A really nice rarity! This is apparently one of only five ever made and is still working! It is a processor card, but using the Z180 – one of the later successors to the Z80. It is still an 8-bit cpu, but this version (Z180181 ?) incorporates a timer, 16 I/O, 1MB MMU, 2 DMAs and 2 UARTs. Having said that, this board is equipped with a 20MHz crystal, so could this be one of the higher spec chips? I don’t know – it’s too difficult to read from the photo.
The 40-pin IC near the top is a MK3801 serial timer interrupt controller. I suspect that it is clocked from the 8MHz crystal at the left hand side and is mostly wired out. It could be the baud rate generator for the UARTs.
The 4MHz crystal at the bottom right will be the bus clock.
This card was designed to be a direct plug-in replacement for the GM813. You could simply swap the cards and everything would run as before, only faster.
I suspect that the memory board (seen here on edge) will be to provide memory that can run at the full speed of this processor, avoiding the bottleneck of the 80-bus.
Credits to Daniel for this picture.
nascom.info 