Interface Card
Network Interface Cards, or NICs (see Figure 14), are used to connect a device to a network.
Cummins 2100, 3100, and 3201 generator set controls and OTPC, BTPC, CHPC and PLT
transfer switch controls have NICs, known as Network Communications Modules (NCM), which
connect these controls to LonWorks networks. The NCMs are sold as a feature with the
generator or transfer switch and are physically mounted on the main control board.Market trends have evolved to where communication capability is expected on even the most
basic controls. Controls have long had integral network capabilities when a standard interface is
required on all controls such as CAN interface from a genset controller to an engine. With the
market expectation of communications capabilities on most controls it is becoming more
common to have the network interface resident on the control rather than as a separate
component. Cummins Power Generation’s 1.X, 2.X, and 3.X generator controls are examples of
controls with integral network capabilities.
FIGURE 14. NETWORK INTERFACE CARD
A044E523 (Issue 1) 23
-
8/16/2019 T-034 - Networking.pdf
32/108
3. Network Devices 4-2013
3.4 Network Management Devices
There are several types of devices that manage traffic on a network. These devices are
commercially available from a number of suppliers. With the exception of the ModLon Gateway,
Cummins Power Generation does not produce any of these devices.
3.4.1 Hub/Repeater Hubs and repeaters are simple devices that repeat all of the network signals to all network
devices to which they are connected. They do not process data or make any type of decision
regarding where to send the data.
A repeater has two ports. It connects two segments of a network. Since they are able to amplify
a weak signal, repeaters are able to extend a network over large physical distances.
A hub has multiple ports and allows devices to attach to a network in the star configuration.
(See Section 4.2 on page 29 for discussion on network topology.) A hub receives data and
broadcasts the data to all of its ports.
Hubs and repeaters are becoming less common as they are not able to meet demands for
faster networks with more devices. More often than not, they are replaced by Ethernet switches.
3.4.2 Switch
A switch is similar to a hub except instead of broadcasting all the messages it sees, it checks to
see that there are no errors in the message packet and if there are no errors, it will transmit the
message only to its intended recipient.
By sending only “good” messages to their intended recipient, switches increase the speed of the
network by eliminating a lot of unnecessary and erroneous traffic. Switches can also improve
network speed by eliminating collisions.
Ethernet switches divide a network into segments called “collision domains”. A collision domain
is defined as a set of network devices whose packets can collide with each other. As discussed
in Chapter 2, if two devices on a network try to “talk” at the same time there is a collision. Whena collision occurs, all the data is lost and each device will delay its message before attempting
to transmit again.
Because a switch manages traffic between ports, there will be no collisions between devices on
separate ports. Only devices that are connected to the same port of a switch can experience
collisions. Devices connected to the same port on a switch are said to be in the same collision
domain. To completely eliminate the possibility of collisions, only a single device is connected to
each port of a switch. There is only one single device in each collision domain.
To maximize speed of network communication, full duplex switching is employed. Full duplex
communication allows devices to both transmit and receive messages at the same time. This is
accomplished by each device having separate transmit and receive lines. By using full duplex
communication with only one device on each collision domain the speed of communication isoptimized. It is limited only by the characteristics of the transceiver and the media.
3.4.3 Bridge
A bridge is similar to a switch except that a bridge has only two ports where a switch may have
many ports. A bridge can connect two network segments separated by large distances or two
segments that are communicating over different types of physical media.
24 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
33/108
4-2013 3. Network Devices
3.4.4 Routers
Routers are similar to switches in that they both manage network traffic. The difference is that
switches direct traffic within a local network; whereas, routers direct traffic between
networks. Where a switch communicates between network segments that are physically
connected to that switch, routers can connect two devices that are physically separated by great
distances with multiple routers in between them. Routers operate based on a device’s logical
address rather than physical location. To illustrate, one can connect his/her laptop to their internet service provider’s server from their home, their office, an airport or their favorite coffee
shop. The physical location does not matter; it is the IP (Internet Protocol) address, the logical
address, that matters.
Routers enable networks that use different media, access, and addressing protocols to
communicate with each other. (They can translate between different Physical, Data Link, and
Network Layer protocols, and layers 1, 2 and 3 of the OSI model. The OSI model will be
discussed further in Chapter 6.) They do not translate between higher level languages
(Application Layer), such as LonWorks and Modbus.
A common example of this is in a campus application where generators, transfer switches, and
switchgear are all located in an equipment room and are to be monitored in a control room a
great distance away. There is an Ethernet connection over fiber optic cable between the tworooms. The equipment communicates via LonWorks over twisted pair wire. In each of the two
rooms there would be a router with one channel connected to the twisted pair wire and the
second channel connected to the fiber. The router does not translate the high level language
(LonWorks), it just manages the transmission of data from the twisted pair network, over the
fiber optic cable, and back to the twisted pair network at the other end. See Figure 15.
