Some links which might give some clues (not to me though, as I’m way out of my depth).
https://gist.github.com/Bouni/803492ed0aab3f944066
Also a thread on cnczone:
https://www.cnczone.com/forums/phase-converters/91847-software.html

I know that a few have managed to get Linux apps to talk to the HY vfds, and that someone wrote a plug-in for Mach3 to do the same. Maybe those endeavours would provide a clue on how to do the same with Mach4, as far as identifying register addresses go at any rate.

We figured it out. They used the wrong function codes! The frames of how they constructed the data is correct... so it’s modbus done wrong lol. We can get it to talk to Mach4... I almost did it in Lua just for fun but we wanted to make it a plugin as that will be a bit simpler to install.

A quote from one of the posts I linked to above said something like ‘I think the Huanyang developers were off having a coffee break when the Modbus protocols were handed out’.

It is the function numbers, they have 3 and 5 swapped. That is the biggest issue. They have the read and write swapped in other words if that was correct we may be able to make it work.  Check sum is a bit odd too. Basically it is not modbus lol