GE 239 Instruction Manual page 34

CHAPTER 2: INSTALLATION
should match the polarity of the master device. When the communications link is active,
COMMUNICATE light will be solid if valid data and relay address are being
the front panel
COMMUNICATE light flashes to indicate invalid data, try
received. If the front panel
reversing the wires to terminals 15 and 16. Each relay must be daisy chained to the next
one as shown in FIGURE 2–8: RS485 Communication Wiring on page 2–28. Avoid star or
stub connected configurations. If a large difference in ground potentials exists,
communication 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 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 terminating resistors and 1 nF capacitor
externally. Although any standard resistor or capacitor of these values 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 . 239PC, a
S1: 239 SETUP\RS485 SERIAL PORT\SLAVE ADDRESS
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 2–8: RS485 Communication Wiring
2–28 239 MOTOR PROTECTION RELAY – INSTRUCTION MANUAL
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