GE 239 Instruction Manual page 28

2 INSTALLATION 2 INSTALLATION
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device. When the communications link is active, the front panel light will be solid if
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valid data and relay address are being received. If the front panel light flashes to
indicate invalid data, try reversing the wires to terminals 15 and 16. Each relay must be daisy
chained to the next one as shown in Figure 2–7: RS485 COMMUNICATION WIRING on page 2–12.
Avoid star or stub connected configurations. If a large difference in ground potentials exists, commu-
nication on the serial communication link will not be possible. Therefore, it is imperative that the
serial master and 239 are both at the same ground potential. This is accomplished by joining 485
2
ground terminal 17 of every unit together and grounding it at the master only.
The last 239 in the chain and the master computer need a terminating resistor and terminating
capacitor to prevent communication errors by ensuring proper electrical matching of the loads. Using
terminating resistors on all the 239s would load down the communication network while omitting
them at the ends could cause reflections resulting in garbled data. Install the 120 Ω / ¼ watt terminat-
ing resistors and 1 nF capacitor externally. Although any standard resistor or capacitor of these val-
ues are suitable, these components can also be ordered from GE Multilin as a combined terminating
network.
Each communication link must have only one computer (PLC or DCS) issuing commands called the
master. The master should be centrally located and can be used to view actual values and setpoints
from each 239 relay called the slave device. Other GE Multilin relays or devices that use the Modbus
RTU protocol can be connected to the communication link. Setpoints in each slave can also be
changed from the master. Each 239 in the communication link must be programmed with a different
slave address prior to running communications using 6  6(783?56 6(5,$/ 3257?6/$9( $''5(66 .
239PC, a communications software package developed by GE Multilin, may be used on a PC to
view motor status, actual values, and view and alter setpoints.
Figure 2–7: RS485 COMMUNICATION WIRING
2-12 239 Motor Protection Relay GE Multilin
Courtesy of NationalSwitchgear.com