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4.2.2 Interference Induced on to Cable
Twisted pair construction and cable screens do not completely shield the communication link from external electro-magnetic interference.
Good practice should be followed when installing communication cables to eliminate problems.
RS485 cables should not be installed in the same duct/tray as power cables, or switching cables. When RS485 cables cross power or
switching cables, this should be at right angles. RS485 should not be installed in areas of high interference, where this is not possible, then
electro-magnetic screening must be adopted for example, metal conduit or cable tray could be used.
The total length of the cable run must not exceed 4000ft. It is only possible to exceed this length by use of repeater units to re-generate
the signal.
4.2.3 Terminating Cables
It is imperative that the cable structure is maintained to a position as close as possible to the termination. This is to maintain the
interference protection and reduce losses due to changes in cable impedance.
The outer sheath and screen must not be cut-back for a distance of more than 100mm (4 inches) from the termination.
The twisted pair must not be un-twisted for a distance of more than 25mm (1 inch) from the termination.
Recommended Cable Manufactured to RS 485 Specifications
BELDEN 9841/9842 (or equivalent) - 24 awg, polyethylene insulation, PVC sheath, UL style number 2919.
9841 Single twisted pair for comms only
9842 Double twisted pair. 1 pair for comms, 1 pair for 24V supply.
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