GE D90 Plus Instruction Manual page 62

ELECTRICAL INSTALLATION
52
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.
Though data is transmitted over a two-wire twisted-pair, all RS485 devices require a shared
reference, or common voltage. This common voltage is implied to be a power supply
common. Some systems allow the shield (drain wire) to be used as common wire and to
connect directly to the D90
common wire is connected to the D90
To avoid loop currents, the shield should be grounded at one point only. If other system
considerations require the shield to be grounded at more than one point, install resistors
(typically 100 ohms) between the shield and ground at each grounding point. Each UR
series device should also be daisy-chained to the next one in the link. A maximum of 32
devices 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 devices 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 isolated 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.
Plus
COM terminal (3a); others function correctly only if the
Plus
COM terminal but insulated from the shield.
Figure 21: RS485 serial connection
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 3: INSTALLATION
Plus
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