A project log for One-instruction TTL Computer

A breadboard-able computer which uses only a single instruction - MOVE

justin-davisJustin Davis 11/28/2017 at 14:230 Comments

I got my parts in hand, and finished the clock circuitry.  Everything works as expected there.  However, I have to jumper my break signal to ground since it goes high when not connected to anything.  I did make a few mistakes in the silkscreen - I got my RUN/STOP labels backwards and my clock selector of FAST/SLOW backward.  Not a huge deal, but still annoying.

I also started on my control circuit.  I found I had to have a lower value pull-down resistor on my reset line.  It was sitting around 1.4V, so it was being read as a logic high instead.   TTL has internal pull-ups, so I have to overcome that if I also want a pull-down.  

And then I discovered that I got my read/write signals backwards.  And looking into it further, some places on the board are correct and some places incorrect.  So it's a big mess that I have to fix.  I think the root of the problem is the active level of TTL is low, and I'm very used to thinking active high.  So I got them swapped when I labelled the nets.  I'll have to start figuring what traces need to be cut and patched.  This will keep me busy for a while.

update: I have to cut 5 traces, and run 5 jumper wires.  I have all my cuts done and 3 jumper wires in.  I'll add the other jumpers when I do those chips.  Fortunately all the jumpers are on the bottom side of the board, and all go to pins of through-hole sockets.  Not a big deal, but still annoying.