Bad RS422 signal?

[Alex_Baker]

I am attempting to read RS422 data  from a Lowrance HDS9 sonar/chart plotter. It can output and recieve data through a 4-wire RS422 interface. I am having troubles reading data and decided to use an oscilloscope. The below image is what I captured by putting the scope probe on the two TX wires.

I do not have much experience with RS422, but this looks bad. Can anybody confirm this suspicion?

[hexreader]

First thing to do is change oscilloscope channel 1 coupling from AC to DC

... or use channel 2, which is already DC coupled.

[Alex_Baker]

Dang, I can't believe I did that.

Probing a "good" rs422 signal looked fine with channel 1 though, so I don't think that is necessarily the problem. Unfortunately I wont be able to do more testing until Monday, this equipment is at a school lab.

[hexreader]

Another thing - trigger on falling edge, not rising

Good luck

[Alex_Baker]

Another thing - trigger on falling edge, not rising

Good luck

That would not change how the signal looks, would it? why would it matter?

[T3sl4co1l]

Note that you need either an isolated bus or scope to probe it that way.  You may find it better (or necessary in general) to tie both probes to circuit ground then probe the two sides of the bus and use MATH to subtract the two signals.

Tim

[hexreader]

That would not change how the signal looks, would it? why would it matter?

The difference is minimal for most circumstances, but try both ways.
Rising edge may be good enough, but see if you can work out for yourself why falling edge would usually be preferred.

[tooki]

Give that there should always be a ground on RS422/RS485, one can just probe each signal individually. Subtracting the two shouldn’t really be necessary, but is certainly doable.