I can't reverse the spindle rotation.

PCW wrote:

I would check the output of the direction control PC817 with a voltmeter/ohmmeter

I checked for continuity between the V output and GND using a multimeter, and even though the LED is lit, there is no continuity. 


Tomorrow I’ll check the resistance at the output. It’s all very strange because I have two brand-new modules, and they behave the same way. 

They always worked perfectly on a parallel port, but I no longer have the PC with a parallel port to test them again...

Deleted as i missed the part about level shifting.
Sorry.

The Isolator is doing 5V to 24V translation so unless the BOB has a relay/24V output
it cannot go directly to the VFD

Sounds like maybe  grounding issue (or damaged OPTO)

Note that a parallel port ground is nearly always frame ground
but the 7I92 ground may be floating
 

Sorry, i missed 5V/24V thing above.
I edited the above posts.

Now the question becomes:
will the servo drive work with 5V on the inputs?
Did you check the manual?
Older drives do not, but all new-ish ones do.
-
Now i wonder if i missed how old is your drive!

The servomotor uses the ECAC XP200 driver, and according to the manual, the digital inputs can only be controlled with 24V.

I also have the ground for the 7i92 tied to the common ground.

Alright, bit of progress. I did some testing and managed to get the PC817 module working — at least for now.

The module has a 302Ω resistor on both the input and output side. When the LED turned on, the output voltage only dropped to about 21V, so the servo driver wasn't seeing it as a proper low signal.

I tried putting another resistor in parallel with the one already on the output side, and bingo — reverse works. With a 3k resistor in parallel, the output drops to around 14V. With a 1.5k, it drops to about 10V. Both values make the servo reverse properly.

So my question now is, what's a safe resistor value to leave in there permanently? I don't want to damage anything, but I need it to work reliably. What would you guys recommend?

Honestly it sounds like you should be using a Darlington OPTO in place
of the low CTR PC817, say a LTV-815 or similar.

( PC817 mas a minimum CTR of 50% vs the LTV-815's 600% minimum )