ABB REB 551-C3*2.5 Installation And Commissioning Manual page 49

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Installing the serial communication cable for
RS485 SPA/IEC
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.
ExV+
Signal B
Signal A
DGND
ExV is supplied by the Node at end of the Bus 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
b) Rt Signal B to Signal A
c) Rd Signal A to GND
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.
Ru = 390 ohm
1/4 W, 2%
Rt = 220 ohm
1/4 W, 2%
Rd = 390 ohm
1/4 W, 2%
en03000112.vsd
= 390 Ω , 0.25 W ± 2.5%
= 220 Ω , 0.25 W ± 2.5%
= 390 Ω , 0.25 W ± 2.5%
Chapter 5
Installing the terminal