Quote Originally Posted by Doddy View Post
I guess you know a bit about signal theory - you're trying to match the characteristic impedance of the transmission line at both ends - just randomly throwing resistors on won't be as effective as a properly matched terminator - and yes, as per 485. What's the driving system?, have a look at that and match it.
Just taken the DDCS apart again, the differential outputs are all driven by a standard quad differential line drivers, AM26LV31, no resistors at the transmitting end. I'm driving this through a 2m db37 cable, not twisted pair, or if it is it is highly unlikely the right pairs are twisted. Still these are EIA-422 spec drivers same as max485, etc, so I reckon I wont go wrong with 120 ohms.

It DDCS has a really nice board, has an onboard FPGA, Altera cyclone3 that does all the realtime stuff. I've taken a few photos whilst it is open.