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.

Rod, awesome video! I actually understood a lot more of that sample INI file, definitely had mistakes. hopefully now it is better (but seems like not yet good because it is not working completely).
I have attached:
1) mb2hal.hal file, which at the moment only loads mb2hal module
2) mb2hal.ini file
3) modbus.png which shows few hal pins of the mb2hal module. what I find a good sign there is that first transaction for reading the registers seem good, no errors are accumulated there
4) modbus error.png shows the error message in terminal when I started LinuxCNC there, maybe that is helpful for understanding what am I missing
5) VFD manual
6) VFD modbus register map

I have a suspicion that "First element" in mb2hal.ini is not correct because of the values that are reported on hal pins.Values of those hal pins match following settings from VFD: P-01 (max speed) is set to 24000 and P-02 (min speed) is set to 1800 and P-03 accel ramp time and P-04 decel ramp are both 6.0 seconds

And now as I was writing this, I checked - if I change P-04 parameter (decel time) to 7.0 seconds, mb2hal.status.coast_stop.int immediately changes value to 700 and changing P-03 to 5.6 seconds changes mb2hal.status.reset.int to 560 - hooray I have some basic communication going!
I chose those names according to functions of first 4 bits of register 1 that show status of VFD - I thought that they would reflect values of the first 4 bits, but now I see that they are integer type hal pins, so now I am confused which registers am I looking at.

I took the "First element" value of 128 from very bottom of page 36 of VFD manual (section 8.4 "Modbus register map")

Could you please hint me what am I doing wrong? Because from that register map I understand that register 4 holds accel / decel (from modbus register map it seems that it is one value used for both accel and decel but I proved that those are 2 separate values received from VFD) and I am somehow reading that but I do not see in that map any register that shows min and max speed settings. Where do those come from (got curious and checked - hal pin values are updated immediately when I change those settings in VFD)?

And it seems obvious that if the first transaction is not reading the register that I thought it would then other 2 transactions will not write the register that I thought they would and they produce those errors...

Thanks in advance!

The error messages indicate that transaction 1 & 2 are bad but in reality all are bad.
the first register is a single 16 bit register. I would map functions to decimals. looks like it will become 0=off, 1= on
0 000 stop
1 001 run enable
2 010
3 011
4 100 coast stop
You don't have any hal plumbing.
You need to scale the desired speed to be a frequency in hal so S12000 should resolve to 200 Hz (2000 in register) - assuming normal 400 Hz VFD

yes, hal pins need to be connected but reading/sending values over modbus come before that. got that working and then also hal pins went quickly and easy

just a little detail in case somebody else will encounter this in the future:
Correct value of "First element" for first register is 0, thanks to Ralph Stirling for sharing a working config.
Attached is working ini and hal files for mb2hal and Invertek Optidrive E3