Product manual

PX Range Product Manual Version 2.0 © Copyright Brainboxes Limited 2010 Page 24 of 37
3.6. Termination Resistors
This RS422/485 termination circuit is based on the recommendation of Texas Instruments, and uses
resistors to provide line termination R
t
, bus-idle failsafing (biasing) R
b
and bus-shorted failsafing, R
f
.
However, this circuit may not be suitable for all RS-485 bus systems. Whilst the circuit includes line
termination to prevent the data signal being reflected, this termination should only be present at each end of
a bus and not at any intermediate nodes.
Also, while this circuit ensures that our PX cards are properly biased when the bus is idle, this circuit may not
provide sufficient bias for any nodes (ie external RS485 devices) which have their receivers connected
directly to the D+ and D- lines. If you do have RS485 devices whose receivers are connected directly to the
D+ and D- lines, you may need to add a couple of resistors to your bus, from D- to ground and from D+ to a
positive supply, with a resistance sufficient to give at least 0.2V between D+ and D- when the bus is idle.