[SiriusHardware]

I've just been looking through the National memory data books, in this case 1994, for generic clues about PROM programming. At the end of the section dealing with the DM74S571, etc, there is this fantastically clear description (attached) of how to program the Nat Semi PROMs. They could hardly have made it any clearer.

It bears some similarity to the procedure I tried to translate from the German data sheet for the (unknown?) chip, the main difference being, apparently, that the unknown device starts with its data bits set to 'H', presumably meaning 'High'.

I wish I had taken the opportunity to read your Telsa devices while I had them (I could have read them as though they were DM devices, no problem) and then we would at least have known whether we were starting from all bits high, or all bits low. When they come back they will (hopefully) be programmed so we still won't know...