P-1
Appendix P
RS-422
Using the TriCoder with an RS-422 board instead of a standard RS-232 serial
port allows multiple TriCoders to be multi-dropped off of a single RS-422 port
at a distance up to 4000 feet. Multiple TriCoders can then be polled periodically
while they are collecting data. To be used with a RS-422 board, the TriCoder
must have the 422 option factory installed.
RS-422 boards are available from a variety of sources. The specifications vary
for different RS-422 boards, so make sure you have proper documentation for
your board.
The biggest problem we see with RS-422 multi-dropping has to do with cabling.
Follow the specifications for the cabling exactly as they are shown. Use only
the wire specified (don’t use whatever is lying around) and be sure to test your
communications thoroughly before going on-line with a large installation.
You can use the following pin-outs to create your own RS-422 multi-drop
cables:
1. Tying
T+
on the TriCoder end to the
R+
on the continuous 422 line.
2. Tying
T-
on the TriCoder end to the
R-
on the continuous 422 line.
Function
Mod 8
Transmit Data + (
T+
)
2
Transmit Data - (
T-
)
3
Ground
4
Receive Data + (
R+
)
5
Receive Data - (
R-
)
6
Table N-1. RS-422 Multi-drop cable
You must have
T+/T-
in one twisted pair and
R+/R-
in the other.
At the end of the string of TriCoders, tie pins 2&3 and 5&6 together with
120 ohm resistors or order our 422 terminator plug. Use Belden 1227AI or
Cat 5 wire for all of the 422 cabling above.
