MADE BY:

ARNOVIA CHRISTINE SABATIANAMARTHA MAULINA

RIZALDI SATRIA NUGRAHA

POLITEKNIK NEGERI BANDUNGREFRIGERATION AND AIR CONDITIONING ENGINEERING

Serial Data Communication

MIND MAP

DEFINITION OF SERIAL DATA COMMUNICATION

SOURCE RECEIVERCOMMUNICATIO

N LINK

Both the receiver and the transmitter must agree on a number of different factors to allow successful communications between them, the most important being: • The type of electrical signals used to transmit the data • The type of codes used for each symbol being transmitted • The meaning of the characters • How the flow of data is controlled • How errors are detected and corrected

TRANSMISSION MODE

CODING OF MESSAGE

Encoding is the process of converting the message data into a standard binary code for transfer over the data communications link. The number of bits in a code determines the total number of unique characters that are possible.

ASCII (American Standard Code for Information Interchange) is the most commonly used code for encoding characters for data communications. It is a 7-bit code with only 2^7 = 128 possible combinations of the seven binary digits (bits). Each of these 128 codes is assigned to a specific control code or character, as specified by the these standards:

• ANSI – X3.4 • ISO – 646 • CCITT alphabet #5

FORMAT OF DATA COMMUNICATION MESSAGE

Data is usually arranged in a particular format, with additional information added so that the message can be effectively transmitted and understood at the receiving end. In summary, the optional settings for asynchronous transmission of characters are: • Start bits 1 • Data bits 5, 6, 7, 8 • Parity bits even, odd, mark, space or none • Stop bits 1, l½ or 2 As there cannot be half a bit, 1½-stop bits means that the mark length is 50% longer than for one stop bit.

DATA TRANSMISSION SPEED

The maximum rate at which data can be transferred from the source to the receiver on a com-munications interface depends on a number of factors:

• Type and complexity of the circuitry at each end (interface)

• Communication link (twisted-pair, coaxial cable, radio etc)

• Distance between the sender and receiver • Amount of data being transferred • The overhead associated with the data transfer • The acceptable rate of error

RS-232-C INTERFACE STANDARD

In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment). It is commonly used in computer serial ports.The EIA-232 standard consists of three major components, which define:

• Electrical signal characteristics• Interface mechanical characteristics • Functional description of the interchange circuits

ELECTRICAL SIGNAL CHARACTERISTIC

At the RS-232 receiver the following signal voltage levels are defined:

• +3 V to +25 V for transmission of logic 0

• –3 V to –25 V for transmission of logic 1

• +3 V to –3 V for an undefined logic level

In practice, many EIA-232 transmitters operate very lose to their margin of safety, e.g. at +7 and –7 volts. This can be acceptable for short cable runs, where it is hoped that there will be no voltage problems. Unfortunately, increased error rates can be expected at the receiver because of induced external interference voltages.

INTERFACE MECHANICAL CHARACTERISTIC

While this pin configuration is likely to be adhered to by manufacturers at the computers communications interface, it is possible (and often likely) that the data receive and transmit lines on remote stand-alone systems are on different pins of the DB-9 connector.

FUNCTIONAL DESCRIPTION OF INTERCHANGE SIRCUIT

Pin 1: Protective ground (shield) Pin 2: Transmitted data (TXD)Pin 3: Received data (RXD)Pin 4: Request to send (RTS) Pin 5: Clear to send (CTS)Pin 6: Data set ready (DSR)Pin 7: Signal ground (common) Pin 8: Data carrier detect (DCD)Pin 20: DTE ready (or data terminal ready)Pin 22: Ring indicator Pin 23: Data signal rate selector (DSRS)

MAIN FEATURE

The following are some of the main features of equipment that use the EIA-232 interface standard:

Communication is point-to-point They are suitable for serial, binary, digital, data

communication (data is sent bit by bit in sequence) Most EIA-232-C communications data is in the ASCII

code, although that is not part of the standard Communication is asynchronous (fixed timing between

data bits, but variable time between character frames) Communication is full-duplex (both directions

simultaneously) with a single wire for each direction and a common wire

RS-485 INTERFACE STANDARD

The EIA RS-485 is the most versatile of the EIA standards, and is an expansion of the RS-422 standard. The RS-485 standard was designed for two-wire, half duplex, balanced multidrop communications, and allows up to 32 line drivers and 32 line receivers on the same line.RS-485 provides reliable serial communications for:

• Distances of up to 1200 m • Data rates of up to 10 Mbps • Up to 32 line drivers permitted on the same line • Up to 32 line receivers permitted on the same line

RS-485 REPEATER

The repeater is a two-port device that re-transmits data received on one side, at full voltage levels, to the network on the other side.

COMPARISON BETWEEN TWO TYPE

PROTOCOL

A protocol is essential to the correct operation of the communication system and determines a number of important features including:

• Initialization • Framing and frame synchronization • Flow control • Line control • Error control • Timeout control

FLOW CONTROL PROTOCOL

Cooperative flow control, in which the transmitter and receiver operate under a common set of rules, is called a flow control protocol. Below are described the two most popular flow control protocols.

Character flow protocols (XON/XOFF) Whole line protocols (ETX/ACK)

ASCII BASED PROTOCOL

The ASCII protocol is normally only used for slow systems with one master talking to a limited number of slaves. ASCII protocols are also popular for stand-alone instruments where a serial interface has been added, with no major design changes, to the existing system.

A simple command/response ASCII protocol, used for communications between a personal computer and a digital transmitter is shown above.

A variation of the short form command and response messages is their long form equivalents. To ensure greater message integrity, and increase reliability, long form messages are included with a block checksum at the end of the message.

ERROR DETECTION

There are three popular forms of error checking used in many protocols. These are, in order of increasing error-detecting capability:

• Character redundancy checks • Block redundancy checks • Cyclic redundancy checks

TROUBLESHOOTING

When trouble shooting a serial data communications interface, a logical approach needs to be followed, to avoid wasting many frustrating hours trying to find the problem. A procedure similar to that outlined below is recommended:

• Check the basic parameters • Identify which is DTE or DCE • Clarify the needs of the hardware

handshaking • Check the actual protocol used

When a data communication link won’t work, the following five very useful devices can be used to assist in analyzing the problem:

A Breakout BoxNull ModemLoop Back Plug PC based protocol analyzer (including

software)