Termination Resistors for PCM bus

[EEcarl]

I have an app that uses a 24" cable with multiple connectors at different intervals on it. It carries a digital audio PCM bus and other signals. The PCM signals are only used on 6" of the cable, from there to the other end they are currently left floating. My question is should I terminate them at the floating end, and if so what resistance?

 

[felixc]

There are possibly terminations at your pc board. Are you able to connect your board at the end of the cable instead of at the 6-inch tap? You would get a better signal quality for sure.

 

[EEcarl]

There are terminations at the boards. Just not on the far end board, it does not use the PCM. The PCM is only transferred within 6" of the cable, the other 18" does not require it, but I can terminate at that end if needed.

 

[felixc]

It is probably the best thing to do, as terminating at the end will prevent signal reflections.
But this is a question of bandwidth too. Do you have a scope? Depending on the rise time of the signals, you may not even need terminations and still have a good signal quality.
Is it a flat cable? Can you just cut the PCM conductors just after your connector?

 

[EEcarl]

It is a flat cable, so cutting it just after the last PCM connector won't work. I will check with a scope. I will populate the board with pull downs and choose the values when I have protos. Thanks.