RS-485 - Eaton Corporation

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Transcript RS-485 - Eaton Corporation

Physical Signaling Differences
Harmonic Spectra
Input
RS-485
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
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
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.
Excerpt (Page 33) from RS-485
Based Installation Manual
Noise Immunity
S1 + S2
RS-485
S2
Delay
S1 + S2
S1
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
S1 + S2
S2
Delay
S1
S1 + S2
INCOM FSK uses a bandpass filter to eliminate signals
substantially above or below 100 kHz. Therefore large
amplitude interfering signals removed
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 and
found 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 relay
requires 1 second to formulate a response to a message. A network with 10 of these relays would update
no faster than once every 10 seconds! INCOM device latency varies from 0.003 seconds to approximately
0.01 seconds.
 485 networks cannot be run nearby power conductors and as such require their signal wire to be installed in
conduit. FSK does not. That’s a minimum $6.65/LF installation cost adder per the 1999 Means Electrical
Cost Data Manual. Also, 485 networks have a distance constraint of 4000 feet, then a repeater/signal
booster of some type must be used. INCOM has a 10,000 foot limit. When you combine 485’s need to
have a data concentrator for every 32 devices (PowerNet supports up to1000, unlimited taps, and 5
branches without using additional hardware) you have further reduced installation costs, opportunities for
failure 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 they
must 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 filled
devices that eventually dry out. The result is limited life and low voltage and temperature rating. C-H
provides 1500V network isolation; RS-485 provides little or none.
A
B
C
1
Install
in PC
RJ-45
D
CONI
E
RJ-11
F
G
H
I
J
1
Cutler-Hammer IMPCABLE or Belden 9463
(or equivalent in Plenum)
This design places brings all network (Lighting Control and
Power Monitoring) back to the Central Plant.
IN 1
IP 2
Shield 3
Ethernet Hub
Phoenix Contact
RJ-11 to Terminals
2
Ethernet
10BASE-T
RS-232
2
Belden 9207
(or equivalent in Plenum)
RS-485/
232
200’ max
Typical Switchboard Tap
RS-485
Device
To PDS
PONI
130
3
IED
IED
IED
PONI
131
Typical
3
Typical
IQ Analyzer
IQ Analyzer
4
4
PONI
131
5
5
IED
Typical
IQ Analyzer
6
Terminal
RS-485
Network
DFTR
DATE
APPD
DATE
1
7
RS-485 INCOM
Network Network
Parking
Garage
Parking Lot
These taps are
legal with INCOM
wiring. These are
not legal with RS485
DGL
11-DEC-98
THE INFORMATION ON THIS DRAWING IS THE PROPERTY OF
EATON CORPORATION. IT IS DISCLOSED IN CONFIDENCE
AND IS NOT TO BE REPRODUCED, USED OR DISCLOSED
EXCEPT FOR THE PURPOSE FOR WHICH FURNISHED.
CUTLER-HAMMER
APPD
DATE
ENG. REF.
DATE TITLE
NEXT REF.
DATE
REVISION
POWER MANAGEMENT PRODUCTS CENTER
SHEET NO
8
1 OF 1
A
B
C
D
E
F
SIZE
SCALE
A
NONE
DIRECTORY
DWG NO
6
RS-485 and INCOM
Wiring Rules
200
feet

200
feet
200
feet
200
feet
200
feet
200
feet
200
feet
Total Cable Length  to 
= 13 x 200’
= 2600 feet
200
feet

RS-485
Node

RS-485
Node
RS-485
Node
RS-485
Node
Conduit space required =
sufficient for 2 twisted pairs
RS-485
Node
200
feet
200
feet
200
feet
200
feet
200
feet
200
feet
200
feet
200
feet
Total Cable Length  to 
= 9 x 200’
= 1800 feet (70% of the RS-485 system)

IQ
Node
IQ
Node
IQ
Node
IQ
Node
IQ
Node
Conduit space required =
sufficient for 1 twisted pairs
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