Rs485 Communications Ports - GE 469 Manual

2.2 ELECTRICAL
Two independent two-wire RS485 ports are provided. Up to 32 469s can be daisy-chained together on a com-
munication 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 Ω (e.g. Belden #9841) and total wire length
should not exceed 4000 ft. Commercially available repeaters will allow for transmission distances greater than
2
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 optocoupled data interface is used to prevent noise coupling.
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 mas-
ter. Failure to do so may result in intermittent or failed communications.
NOTE
The source computer/PLC/SCADA system should have similar transient protection devices installed, either
internally or externally, to ensure maximum reliability. Ground the shield at one point only, as shown in the fig-
ure below, to avoid ground loops.
Correct polarity is also essential. The 469s must be wired with all the '+' terminals connected together and all
the '–' terminals connected together. Each relay must be daisy-chained to the next one. Avoid star or stub con-
nected configurations. 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 provides a
reliable communication system immune to system transients.
2-20
469 Motor Management Relay
Figure 2–23: RS485 COMMUNICATIONS INTERFACE
469 Motor Management Relay
2 INSTALLATION

2.2.14 RS485 COMMUNICATIONS PORTS

469 Motor Management Relay 469 Motor Management Relay
GE Power Management