VFD mb2hal

To enter numeric constants in hex, just put them in the HAL file as 0x800A (for example)
HAL interprets any constant beginning with 0x as hex.

Or, at least, I think it does. I can't immediately find any reference to this in the documentation.

It is also likely that integers beginning with "0" will be interpreted as octal. Which I can imagine might occasionally be unexpected.

There are multiple ways to do this, Andy showed you one way.
In the machine config I posted I used Classicladder to convert the control bits to the numbers needed to write to the modbus registers.

I think I would do it using a Weighted_sum component.
Then make the connections in hal, something like this. The weighted sum component lets you toggle the individual bits of the 16-bit register number.

Connecting your pyvcp controls to Linuxcnc is also done in your hal file(s) and is covered in the manuals, and there are examples in the example configs included with Linuxcnc.
linuxcnc.org/docs/2.7/html/gui/pyvcp.html
linuxcnc.org/docs/2.7/html/gui/pyvcp-exa...tml#_the_connections

I'm trying to connect my VFD (chinese yl620) so it will respond to M3-M5,Sxx commands.
I don't want any new GUI elements, don't even need any feedback. At the moment VFD fault relay is in Estop circuit, if VFD trips it stops motion preventing cutter breakage.
I have written YL620.ini to write and read relevant registers

But I'm at loss with what next.
edit custom.hal ? how ?
how to translate M3 to writing 18 to control register and M5 to writing 1 to the same control register?
Any pointers please, I know it's all in WIKI, but it's sooo huge.

I would start with a mux4 component to send numbers to a register based on the spindlle-fwd and spindle-rev bit outputs.

linuxcnc.org/docs/2.8/html/man/man9/mux4.9.html

You may need to convert to int from float too. (conv_float_u32 component)

An alternative is to do bit to float conversion in two steps like this Not saying this is better than anything else, it is just another way.
One kind of whishes mb2hal could take a bit as input.