Hi Paul
As you had also put through your question on Facebook I assumed you were UK based.
I used the coms sniffer on a V570 to look at a known working Modbus command, read holding registers 0-31 and then tried to duplicate the message using the Protocol FB. I came across a small snag in that the CRC byte order in wrong, it's back to front.
The working message is
01 03 00 00 00 20 44 12
which the Protocol FB generates as
01 03 00 00 00 20 12 44
I cannot see a way to reverse the CRC bytes automatically but as I guess you do not need to have many messages you could work out the CRC using the above method and then hard code it in the Protocol FB. The method for generating the CRC for Modbus appears to be binary, zero forward offset, zero backward offset, CRC-16, 256 (100H), 2 bytes.
Note that the message is only 8 bytes long including the CRC, the screen shots added by Nouy would look like this for my message, two extra bytes at the front of the message.
00 01 00 03 00 00 00 20 xx yy
Hope this helps but if you still have issues them please get back to me.
Tony