[CAD_CAM_EDM_DRO] Re: Major encoder trouble, need advice.

Hi guys,
Thank you for all the advice, I'll mix some comment here...

First, the termination is not in series with the encoder-lines, it is one
120ohm resistor in series with a 4.7nF capacitor ACROSS each signal
pair. This is what USDigital recommends for my particular encoder. I
haven't measured the encoder to actually verify there's ~120ohm but
I'll do that just to make sure. I've tried without termination, with
only the 120 ohm resistor and with resistor-capacitor - Nothing works.

(Yep, the SN75115's internal termination is "active" so there really
shouldn't be any external termination but I had to try it.)

Interesting about the hysterises, perhaps I should get a couple of 26C32
and wire them in to the SN75115 socket just to try it. I think I'll do
that if the test with external power fails.

The encoder cable shield is grounded to logic level GND on the drive and
only there. There are no unused wires in this cable.

Regarding the ground-loop and faulty motor. I'm not saying that isn't
the case but by just swapping the encoder it all suddenly starts to
work. Same drive, same motor, same motorcables, same powersupplies. The
only thing changed is the encoder and encoder cable. With the
"working" encoder I can run 15 meters of unshielded encoder cable
without seeing any problems.

So, I've tried two different motors with three different encoders in
total, "everything" works except when using the USDigital encoder on
any of the motors. And I've tried two different E7P encoder thinking I
damaged the first one - still no go.

And finally, regarding the filtercaps on the SN75115 I mentioned.
Originally there was 2.2nF, if I read the graph correctly this should
give around 3MHz operating frequency. I tried 10nF, 4.7nF, 2.2nF, 1nF,
220pF, 22pF and 2.2pF but it doesn't work properly. Only when removing
these caps completely does it start to work. It's strange and totally
expect it come back and bite me (perhaps it has) but again, it DOES work
with both the Hengstler 3600lines encoder AND the AMT encoder set to 500
or 1000lines.

Will try powering the encoder externally today and report ASAP. Sorry for
the long post, tried to squeeze in comments on all the good suggestions,
thanks guys!

/Henrik.