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.
It's executing code fine. I can single step and see it working. Interestingly if I remove the delay loop at the end of the test program and do an unconditional jump back to the INCB instruction I am able to single step and watch it count. It works perfectly if I single step. The output port counts up as expected.
But run it at full speed and it fails.
If I scope the the enable lines on the latch with some of the input I get something like this:
This is IC 7 on the LED output board. It's one of the 74LS75 latches. The scope shows the enable lines (in yellow) with the input from pin 7 that's the D0 data line.
I set my scope to trigger on the enable lines rising edge.
Have to say I am not sure about this? The enable pulses seem very quick? It's not latching that data either.
Is this an issue with the enable line signal or a faulty batch of 74LS75s? I have swapped out IC 10 a 74LS02 and IC9 a 74LS42 with known good ICs and things don't change.
Any thoughts?
Gavin.
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!