Thanks for the responses.
I understand the rs232 / ttl level conversion requirement, but why would I need more than one serial input? At this stage, I can only forsee this gadget emulating one device, but you've given me food for thought.....and I've probably just answered my own question.
Firstly, though, I have to work out how to recognise an incoming string and produce a reaction. Like maybe send some text to an lcd display.
Any thoughts greatly appreciated.
Cheers