[Dr_C2]

Thanks Andy,

And just to add, the ‘riko cable’ is a well established and well liked solution. For most, a mean time to failure of 5 years sounds pretty good. The opto-isolator approach is simply another option for consideration. As mentioned the module is available online from computer stores. 

‘DSD TECH SH-U11F Isolated USB to RS485 RS422 Converter Built-in FTDI chip for Industrial Application’

If anyone is interested, it does require a three wire connection into the APB i.e. A,B and 0V. And of course, these three wires are fully electrically isolated from the incoming USB wiring - which is the whole point :)

c