Hi Mark,
think I've tumbled to the solution.
What gave it away is: have a look on the Modbus function page where you have declared your registers, the type is listed as Mach Output Register
whereas on the read coil function page it is listed as Mach Output. You will note that I declared a name, fwdrevbit, as the name of the input
to the modbus function which is a Mach output. In Machs outputs page I elected to use Output#1 but could have in fact any valid Mach output...
A picture is worth a thousand words.
Craig