Just to update - I haven't had a chance to try different termination yet. I have a couple of passive terminator blocks, but no active ones. I'll give them a go first and if it doesn't work, try to pick up an active terminator. If that doesn't work, then it's probably into the 'box of bits' it goes... until I can get one of my old PC's resurrected to get my eprom reader/programmer working again.