Intelligent RS-485 and RS-422 Adaptors
3
The cable's outer shield must be continuous from one end of the cable
to the other, and not connected to anything other than the ground pins at
the Intelligent RS-485 and RS-422 Adaptor and each networked unit.
It is vitally important that the twisted-pairs of the cable be used as pairs
- do not use one wire of one pair and another wire of another pair.
Unless you’re practiced in handling multi-pair cable, you’ll probably
find it easiest to identify the twisted-pairs if you strip the cable about 20
to 30cm back from the end.
Care must be taken with the routing of the cable when it is installed.
The ideal situation is for the network cable to be kept as far away from
other cabling (ie mains power, telephone, alarm, RF, etc) as possible.
Where possible, the network cable should cross other cables at a right-
angle.
ASP recommends the use of MCP-2S (for RS-485) or MCP-3S (for
RS–422) shielded low-voltage twisted-pair computer data cable. These
cables, which contain two or three twisted-pairs of wires respectively,
are available in 100 metre and 500 metre rolls from A.G. Garlands &
Co Pty Ltd in Melbourne (phone (03) 9543 6411, with offices in other
states). Other suppliers may have equivalent twisted-pair data cable.
Category 5 or Category 3 twisted pair cable is also suitable.
Although the Intelligent RS-485 and RS-422 Adaptors are normally
used at one end of the cable, there is no reason why the adaptors cannot
be installed at some point along the cable. This method of wiring may
be more convenient in some installations. Note, however, that the cable
must be terminated only at the ends.
Cable Termination
For proper operation, each end of each pair of wires in an RS-485 and
RS-422 network cable must be terminated with a 120 ohm ¼ watt
resistor.
The Intelligent RS-485 Adaptor is fitted with an internal terminating
resistor, enabled via a jumper. The Intelligent RS-422 adaptor is fitted
with an internal terminating resistor on the receive data line only, also
enabled via a jumper - remember that the RS-422 transmit line must
also be terminated. See page 8 for jumper details.
Don’t forget that the far end of the cable also needs to be terminated!
