While I've seldom encountered defective IC's of any sort, it may be time, just as a confidence-building measure, to request a sample of the current version, i.e. DS89C430, since, first of all, it's clear that IAP doesn't work in the '420, among other problems that were fixed in the current silicon of the 430..450, but perhaps not in the '420.

If it still doesn't allow you access to the internal XRAM block after you've substituted the '430, perhaps you'll become convinced that the problem's in your code. Otherwise, it will simply be "fixed."

RE