I haven't got a scope to check but it's a meaty 4A regulated PSU which I've used for lots of other circuits with no problems.

I just tried it with an alternative PSU with the same result: it responds but repeats its identity.

I wondered whether the reset circuit was affected by something else but it has the banner including the copyright notice,just once, followed by the id string repeated forever. If it was resetting each time, it would repeat starting with the banner line and there would be some randomness in the repeats.

Is it a corrupted loader ROM? I sent for another one ('420 only available) to see if it's any different.

Thanks for the suggestion to talk to Dallas but would they be bothered about someone with one dud "new" 5 year old chip not got from them?

DS89C430 LOADER  VERSION 2.1 COPYRIGHT (C) 2002 DALLAS SEMICONDUCTOR

DS89C430 LOADER  DS89C430 LOADER  DS89C430 LOADER  DS89C430
 LOADER  DS89C430 LOADER  DS89C430 LOADER  DS89C430 LOADER
  DS89C430 LOADER  DS89C430 LOADER  DS89C430 LOADER  DS89C430
 LOADER  DS89C430 LOADER