Additional Installation Tips
6.7 Cable and Bus Termination RS485
•
Use shielded and twisted pair cable. The pair capacitance may not exceed
52.5 pF/m and area at least 0.25 mm
2
(AWG 24), if you want to use the
maximum transfer distance and maximum transfer speed.
•
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 plastic
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).
6.8 COM Port Common-Mode Voltage
The COM port I/Os have special restrictions for common-mode voltage with
respect to ground.
COM Port
Maximum Common-Mode Voltage Range
COM1 - RS232
-15 V to +15 V
COM2 - RS422/RS485
-7 V to +7 V
Beijer Electronics, MAEN166
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