FIGURE 15. ROUTERS IN MULTIPLE NETWORKS
3.4.5 Gateways A gateway is a device that enables two network segments of different protocols to communicate
with each other. A gateway manages all layers of the OSI protocol stack from the physical
media to the high level language. Cummins Power Generation’s ModLon Gateway (see Figure
16), which converts LonWorks to Modbus data for Cummins products, is a good example of a
gateway. It should be noted that gateways can slow down communication and in some cases
can compromise reliability as they introduce single points of failure into the system.
A044E523 (Issue 1) 25
-
8/16/2019 T-034 - Networking.pdf
34/108
3. Network Devices 4-2013
FIGURE 16. MODLON GATEWAY
In some cases, gateways that connect serial protocols, such as RS-232, to Ethernet are known
as Device Servers. They are typically used to connect devices such as printers and fax
machines which have only a serial connection to an office LAN so that the equipment can be a
shared resource.
3.5 Web Servers
Web servers are devices that reside on a network and serve web pages consisting of data
acquired from equipment on that network. In most cases web servers will be capable of sending
emails based on alarm conditions. Typically, with a web server in a power generation
application, the owner of the system needs to provide internet access at the site where the
generator is and will need to provide the appropriate IP and email server addresses to connect
to the internet.
Cummins’ PowerCommand 500 and 550 and iWatch 100 are examples of web servers (see
Figure 17). The PowerCommand 500/550 and the iWatch 100 communicate with generator and
transfer switch controls over LonWorks or Modbus and serve web pages that display the currentstatus of the equipment. Operators can view this data with their own web browser. If operators
have the proper security access they can also start the generator set or initiate a test of the
transfer switch through the web page. Web servers will also send text messages or emails on
alarm conditions so that service personnel can be alerted that there has been some event in
their power system. Cummins Power Generation’s DMC200 and DMC300 digital master
controllers also have integral web servers. Components in some equipment may also have web-
servers. For example, it's physically possible for a generator set control to have an on-board
web server. However, it is more practical to have a web server for a system than a single
component in a multiple component system.
26 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
35/108
4-2013 3. Network Devices
FIGURE 17. NETWORK INCLUDING IWATCH WEB SERVER
A044E523 (Issue 1) 27
-
8/16/2019 T-034 - Networking.pdf
36/108
3. Network Devices 4-2013
This page is intentionally blank.
28 A044E523 (Issue 1)
-
8/16/2019 T-034 - Networking.pdf
37/108
4 Network Characteristics
4.1 Ov ervi ew
This chapter defines physical network characteristics, topology, bandwidth, and distance. Itdescribes the interaction between these characteristics and provides a baseline for comparison
of various media and protocols.
Selection of topology, bandwidth, and distance impose tradeoffs on a system. Typically,
bandwidth and distance conflict with each other. Increasing the physical distance between
devices decreases the maximum speed of data transmission. Topology choices will affect the
other capabilities as well, with the simplest topology allowing for the greatest distance and
bandwidth, while more complex topologies will impose limitations.
4.2 Network Topology
Topology refers to the physical shape and interconnection scheme of the network. The threemost common topologies used in control networks are known as “bus”, “ring”, and “star”. Some
of the more complex topologies such as “tree” and “mesh” topologies are combinations of the
three basic topologies. Echelon Corporation, creators of the LonWorks network platform, uses
the term “Free Topology” to position LonWorks as flexible enough to be configured in any
topology. Some topology examples are shown below (Figure 18).
FIGURE 18. COMMON NETWORK TOPOLOGIES
The main criteria for choosing a network topology are:
• Distance/bandwidth capability of the media and hardware,
• Degree of difficulty in wiring it,
• Robustness in the event of a break/failure,
• Capability for redundant communication paths,
• Ability to segment traffic (does all traffic have to pass through one spot? are there
dedicated paths between individual devices?).
A044E523 (Issue 1) 29
-
8/16/2019 T-034 - Networking.pdf
38/108
4. Network Characteristics 4-2013
4.2.1 Bus Topology
With a bus topology, all of the devices are connected to a common backbone or bus. A bus
topology is the simplest and least expensive to install. A bus topology will allow for the greatest
distance and bandwidth. With Cummins Power Generation networks, we always recommend
using a bus topology except where redundant communication paths are required.
Bus topology is very similar to a “daisy chain” topology. The terms “bus” and “daisy chain” areoften used interchangeably but there is a difference. With a bus topology, each device is
connected to the bus, and with a daisy chain, each device is connected directly to the adjacent
device. With a bus network,