I have been able to get the modbus to work with tons of VFD's. It may take some messing about but I am sure we can make it work. If you would like I can remote in and look at it with you... All I need to see is the manual !

Thanks Brian. I’ve just sorted out some twisted pair cable, and I’m waiting for the rs485 adaptor to arrive, hopefully this week sometime. I found this https://caldwellfam.net/bill/wp-content/uploads/2019/04/Connecting-a-VFD-to-Mach4-through-the-Modbus-v5.pdf#page21 which looks useful.

Just spent some time attempting to get modbus setup, and unsurprisingly failed, at least so far. The modbus diagnostics show there is no communication. The parameters I’ve set on the Huanyang are pd01 and 02 both set to 2, pd163 = 01, pd164=1 (9600 baud rate, though have also tried 19200), and pd165=3 (8N1).
The link in my last post referred to register addresses, and for his particular model (GT series Huanyang) they are Command register address 4096 decimal, Status register address 4097 decimal, and Speed register 8192 decimal.
I can’t find the correct addresses for my vid model (HY02D223B) though I did try some alternative values (8193, 8203 and 8192) which didn’t work.
Oh well, for the total sum of £4 it was worth a try.

swap the send and receive lines . Most people so them backward the first time. 

Already tried!
As far as I can tell I don’t think anyone has managed to get this particular series of HY inverters running via modbus successfully with Mach4, although quite a few have done so with LinuxCNC.
There was a plug-in available for Mach3, but of course that’s no use for Mach4. Although I did wonder whether there is anything in the plug-in that might give a clue about some of the values needed, maybe?

The modbus in Mach4 will run way more devices ... I have not had anything that could not be run

You want me to remote in at some point and have a look ?

Are you using RS-, RS+ and DCM at the VFD?

https://store.chipkin.com/articles/rs485-rs485-cables-why-you-need-3-wires-for-2-two-wire-rs485

MN300 - Just two wires and a shield which I've grounded to the chassis of the enclosure holding all the control gear. Only two terminals on the usb/rs485 converter so nowhere for a DCM connection to go at the computer end.

Brian - I don't have an internet connection at the workshop unfortunately. Would it be useful to grab some screenshots?

Not really... The issue is sometimes you have to be 1 register off (base zero base one issue) Subtract the table location from the value (sometimes, depends on the manual).  It's just not that much fun! I am ordering one and I will beat that thing into submission ! Can you give me a link that will show me the correct VFD ?

This is the one as far as I can tell https://www.ebay.com/itm/3HP-10A-2-2KW-220V-Variable-Frequency-Drive-Inverter-VFD-VSD-Single-To-3-Phase/274338564112?hash=item3fdfda4810:g:cCkAAOSwel9emA~7 - it's a 220v input 2.2kW model. Serial number HY02D223B. If its anything like the UK there are a number of sellers.