Do you have an oscilloscope? What I'm wondering is whether it's oscillating when you connect OUT1 to VM with a very low impedance like this. Could be worth looking at.
That's a really interesting possibility. Seems unlikely. But I can't rule it out. I would've thought a low impedence connection would be fine, but in my case I probably have some inductance, so maybe something freaky is going on.
I do have a scope, an annoying USB one, but it ought to do the trick.
This part is not designed to have 24V (or a load connected to the supply) connected to the Out1 pin. You may be back feeding some internal circuitry in the part.
It's an H-Bridge. The datasheet shows it as being an H-Bridge, along with two FETs on the output. When not in Hi-Z it will have VM connected to the Out1 pin through a low impedence.
And even if this particular H-Bridge implementation doesn't like VM being connected to Out1 when in Hi-Z mode I'd like to understand why. Sounds like a good learning experience.
If you just need to switch a load to ground, a simple logic level FET would suffice.
I don't just need to switch a load to ground. I need a H-Bridge.