Thanks for the response.
No, I don't have a scope, or an capacitors either.
It's actually probably good enough for a wood router, but I'll have to wait until I can cut something to know for sure.

I was really just wondering if anyone else has tried controlling these with an analog signal? I'm curious if all of these Huanyang VFD's suffer from this issue?
I found one other post with the same issue on the Mach3 forum, but it had no replies.

These Huanyang VFD's use a non standard modbus implementation, so you can't control them with Modbus. But someone did write a UCCNC plugin, that supposedly works very well.
Since my breakout board has the speed control already there, I set it up that way first.

I'll be ordering an RS485 converter, and testing with the plugin when it comes in.
Thanks again.