Rs485 Communications Port - GE GEK-113045B Manual

2.3 ELECTRICAL INSTALLATION 2 INSTALLATION

2.3.12 RS485 COMMUNICATIONS PORT

One two-wire RS485 port is provided. Up to 32 SPMs can be daisy-chained together on a communication
channel without exceeding the driver capability. For larger systems, additional serial channels must be added.
It is also possible to use commercially available repeaters to increase the number of relays on a single channel
to more than 32. Suitable cable should have characteristic impedance of 120 Ω (e.g. Belden #9841) and total
wire length should not exceed 4000 ft. Commercially available repeaters will allow for transmission distances
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greater than 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 is
used to prevent noise coupling. To ensure that all devices in a daisy-chain are at the same potential, it is imper-
ative that the common terminals of each RS485 port are tied together and grounded only once, at the master.
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, to ensure maximum
reliability. To avoid ground loops, ground the shield at one point only as shown below.
Correct polarity is also essential. SPMs must be wired with all '+' terminals connected together, and all '–' termi-
nals connected together. Each relay must be daisy-chained to the next one. Avoid star or stub connected con-
figurations. The last device at each end of the daisy chain should be terminated with a 120 Ω, ¼ watt resistor in
series with a 1 nF capacitor across the '+' and '–' terminals. Observing these guidelines will result in a reliable
communication system immune to system transients.
Figure 2–6: RS485 WIRING
2- SPM Synchronous Motor Protection and Control GE Multilin
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