Normally if I look at a digital signal I see a nice, relatively sharp pulse train on the screen, but when scoping the outputs of a PS/2 keyboard things get a little weird and I'm not sure what it means (though I suspect it means I've got something to learn here!)
If I don't hook the PS/2 outputs to anything other than the scope I get a rather normal looking pulse train on keypresses... it's a little odd in that it seems to slope at a small angle, but it's recognizable. However, when feeding an input line to a microcontroller (or using a 2K2 or so pull-up resistor to 5V) what you see below is what I get... (this is in AC mode for ease of display. DC mode looks similar except these oddly miniscule pulses are from 5V, though the pulses themselves are still <1 V peak-to-trough).
(Red = DATA, Yellow = CLK)
No matter what I do, the signal is surprisingly weak. Just for grins I started a normal pulse train out of the microcontroller and as expected it looked perfectly normal. I understand that PS/2 devices use an open-collector design, if that helps. So: what am I seeing here? And how do I get a relatively normal logic-level pulse train out of this signal?
Note: I tried another PS/2 device - same results.
Edit: updated pictures! All the outputs I got, normal-ish or not, are around 600mV peak-to-trough.
Edit 2: This all started because what should've been an extremely simple project (hook-up of a PS/2 keyboard to a microcontroller - with software applied to it and monitoring/driving the correct pins) didn't work... I was surprised at that first and foremost, and then started snooping at the signals (which I planned to do anyway AFTER it was supposed to be up and running, to learn more.) I keep thinking maybe it's the keyboard but I get the same effect from an old mouse as well (though it requires more handshaking to initialize it does at least send a clock burst on startup, and that burst is virtually the same in all characteristics as what I saw with the keyboard. The wiring seems sound and there really isn't much at all between the microcontroller and the DATA/CLK lines... it's virtually a straight shot to power and I/O.
And yet these patterns look vaguely familiar to me from a long time ago (different projects) and I suspect it'll be something simple.
Edit 3: I asked someone I know who is more analog-oriented and they think it looks an awful lot like a signal being clamped by a diode or such (the fact it's in a 0.6V range rather than a 5V range makes more sense in that case... now I know why it looked familiar). More later when I've tested things... might be that the cable I'm using to plug into isn't so passive after all (I thought it was... but *facepalm* I'll bet it's got a diode in it for some reason. Note it's salvaged from an old CueCat FWIW, and I am wondering if it's the splitter that's not in use that's the ultimate cause... and yes, I tested the connections and they seemed normal but I could've gotten lucky and forward-biased a hidden diode without knowing it).
Scope output (with pull-ups, AC mode probes, each centered at 0V):
Scope output (with pull-ups, DC mode probes, each centered at 5V):
Scope output (direct read (no pull-ups), DC mode probes, each centered at 0V):
Scope output (direct read (no pull-ups), DC mode probes, each centered at 0V) (overlay view of the signal above):