40
Installing the serial communication cable for
RS485 SPA/IEC
Chapter 5
Installing the terminal
Three isolation options exist:
a) The entire node electronics can be galvanically isolated
b) The bus interface circuit can be isolated form the rest of node electronics by optoisola-
tors, transformer coupling or otherwise.
c) The RS485 chip can include built-in isolation
2.3
Bus excitation and signal conveyance
2.3.1
Requirements
a) The RS485 specification requires the Signal A and Signal B wires.
b) Each node also requires (5 V) Excitation of the RS485 termination network.
c) Vim - the common mode voltage between any pair of RS485 chips may not exceed 10 V.
d) A physical ground connection between all RS485 circuits will reduce noise.
2.3.2
Bus segment termination network
The termination network below required at each end of each Bus Ph-segment.
Figure 11:
RS-485 bus segment termination
ExV is supplied by the Node at end of the Bus Segment
The specifications of the components are:
a) Ru
+ 5 V to Signal B
= 390
Ω
, 0.25 W
±
2.5%
b) Rt
Signal B to Signal A
= 220
Ω
, 0.25 W
±
2.5%
c) Rd
Signal A to GND
= 390
Ω
, 0.25 W
±
2.5%
2.3.3
Bus power distribution
The end node in each Ph-segment applies 5 V bus excitation power to the Termination network via
the Excitation pair (ExV+ and GND) used in the Type 3 Physical layer specification.
en03000112.vsd
ExV+
Signal B
Signal A
DGND
Ru = 390 ohm
1/4 W, 2%
Rt = 220 ohm
1/4 W, 2%
Rd = 390 ohm
1/4 W, 2%
ExV is supplied by the Node at end of the Bus Segment
Summary of Contents for REB 551-C3*2.5
Page 9: ...Contents ...
Page 21: ...12 Introduction to the installation and commissioning manual Chapter 1 Introduction ...
Page 27: ...18 Note signs Chapter 2 Safety information ...
Page 53: ...44 Installing the 56 64 kbit data communication cables Chapter 5 Installing the terminal ...
Page 59: ...50 Checking the binary I O circuits Chapter 6 Checking the external circuitry ...
Page 147: ...138 Repair support Chapter 16 Fault tracing and repair ...