GE B30 Instruction Manual page 81

3 HARDWARE
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 B30 COM terminal (#3); others function cor-
rectly only if the common wire is connected to the B30 COM terminal, but insulated from the shield.
To avoid loop currents, the shield should be grounded at only one point. 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 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 have more than 32 relays on a single channel. 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.
c) 10BASE-FL AND 100BASE-FX FIBER OPTIC PORTS
Ensure that the dust covers are installed when the fiber is not in use. Dirty or scratched connectors
can lead to high losses on a fiber link.
GE Multilin
Figure 3–28: RS485 SERIAL CONNECTION
B30 Bus Differential System
3.2 WIRING
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