RS-485 Modbus Serial Cables 217
Appendix A Pin-Outs for Cables and Connectors
Cable Selection
When selecting an RS-485 cable for your Modbus network, you should observe the
following guidelines:
• Maintain a cable impedance of greater than or equal to 100 ohms.
• Supply a separate shield for each twisted pair.
• Use double twisted-pair networks to house the pairs in a single sheath or in
separate sheaths.
• Use branch cable of the same quality as the trunk cable, but of less rigid
construction. For example, use Belden 9182 (150-ohm) for the trunk and
Belden 9729 (150-ohm) for the branches.
• Follow all applicable local codes.
• Terminate the cable in the characteristic impedance of the cable.
Cable Termination
RS-485 trunk cable termination (point-to-point or multi-point) greater than 200
meters (650 feet) requires termination at each end. Traditionally, resistors are
connected to each end of the cable. This technique matches the cable impedance
and prevents signal reflections which could cause data errors. However, this
technique has the following three undesirable side effects:
• When no driver is active on the pair, the resistors pull the two wires
together. Noise—even very low-level noise—can appear to be data when
the wires are in this state.
• When a driver is active on the pair but not sending data, the resistors cause
33 mA of DC current to flow in the cable. This is an excessive load on the
driver.
• When a driver is transmitting data over the cable, the resistors lower the
signal level and consequently lower the immunity to noise.
A better technique for terminating the cable pair is to use an RC network and
pull-up/pull-down resistors. This reduces power consumption and forces the pair
to a valid data state when no driver is active.
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