Trouble with the registers

A project log for Yet Another TTL Computer

Yet another TTL computer, inspired by Ben Eater's YouTube video series.

AmosAmos 01/10/2019 at 11:031 Comment

Grrr! I spent today tidying up some wiring on my A and B registers and building the Instruction Register. Everything seemed to be working fine until the B register started acting up. Reading data from the bus is way, but when I enable to the output latch, not all the bits get put on the bus properly. I bit of poking and prodding at the wiring and sometimes (but not always) the data will appear. I know it isn't a power problem - I have checked the power distribution and it is nice and stable across the boards I have connected so far.

The culprit is most likely the wiring. I have been unhappy with one of the spools of hookup wire I have - the wire is difficult to work with and tends to break too easily. I need to toss the spool and replace the wiring, but that is not so easy. Where I live it takes around a week to get anything via mail order (even longer if it is from overseas) and there are no local electronics stores I can turn to. Plus I'm going away next week... In the meantime I will use a different colour (I like to colour code my wiring and I don't have any spare colours (yet)) to confirm that the wire is the problem and replace it (again) when I am able to get some new spools of hookup wire.


Amos wrote 01/10/2019 at 22:37 point

It turns out the wiring is okay (but I am still ordering some new wire and will replace the suspect wiring anyway) but I had a dud 74LS245. I did some probing around this morning and I noticed that the chip was outputting low signals to the bus even when !OE was high. Swapping out the chip with another fixed the issue.

Looking at the dud chip I can see that the ground pin was bent and wasn't making contact with the breadboard socket. I have no idea how this register ever worked TBH - when I tidied up the wiring for this module I did not touch any of the chips, so the ground hasn't been properly connected at all, yet the module passed all the tests before I connected all the modules together. Oh well, it is working now, on to the ALU...

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