Thanks for the reply,

Comms settings are good, the PIC appears to receive the correct data.
DTR is negative volts

An example of the reply to i0, in hex :)

0918 0820 0802 494E 4162 7908 094E 4340 ..INAby.NC@
3A

And zx command

0018 3A
0318 3A02 103A ::

looks like I have a bit missing, literally.

1.9V power to the callipers is ok?

Anyway, back to playing :)