Additional Installation Tip s
6.7 CableandBusTerminationRS485
• If maximum transfer distance and maximum transf er speed is needed,
shielded and twisted pair cable should be used. The mutual capacitance
may not exceed 52.5 pF/m, an d 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,
theshieldshouldbeconnectedtothegroundvia0.1μF/250Vplastic
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. The RS485
standard does not describe how the “Fail Safe” function would be carried out,
justthatthesystemshouldbeabletohandletheerror.
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 s ignals are detected when the
bus is in resting mode (all transmitters are disconnected).
BeijerElectronics, MAEN181B
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