I recently had an idea about adapting USB-FIFO modules into the many different retro UARTs. 

Then I realised that adapting modules to a static RAM chip pinout was a better idea, because most retro machines have at least one of these. Even a ROM chip would have D0-7, A0 and /RD in the same position.

Using a memory chip socket allows an adapter to fit boards that don't even have a UART.

Like the Trump Card!

This is very desirable for me, as it avoids having to 

• get hold of an ISA bus PC
• install and run MSDOS
• install the Trump card software
• find an analogue CRT based monitor

I know for some people that is part of the retro fun, but this board remake is already using a CPLD instead of TTL.

I just want to be able to talk to a Z8000 processor.

Having a modern PC replacing the original PC has these advantages:

• No ISA bus hardware needed
• Just needs a USB communication link
• Can be Windows or Linux
• Could drive a virtual Tektronix graphics terminal

I have a project about retro-fitting USB-FIFO modules to existing machines, including a log about fitting a module using an SRAM socket.

Hardware

The USB module needs to appear at Z8000 address 1. 

The Trump card has a single FIFO which it calls a 'bucket', and has to handshake to give ownership to one processor at a time.

A USB-FIFO module has two separate FIFOs, one for transmit and one for receive. So no ownership exchange needed.

The Z8000 simply needs to read the status flags to see if the receive FIFO has data or the transmitter FIFO is ready to accept data. The Trump card already decodes address 3, so this seems the sensible place to put it.

The Trump card had the ability for the PC to give an NMI to the Z8000, e.g. after filling the 'bucket' with data bytes  and telling the Z8000 to collect them.

The USB module has no way to choose whether or not to interrupt its CPU. It might be simplest to interrupt the Z8000 every time the receive buffer has data. The PC can then never be ignored. The Z8000 has no time-critical duties to do (like managing a disk drive), so an NMI should not be a problem.

Software

As with all the best toys say, "some assembly required". You will need to modify the Z8000 firmware but you will only be simplifying the communications routines a bit rather than adding lots of code.