Help with Modbus needed

Hello!

I need to control Invertek Optidrive E3 VFD from LinuxCNC. I have start/stop signal working from Mesa board output and I can set the speed from potentiometer but the actual user wants to set the speed from g-code to avoid potential mistakes, especially if there will be toolchange and different spindle speeds would be needed for different tools.
The thing is that all 4 analog outputs on Mesa 7i33 board are already used for servodrives, so I was thinking that I could hopefully set the frequency via modbus instead of getting new board just for analog output.
I have usb-to-rs485 converter, lsusb shows it as CH340 chip and I was able to determine that it is using ttyUSB0 port.
Here is the section of Optidrive E3 manual that describes the modbus message contents:
ibb.co/PzFsq8fP

And according to manual I can still have start/stop signal the way I have now even when the drive is in Modbus mode so what I would need is
to send the frequency value (optionally - read actual frequency to determine "spindle-at-speed" and "spindle-is-stopped"):
ibb.co/PGXK2gW5

I tried searching web of how to start and went for mb2hal route.
In HAL file I have:
loadusr -W mb2hal config=mb2hal.ini

Here is contents of mb2hal.ini file:
pastebin.com/h9qHcPR1

I tried looking at pins of mb2hal component in HalShow but the only thing that was changing was a pin that shows number of errors.
mb2hal.ini file has 2 entries about debug messages but I did not even understand where to find them so at the moment I have exactly no idea what is wrong and how to proceed so I would appreciate if someone could hold my hand and then gently poke in correct direction. It seems like a basic thing of what people are doing with VFDs over modbus but I have never done that.

Looks like register 2 sets the frequency. 7 is the actual frequency
at speed will use the near component to compare the two.
I did a detailed Youtube video showing how to use mb2hal recently. You will find it on my @MrRodW channel,
modbus lete you read concecutive registers in one call.