I have just seen these three isolated boards which plug into the Raspberry Pi, which were reviewed in the EPE magazine. They can also be connected to any micro and embedded system. They give 128 opto inputs and 128 opto outputs, but best of all there is a 24-bit multi-channel ADC board with additional i/o, which is the first I have come across. I have downloaded the tech manual from Zeal Electronics web site and they look interesting because they will eliminate the ground loop problems I come across.
Has anybody used them yet?
http://zeal-electronics.co.uk/rpi/index.html