Thx for your time but I have decided to simplify the software side and design an electronic interface for the 4-20 mA signal. I will use 4 digital I/O outputs to drive 4 constant current sources (1mA) (2mA)(4mA) (16mA) utilising opto-couplers for voltage isolation. These I will then combine to give me a 4-20mA output with a 1 mA resolution which is more than satisfactory for my application. I will select the combination of outputs required from simple logic on the PLC side. In fact, this will probably result in a better & more robust result than utilising a PWM as my initial driver. As I am driving a hot water element, PID type motion control is simply not required.

Thx again for your support / time

Regards

Bernard

PS As soon as I tried adding the functional blocks (comparators) to the previous software the PLC 'Build' request called up missing Libraries ... as a hobbyist I  do not want to spend money on additional Libraries licences when I may not have the skill to utilise them anyway. I will simply stick to what I do best ... electronics & power engineering with simple PLC logic.