a) RS485 PORTS
RS485 data transmission and reception are accomplished over a single twisted pair with transmit and receive data alternat-
ing over the same two wires. Through the use of these port(s), continuous monitoring and control from a remote computer,
SCADA system or PLC is possible.
To minimize errors from noise, the use of shielded twisted pair wire is recommended. Correct polarity must also be
observed. For instance, the relays must be connected with all RS485 "+" terminals connected together, and all RS485 "–"
terminals connected together. The COM terminal should be connected to the common wire inside the shield, when pro-
vided. To avoid loop currents, the shield should be grounded at one point only. Each relay should also be daisy chained to
the next one in the link. A maximum of 32 relays can be connected in this manner without exceeding 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. Star or stub connections should be avoided entirely.
Lightning strikes and ground surge currents can cause large momentary voltage differences between remote ends of the
communication link. For this reason, surge protection devices are internally provided at both communication ports. An iso-
lated power supply with an optocoupled data interface also acts to reduce noise coupling. To ensure maximum reliability, all
equipment should have similar transient protection devices installed.
Both ends of the RS485 circuit should also be terminated with an impedance as shown below.
OR AT UR RELAY
(*) TERMINATING IMPEDANCE AT
EACH END (TYPICALLY 120 Ohms and 1 nF)
GROUND SHIELD AT
UP TO 32 DEVICES,
MAXIMUM 4000 FEET
Figure 3–20: RS485 SERIAL CONNECTION
L90 Line Differential Relay
GE Power Management