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RS-485. Physical Signaling Differences. Harmonic Spectra. Input. Output. 1st. Sum. 3rd. 5th. INCOM. Because different frequencies are delayed and attenuated by different amounts, a waveform rich in harmonics is distorted after passing through an LC circuit.

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rs 485
RS-485

Physical Signaling Differences

Harmonic Spectra

Input

Output

1st

Sum

3rd

5th

INCOM

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

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

detecting rs 485 1 s and 0 s
Detecting RS-485 1’s and 0’s

11

00

11

01

0?

1?

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.

incom decoding
INCOM Decoding

0

1

0

1

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.

noise immunity
Noise Immunity

S1 + S2

S2

Delay

S1 + S2

S1

S1 + S2

S2

Delay

S1 + S2

S1

RS-485

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

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

slide7

A

B

C

D

E

F

G

H

I

J

1

1

Cutler-Hammer IMPCABLE or Belden 9463

(or equivalent in Plenum)

RJ-11

Install in PC

CONI

This design places brings all network (Lighting Control and

Power Monitoring) back to the Central Plant.

RJ-45

IN 1

IP 2

Shield 3

Phoenix Contact

RJ-11 to Terminals

Ethernet Hub

Ethernet

10BASE-T

RS-232

2

Belden 9207

(or equivalent in Plenum)

2

RS-485/

232

200’ max

Typical Switchboard Tap

RS-485

Device

To PDS

PONI

PONI

130

131

3

3

IED

IED

IED

Typical

Typical

IQ Analyzer

IQ Analyzer

4

4

PONI

131

5

5

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

IED

RS-485

Network

INCOM

Network

Typical

IQ Analyzer

6

6

Parking

Garage

Parking Lot

Terminal

RS-485

Network

7

DFTR

DATE

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.

1

APPD

DATE

DGL

11-DEC-98

CUTLER-HAMMER

APPD

DATE

POWER MANAGEMENT PRODUCTS CENTER

ENG. REF.

DATE

TITLE

REVISION

NEXT REF.

DATE

8

SHEET NO

1 OF 1

SIZE

SCALE

DIRECTORY

DWG NO

NONE

A

A

B

C

D

E

F

rs 485 and incom wiring rules
RS-485 and INCOMWiring Rules

200feet

200feet

200feet

200feet

Total Cable Length  to 

= 13 x 200’

= 2600 feet

Conduit space required = sufficient for 2 twisted pairs

200 feet

200 feet

200 feet

200 feet

RS-485

Node

RS-485

Node

RS-485

Node

RS-485

Node

RS-485

Node

200feet

200feet

200feet

200feet

Total Cable Length  to 

= 9 x 200’

= 1800 feet (70% of the RS-485 system)

Conduit space required =

sufficient for 1 twisted pairs

200 feet

200 feet

200 feet

200 feet

IQ

Node

IQ

Node

IQ

Node

IQ

Node

IQ

Node

reduce total cost of ownership
Reduce Total Cost of Ownership
  • Reduce initial installed cost
  • Lower operating cost
  • Reliability & Safety
  • Maximize capital utilization
  • Better customer service