I'm building a retro computer around the 68008 CPU The main board will be simple, The CPU, 1MB of RAM, and a 32kB ROM (Bios/boot rom)

here is the adress map

  • $000000 - $07FFFF : RAM0
  • $080000 - $0FFFFF : RAM1
  • $3F8000 - $FFFFFF : ROM

The rest of the address space is free for any other use, I/O, Floppy, ...

I have in mind a modular computer like the IMSAI 8080 but around 68K CPU, that's why the bus connector as 16 data lines and some other feature for the 68000 CPU

So I'm looking for some review, advice for my design and point me the weakness of it

enter image description here

  • 1
    \$\begingroup\$ We discourage broad, open-ended design review questions here on EE.SE, because the answer(s) tend to become long strings of unrelated edits and/or comments. While this might help you with your immediate problems, it is of no value to the site overall. We DO allow design review questions in which you explain your choices and then focus on a few points about which you still have doubts. To get a better feel of what is or is not acceptable, search for "design review" on the meta site. \$\endgroup\$
    – Dave Tweed
    Aug 6, 2019 at 17:42
  • \$\begingroup\$ Decoupling caps. I'd use 244's for buffering address lines instead of 245's (that's just me). Your processor has 8 bits, bus connector has 16. In general I'd multiplex 8 to 16, so processor could access upper and lower. Doesn't really make it modular otherwise. Not sure what that would entail for you. \$\endgroup\$ Aug 6, 2019 at 22:01
  • \$\begingroup\$ @StainlessSteelRat the decoupling are not present here, but are present for each chip I used 245 for adresses cause I have in mind to implement DMA on évolution of this board, and 16 data line, it's just a generalization of the data line in 68000 family CPU, but what dou you mean by multiplexing 8 to 16 ? \$\endgroup\$ Aug 7, 2019 at 17:23
  • \$\begingroup\$ If they are meant to be bidirectional, then hardwiring 245's as 68008 output makes no sense. Your 68008 is 8 bits and RAM/ROM are 8 bits, but your bus is 16 bits, which means your 68008 cannot access D8 to D15, which does not make it modular. As for option bus, I'd go with some standard like VME, which has a lot of the grounding/power worked out. \$\endgroup\$ Aug 7, 2019 at 17:59

1 Answer 1


A few comments:

Don't use data strobe on your data buffers.

RAM/ROM CE and WE should use data strobe (ANDed with other signals). Otherwise, spurious writes will occur during signal transitions.

Your I/O connector should have a lot more grounds. I recommend 20%.

Do you have a plan to get it up and running? The 68008 doesn't have a test port, I always had a hardware emulator available to help with this.

Grounding, clocks, and sneak paths, these are the things that have bitten me the most in my career. Your clock is point-to-point, that is good, but consider a series terminator. All your circuits are on the same Vcc, sneak paths shouldn't be a concern. Use ground planes and lots of grounds in connectors.

  • \$\begingroup\$ I will rethink the adress decoding logic to get DS into, and R/!W too Yes I plan to get it running . I gonna make an extension board, to have a serial connection and even a "video" card The PCB will use ground plane, for ground pin on the Bus connector I get inspired by the ECB Bus (or VMEbus) wich have only 1 ground 1 one VCC by pin row. what do you mean by series terminator ? \$\endgroup\$ Aug 7, 2019 at 17:30
  • \$\begingroup\$ Regarding the series terminator - at high frequencies signals begin to behave like waves and the reflections can cause issues. Keep you clock nets really short and you might be OK for this design. Or, you can put an option in for a series terminator. Put a small value resistor in series at the source of the signal. You could start with zero ohms and go up to 20-30 ohms if you have issues. Entire books have been written on transmission line theory. For a start, Google: series terminator transmission line. \$\endgroup\$
    – Mattman944
    Aug 7, 2019 at 19:25
  • \$\begingroup\$ Regarding grounds - VMEbus has 8 grounds in P1, about 8%. If you truly use legacy parts that might be OK. Modern parts have faster edge rates and will have more issues with ground bounce. 20%, distributed somewhat throughout the connector will save you a lot of heartache. \$\endgroup\$
    – Mattman944
    Aug 7, 2019 at 19:38

Not the answer you're looking for? Browse other questions tagged or ask your own question.