Product manual

PX Range Product Manual Version 2.3 © Copyright Brainboxes Limited 2013 Page 27 of 45

Terminating Impedances

RS-422 and RS-485 lines should be terminated at the end of the main branch of the RECEIVER, in the

cables characteristic impedance. These terminating impedance’s stop echoes caused by the serial data

being reflected back at the cable ends. It is not necessary to terminate the transmitter end of the twisted pair.

Brainboxes cards have the correct 120 Ohm (nominal) terminating resistors for the RXD twisted pair line and

the CTS twisted pair line fitted on the RS-422/485 card for the serial ports on the card.

There is no need to add any more at the PC end.

The terminating impedance’s shown later in the wiring diagrams are automatically provided by the on board

resistors and do not have to be added by the user.

This RS-422/485 termination circuit is based on the recommendation of Texas Instruments, and uses

resistors to provide line termination R

, bus-idle failsafing (biasing) R

and bus-shorted failsafing, R

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 (i.e. external RS-485 devices) which have their receivers connected

directly to the D+ and D- lines. If you do have RS-485 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.