Serial Communications - GE 845 Instruction Manual

ELECTRICAL INSTALLATION

Serial Communications

CAUTION:
2–22
TERM # MASTER
20 RELAY_7
21 RELAY_7
22 RELAY_8
23 RELAY_8
24 RELAY_8
One two-wire RS485 port is provided. Up to 32 8 Series IEDs can be daisy-chained together
on a communication 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 ohms (for example, Belden #9841) and total wire length
should not exceed 1200 meters (4000 ft.). Commercially available repeaters allow for
transmission distances greater than 1200 meters.
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 opto-coupled data interface is used
to prevent noise coupling.
Figure 2-26: RS485 wiring diagram
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 master or at the 845. 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. Ground the shield at one point only, as shown in the
figure above, to avoid ground loops. Correct polarity is also essential. The 845 IEDs must be
845 TERM #
20
21
22
23
24
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 2: INSTALLATION
MASTER 845
RELAY_15
RELAY_15
RELAY_16
RELAY_16
RELAY_16