The Adventures of a home brew 8Bit Computer

Got a few parts in such as the MC6800 and the 74s189 chips. For some odd reason I thought the 74s189 was a Static RAM chip. It’s not, it’s a Dynamic RAM. So now I need to find some proper Static RAM.

Also the 6821 chips I had in my junk box don’t work. I got these chips from a box of electronic parts from a friend when he went to ITT Tech and I have come across a few 74 series logic chips that are dead so it wasn’t a surprise the 6821 ICs were dead as well.

However this is great news so now I can go ahead and go with a z80 CPU. After all I bought ten z80420A PIO chips when I bought the 74s189 chips.

The overall form factor of the system is going to be a back plan with a CPU card, Memory/ROM card, video card and a general IO board. Maybe a switch console or parallel LCD. The video card will be the last item to build so I’ll mostly be using a serial terminal. I guess I should also find some z80 SIO chips to toss in as well. If I do that I will have the PIO, SIO and RTC on their own board.

I plan to wire wrap the computer boards but for the back plan I might etch a PCB or strip board it. Since the cards need to be inserted and removable wire wrapping the sockets on the back plan would be a back idea. I would love to use the old ISA style edge card design but the card edge sockets cost too much so I might just use double row or right angle single headers.

In all I don’t plan to try to do the smallest chip count or making it small as possible. I also don’t plan to build this overnight and expect it to work. This is going to be long-term project and once in a while chime in on here about the progress of it. The overall goal is to getting a standalone system up and running CP/M or MP/M.


DIY 8bit computer

I’ve had a couple of 680x PIA chips laying around. Every now and then I come across a DIY retro 8bit computer project and makes me want to build one. So I went ahead and bought a 6800 CPU and a couple of 74s189 SRAM chips.

A while back I had sent off for some TerminalScope PCBs and they have been sitting collecting dust in a box somewhere. I guess I can toss one together for the 8bit computer.

While I was shopping for the SRAM chips I came across some z80 compatible PIA chips. It was 15 bucks but it comes with ten ICs. So maybe down the road I can build a Z80 based system with CP/M

In all I got a lot of info from Grant Searle’s website. He has a few small chip count mini 8bit computers that can be built on bread boards.

Right now im awaiting parts so I can start. In the meantime I am researching and thinking of a few things such as power supply and storage. I am thinking of a simple linear supply and making sure I have filter caps at every chip’s V+ and ground pins. Also im leaning towards Compact Flash for storage since it is a Parallel device. Doubt I could get Ethernet with a TCP/IP stack on it so simple direct connection via Serial should be plenty. I think I have a MAX232 chip in my junk box. If not I could use a 7404 Hex Inverter for it or for fun use some switching transistors and resistors. It would be fun to have an array of switches to control functions kinda like the Altair 8080. I’ll just add some LEDs to the inputs and outputs for some blinken lights.

If anything on the final build I can add the TerminalScope circuit to the mainboard. It’s a dirty way but the easiest without using too much system resources.