RS-485

INCOM

Sum

Because different frequencies are delayed and attenuated by different amounts, a waveform rich in harmonics is distorted after passing through an LC circuit.

Harmonic Spectra

1st

3rd

5th

Input Output

On the contrary, a pure sinusoid, while delayed and attenuated, keeps its original waveshape after passing through an LC circuit.

Physical Signaling Differences

Detecting RS-485 1’s and 0’s

Receivers than convert serial bit streams into parallel data are called UARTs (Universal Asynchronous Receiver Transmitters). A UART determines whether the serial stream contains a 1 or a 0 by sampling at the bit rate (9600, 19.2 k, etc.) For better error immunity it typically samples 2 or 3 times per bit. Two bit readings in a row must be the same for the UART to declare a 1 or a 0.

With distorted wave shapes, the UART will typically be observing a data stream where the bit value changes from within a bit time.

This can cause the UART to declare various errors (such as framing or parity) and discard the byte.

11 00 11 01 0? 1?

INCOM Decoding

Since the waveshape is not changed, it is still easy to detect the difference between a 1 and 0 from an Frequency Shift Keyed (FSK) signal.

INCOM signals are not as affected by impedance mismatches (taps, T’s, Y’s, etc.). Consequently the cabling rules are more relaxed than with RS-485.

0 1 0 1

Excerpt (Page 33) from RS-485Based Installation Manual

Noise Immunity

RS-485

INCOM

Since RS-485 decides if a signal is a 1 or a 0 by examining the amplitude, external events can mask the true value.

INCOM FSK uses a bandpass filter to eliminate signals substantially above or below 100 kHz. Therefore large amplitude interfering signals removed

Delay S1

S2

S1 + S2

DelayS1

S2

S1 + S2

S1 + S2

S1 + S2

INCOM Vs RS-485

Companies using RS-485 claim an advantage of higher bit rates (19.2K or faster) compared to INCOM(9.6K). One consultant tested the distance constraints for a leading vendor using RS-485 at 19.2K andfound reliable communication could be guaranteed only if the network length was less than 4 feet. In addition, response time is based on the latency of the device. One leading vendor’s motor protective relayrequires 1 second to formulate a response to a message. A network with 10 of these relays would updateno faster than once every 10 seconds! INCOM device latency varies from 0.003 seconds to approximately0.01 seconds.

485 networks cannot be run nearby power conductors and as such require their signal wire to be installed inconduit. FSK does not. That’s a minimum $6.65/LF installation cost adder per the 1999 Means ElectricalCost Data Manual. Also, 485 networks have a distance constraint of 4000 feet, then a repeater/signalbooster of some type must be used. INCOM has a 10,000 foot limit. When you combine 485’s need tohave a data concentrator for every 32 devices (PowerNet supports up to1000, unlimited taps, and 5branches without using additional hardware) you have further reduced installation costs, opportunities forfailure and additional maintenance overhead for hardware and software.

IQ devices use an isolation transformer between the network and its circuitry. 485 devices can’t, as theymust be DC coupled. Otherwise there would be severe distortion of the shape of their square wave.Typically they use an electrolytic capacitor as the isolation device. Electrolytic capacitors are liquid filleddevices that eventually dry out. The result is limited life and low voltage and temperature rating. C-Hprovides 1500V network isolation; RS-485 provides little or none.

A B C D E F G H I J

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A B C D E F

THE INFORMATION ON THIS DRAWING IS THE PROPERTY OFEATON CORPORATION. IT IS DISCLOSED IN CONFIDENCEAND IS NOT TO BE REPRODUCED, USED OR DISCLOSEDEXCEPT FOR THE PURPOSE FOR WHICH FURNISHED.

CUTLER-HAMMERPOWER MANAGEMENT PRODUCTS CENTER

DWG NO

TITLE

DFTR DATE

APPD DATE

APPD DATE

ENG. REF. DATE

NEXT REF. DATE

SHEET NO

SIZE

A

RE

VIS

ION

1

SCALE

NONEDIRECTORY

DGL 11-DEC-98

1 OF 1

ParkingGarage TerminalParking Lot

IED

IED

IN 1

Shield 3 IP 2

RJ-11CONIInstall in PC

Phoenix ContactRJ-11 to Terminals

Cutler-Hammer IMPCABLE or Belden 9463 (or equivalent in Plenum)

PONI

Typical Switchboard Tap

IQ Analyzer

PONI

IQ Analyzer

Typical Typical

130 131

200’ max

RS-485/232

Belden 9207(or equivalent in Plenum)

RS-232

IQ Analyzer

Typical

PONI

131

RS-485Device

Eth

ern

et H

ub

To PDS

Ethernet10BASE-T

RJ-45

IEDIED

RS-485Network

This design places brings all network (Lighting Control andPower Monitoring) back to the Central Plant.

These taps are legal with INCOM wiring. These are not legal with RS-485

RS-485Network

INCOMNetwork

RS-485 and INCOMWiring Rules

RS-485Node

IQNode

200

feet

200feet

IQNode

IQNode

IQNode

IQNode

Total Cable Length to = 13 x 200’= 2600 feet

Conduit space required = sufficient for 2 twisted pairs

Total Cable Length to = 9 x 200’= 1800 feet (70% of the RS-485 system)

Conduit space required = sufficient for 1 twisted pairs

RS-485Node

RS-485Node

RS-485Node

RS-485Node

200

feet

200

feet

200

feet

200feet

200feet

200feet

200

feet

200feet

200

feet

200

feet

200

feet

200feet

200feet

200feet

Excerpts (Page 34) from RS-485 Based Installation Manual

Reduce Total Cost of Ownership

Reduce initial installed cost Lower operating cost Reliability & Safety Maximize capital utilization Better customer service