Mark-8

Public·2 members

October 27, 2024

Output port not working

Hey,

So following on from my earlier post regarding some LS logic ICs not working in my Mark-8. I think I may have an issue with the 74LS75 latches that I have.

Firstly they seem to work fine on a breadboard test circuit. I can get them to latch data without any issues.

However if I input the test program on page 9 of the construction guide and run it the display shows this:

Rather than counting up on the output port bit 3 gets stuck on and all the other bits show very brief flickering.

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Artem Kalinchuk

Nov 03, 2024

Wow, that’s awesome! Thanks for sharing the details. I don’t remember having to add a pull-up resistor but I think it may be a difference between using the LS version or not. It might also be different depending on the manufacturer, which is why there are other areas in the computer that needed fixing of floating inputs but appeared to work for Jon without pull-up resistors.

In essence, it’s a good idea to tie all inputs to something and not leave them floating, otherwise you might get strange outputs.

Great work!

Gavin

October 27, 2024

Problem LS ICs on my Mark-8 Build

Hey,

I have been building a Mark-8 Minicomputer since early this year. It has been taking me a long time to finish as I have a number of retro computing projects on the go and don't work on the same project all the time.

Another issue that has delayed me is having to deal with LS logic ICs that don't seem to be happy to work in the Mark-8.

For example here was my first pass at the CPU clock:

Yes, you read that right 4MHz and yes a very messy sine wave. Also you will see that both phases of the clock are in sync.

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Artem Kalinchuk

Oct 28, 2024

Very nice write-up! Thank you for sharing your findings and insights. I think I used LS chips for my clock and it appears to be working fine. It may just be some bad chips.

Finding issues with the build is not always pleasant but it encourages the builder to dig in and learn about the inter-workings of the computer, which is a plus.

I have to say, your computer is looking amazing so far!

wickert

September 1, 2024

ROM for MARK-8?

Currently getting parts together for a build. There are four extra slots. I'm going to use them for a serial board and extra RAM. Is there any PCB for ROM or is that going to be a DIY?

Also are there any Operating Systems that will work with the 8008?

Got one 8263 on FleaBay still looking for the rest of the multiplexers.