Joe's 1000x600x200 steel and aluminium router started

[devmonkey]

Question for Doddy,

I've scoped the common mode noise and it isn't sufficient to cause a phantom step to be read at the 328p. I think the phantom steps I was detecting that I have 'fixed' by debouncing the read are actually being caused by reflections and/or induced voltage spikes as the differential pair switches. If this is correct placing a termination resistor across the pair as you do for rs485 should help do you concur?

Cheers, Joe

[Doddy]

Question for Doddy,

I've scoped the common mode noise and it isn't sufficient to cause a phantom step to be read at the 328p. I think the phantom steps I was detecting that I have 'fixed' by debouncing the read are actually being caused by reflections and/or induced voltage spikes as the differential pair switches. If this is correct placing a termination resistor across the pair as you do for rs485 should help do you concur?

Cheers, Joe

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.

[devmonkey]

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.