Some initial design thoughts

As mentioned in the last post, the design I came up with back in the day ended up being across multiple boards and, to an extent, followed what “Build Your Own Z80 Computer” described. There was a discrete clock circuit, a de-bounced reset circuit, the Z80 CPU, a few EPROMs, a few static RAMs, and some address decoder, buffer, and glue logic circuits. It was quite a lot of stuff for just the basic workings of a computer system.

The EPROMs were a huge issue in themselves. First, I didn't have a way to program them. I did build up a circuit to provide the correct voltages and timing but I still had to run everything manually. Imagine having to  set the address on a set of DIP switches, then set the data byte on another set of DIP switches, then press a button that would apply the correct programming pulse. This had to be done for every byte programmed. Miss a bit? Start over! And by starting over, I mean putting the EPROM in the UV eraser (at least I had one of those) for a few hours.

I still don't have a chip programmer but I believe I have come up with a design that eliminate the need for one, at least, directly. The schematic is linked at the end of this post. The design has been simplified as follows. The discrete clock circuit has been replaced by a 2 MHz crystal oscillator (Digikey XC232-ND, $2.98 USD). The Z80 is a standard, 40 pin DIP. The memory will consist of a single 128Kx8 static RAM chip (Digikey 1450-1017-ND, $2.53 USD). There will be no non-volatile storage. There will be an integrated programmer that will load the RAM chip through a USB serial connection. The chip has both an active high and active low enable and that made it extremely easy to interface to the Z80 by using both IORQ an MREQ lines. Since the Z80 only has 64K of memory address space, I will be "wasting" half of the chip but the ease of implementation makes up for it.

I initially had the Z80, RAM, and programmer all on a single board. I ended up splitting the programmer from the CPU section out of convenience. By splitting the circuit, I was able to piggy back the smaller boards onto other orders rather than having to place a unique order for the one board. It's a long story but it works the best for me.

The remainder of the schematic is the interface to the programmer and an expansion port for future I/O.

Z80 CPU Board Schematic