Additional Installation Tips
7.7 Cable and Bus Termination RS-485
•
If maximum transfer distance and maximum transfer speed is needed,
shielded and twisted pair cable should be used. The mutual capacitance
may not exceed 52.5 pF/m, and the cable area should be at least 0.25 mm
2
(AWG 24).
•
0 V, the reference voltage for communication should be included in
the cabling. With two-way communication use two pairs; one pair for
communication and one pair for 0 V.
•
The shield must be grounded at one end. The other end is usually grounded,
but with longer distances or when there is a difference in the ground potential,
the shield should be connected to the ground via 0.1 μF/250 V film capacitor
to prevent ground current in the braided shield. A number of manufacturers
recommend that the shield be grounded at each node. Various manufacturers
have different systems for bus termination.
Depending on the recipients’ design, the bus wires may be on the same level or
require pull-up or pull-down to ensure that no faulty signals are detected when the
bus is in resting mode (all transmitters are disconnected).
Beijer Electronics, MAEN329
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