GE Multilin 369 Quick Start Manual page 17

CHAPTER 2: INSTALLATION
369 MOTOR MANAGEMENT RELAY – QUICKSTART GUIDE
Up to 32 369s (or other devices) can be daisy-chained together on a single serial
communication channel without exceeding the driver capability. For larger systems,
additional serial channels must be added. Commercially available repeaters may also be
used to increase the number of relays on a single channel to a maximum of 254. Note that
there may only be one master device per serial communication link.
Connections should be made using shielded twisted pair cables (typically 24 AWG).
Suitable cables should have a characteristic impedance of 120 ohms (e.g. Belden #9841)
and total wire length should not exceed 4000 ft. Commercially available repeaters can be
used to extend transmission distances.
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 optocoupled data interface is used to prevent
noise coupling. The source computer/PLC/SCADA system should have similar transient
protection devices installed, either internally or externally, to ensure maximum reliability.
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 in one
location only, at the master. Failure to do so may result in intermittent or failed
communications.
Correct polarity is also essential. 369 relays must be wired with all '+' terminals connected
together, and all '–' 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 '+' and '–' terminals. Observing these guidelines will result in a reliable
communication system that is immune to system transients.
FIGURE 2–5: RS485 Wiring
ELECTRICAL INSTALLATION
2–13