Not too long ago I came across a couple of unbuilt Solid State Music CB1A, 8080 based, S-100 single board computers. This board has 2K of ROM, 1K of RAM, and a parallel input port for an ASCII keyboard.
I could not bring myself to build either kit, so I decided to create a reproduction version. While doing so, I could not resist the urge to make this into an honest single board computer by adding a serial I/O port so it can directly communicate with a terminal. I made two versions of the board, both are workalike to the original SSM board but with the option to add a bi-directional, TTL level serial port.
I wanted the first version to look very much like the original SSM board, but just have a serial port. I used the address decoding for the existing keyboard input port, but there is an additional socket under the keyboard 8212 latch for a 7400 NAND gate. If using the keyboard as in the original design, the 8212 is inserted, if using the serial port the 8212 is left out and the 7400 is inserted. this 7400 creates an RS latch that is toggled to control the transmit bit of the serial output. The RxD bit is read when either the keyboard port or the keyboard status is read.
The second version goes a bit further from the original intent of the SSM board and has a full 8-bit output port. One bit is peeled off from the input port and the output port to create the TTL level serial port. The other 7 bits are available for general I/O use. I was able to move around some of the existing logic gates, but to get the glue logic to work I needed to swap out a couple of chips. In addition, an 8-bit output latch and a DIP header for the output port were added directly above the 8080.
