This experimental system was built based on a Mostek Z80 CPU board and a Mostek Z80 SIO board. I bought the boards in 1984 directly in the US at Jameco near highway 101 in Belmont, because I could not get them at home.
These boards did not come with any software, just the HW documentation and schematics. I constructed a (very) simple backplane by wiring two connectors together. This allowed for the connection of the CPU and one SIO board. Later, I modified the ROM/RAM adressing logic (the picture shows a little piggyback board) which gave me more flexibility in mixing ROM and RAM.
The modified "Bigboard" monitor was the basis for this project's monitor, which was enhanced by debugging-support routines in EPROM. These routines allowed setting breakpoints in RAM and single-stepping functions. In addition, I implemented the X-modem protocol so I could download software into the RAM easily, before committing it to ROM.
Due to the various HW limits (2708 EPROMs and 4116 SRAMs) it never ran any "application", but it helped me realize the need for a higher level language (than a Z80 assembler) to constuct sensible software. Thus my "PL/E" software project was born.