Serial Communications - GE MI-869 Instruction Manual

ELECTRICAL INSTALLATION

Serial Communications

CAUTION:
2–26
One two-wire RS485 port is provided. Up to 32 8 Series IEDs can be daisy-chained together
on a communication channel without exceeding the driver capability. For larger systems,
additional serial channels must be added. Commercially available repeaters can also be
used to add more than 32 relays on a single channel. Suitable cable should have a
characteristic impedance of 120 ohms and total wire length should not exceed 1200
meters (4000 ft).
Voltage differences between remote ends of the communication link are not uncommon.
For this reason, surge protection devices are internally installed across all RS485
terminals.Internally, an isolated power supply with an opto-coupled data interface is used
to prevent noise coupling.
Figure 2-27: RS485 wiring diagram
To ensure that all devices in a daisy-chain are at the same potential, it is imperative
that the common terminals of each RS485 port are tied together and grounded only
once, at the master or at the 869. Failure to do so may result in intermittent or failed
communications.
The source computer/PLC/SCADA system should have similar transient protection devices
installed, either internally or externally. Ground the shield at one point only, as shown in the
figure above, to avoid ground loops. Correct polarity is also essential. The 869 IEDs must be
wired with all the positive (+) terminals connected together and all the negative (–)
terminals connected together. Each relay must be daisy-chained to the next one. Avoid
star or stub connected configurations. The last device at each end of the daisy-chain
should be terminated with a 120 ohm ¼ watt resistor in series with a 1 nF capacitor across
the positive and negative terminals. Some systems allow the shield (drain wire) to be used
as a common wire and to connect directly to the COM terminal; others function correctly
only if the common wire is connected to the COM terminal, but insulated from the shield.
Observing these guidelines ensure a reliable communication system immune to system
transients.
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 2: INSTALLATION