© 2005 Carlo Gavazzi Industri A/S. All rights reserved 108
G 3800 X036 Operation Manual
Appendix C
RS485 Communications
The cable
RS485 communication cable is a shielded, twisted pair cable. Many cable manufacturers supply cable
meeting the RS485 standard (e.g. Belden 9841). Other cabling media such as telephone cable, coaxial cable
and multi-core wires could prove problematic and not provide satisfactory performance.
Topology
The cable must be installed to pass close by each node. Stubs (cables joining the node to the cable) or stars
(multiple cable segments brought back to a single point) are not permitted.
Cable Length
The maximum length of a single cable run is 1200 meters.
Connection of the cable shield
The shield of the RS485 cable establishes a reference voltage for the RS485 signal conductors. For this
reason the cable shield must be connected to V- (terminal 32) on each Master Generator on the network. The
"screen pig-tails" going into the terminals should be as short as possible. The shield should be continuous
throughout the installation, but only connected to proper ground at one point with an earth contact resistance
of less than 2 Ω.
Termination and Fail-safe resistors
The cable must be terminated at each end. This is to prevent reflections which would disrupt communica-
tions. Also, the use of so-called fail-safe pull-up and pull-down resistors in one of the ends is required to
ensure safe and reliable communication. The Master Generator has built-in termination and fail-safe resistors.
The termination resistor is connected to terminal 30 (B) and terminal 27 internally. Therefore pin 27 needs to
be connected to pin 31 (A) in both of the cable endings in order to make the terminations effective.
The fail-safe resistors shall only be used in one cable end. By connecting pin 31 (A) to pin 29, and by
connecting pin 30 (B) to pin 28 the fail-safe resistors are made effective.
Cable Isolation
The communication cable must not be run in cable trays carrying power wiring nor in close proximity to
power wiring. Current surges in power wiring due to high equipment starting currents or to faults can disrupt
communications.
Wiring diagram
Find below an example of a complete wiring diagram for a system with 3 Master Generators connected in an
RS485 network.
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