Digital IO where low state is 2.37 volts - what is this???

I’m using an absolute rotary encoder with 26 digital output lines, a reference line, and a + power in line requiring 10 to 30 VDC (I’m using 24). The mfgr is kind of vague about the digital output lines but says they’re open collector, which is typical.

Now I have observed that the output lines are at 2.35 to 2.39 V with respect to the reference line when they are in the low state. In the high state they go up to about 22 V.

Today my DAQ equipment reads the 2.37 as a high state and so sees all the lines high no matter what the encoder position, though last week it was working fine (I had not tried measuring their voltage then).

I added a power supply to drive the reference line for the encoder about 2 V below ground, so it in effect has a power supply that is +22 and -2. The DAQ works again.

What the hell is this? Who ever heard of a 2.37 V low state? Is this some weird trick specific to encoders?

What value resistor are you using for a pullup?

Sounds like 2.37 is in the indeterminate range. I’ve had plenty of noise issues with encoders which required forcing the negative reference a bit low or adding a pull-up resistor, especially the ones using TTL.

An engineer with the manufacturer says they aren’t open collector, they’re push pull. I think this means there is almost nothing wrong I can do with a digital input to give their output the wrong voltage.

When I disconnect my data acquisition equipment all the encoder outputs go up to 22 volts or so.

My equipment says it has 50 k pulldown (not pullup) resistors on digital inputs. I’m not adding any components.

In careful experiments I am having trouble reproducing things I’m sure I saw an hour earlier. My notes are getting bigger and bigger. Meanwile I have found there are actually two encoders behaving this way. One worked right for at least a year and suddenly started doing this, and mgr tech support said it was dead so I replaced it. The replacement just started doing it today after several good weeks.

Mfr says they want to look at them, so away they go. I wonder what would have happened if I had started adding resistors to ground to try to force the low state lower. In most recent experiments the low state is about 5.8 volts with the DAQ attached, as if the DAQ pulldown is nowhere near conductive enough to get the voltage right. Can’t reproduce things. Go out to the plant and crawl around with a DMM and hurt my knees, come back up here and call people and send email notes, go back out again and it seems like it’s all changed.

Maybe have to leave it in mfr’s hands for now, and search out competing products.

I had a similar problem once which wound up being caused by an ac component leaking through the power supply and offsetting my reference voltage, that might be worth looking at if the problem persists.