> The 3.5" square (exactly) daughter board plugs directly into and > only into where the Z80 CPU normally resides. The board contains: > Z80 CPU, AM25LS252IPC, 2 x SN74LS245N, SN74LS00N, 74F32PC chips > with assorted resistors etc. There is a 26 way berg connector to > take the cable to the IEEE-488 port. Also there is a 8-way link > box with 5 unbroken links, 2 broken links and 1 remade link.

I don't like to say things like this, but I think I am justified... 'Impossible!' You've got an address decoder (AM25LS2521 == 74LS688 IIRC), a couple of bidirectional bus buffers ('245s), and a few gates ('00 NANDs and '32 ORs). What you don't have is any form of latch, which is what you'd need for an output port. Since the IEEE-488 bus can run a lot more slowly than the Z80 bus, it's going to need such latches, at least for output. There are several possibilites. Either you've missed out some chips (like '374s, or even a true IEEE-488 chip like the 9914 or 7210). Or this is _not_ the IEEE-488 option. I suspect the latter, actually. I would be very suprised if the RML IEE-488 option was a daughtercard that fitted under the CPU. Everything I can find out about it suggests it's a normal Z50 bus card. This sounds like some kind of bus buffering/extension card, sort of like the IDE interface (although obviously incompatable with the PC type of IDE). Are you sure this is not some kind of strange winchester interface. It could well be the host adapter for (say) a WD1001 card or something like that.