>Can anyone tell me from where to get information on 'How to program OPOS compliant devices', such as Printers, Scanners & Customer Displays

I use Epson POS products and their free developer's kit. If you go with their equipment and kit, you can look at my demo code designed to work with their ActiveX control.

http://pos.epson.com/pointofsale/

http://www.mctweedle.com/downloads/oposdemo.zip