RetroChallenge 2017/10 – Purely by coincidence

So here’s a thing. I recently met up with an old acquaintance who I hadn’t seen in years and as we’re both retro-computing fans we were talking about old machines.

I have just received an email from an old acquaintance of mine who is a dedicated retro-computing fan and has spotted my plight in these pages after following me on Twitter (@acollins22 if you’re interested).

He mentioned that he had had a similar problem on a Nascom a few years ago and it was caused by a buffer failing. Now I know this is wishful thinking but it’s worth a look.

The chip in question was an 81LS97, a 3-STATE Octal Buffer.

I’m not the most familiar with this to be honest, a keen amateur. My take on this is that Y1 will equal A1 when _G1_ is low. At other times it will be tri-stated. The other pairs (A2-Y2, A3-Y3 and A4-Y4 will do the same).

Is this right.

I wired up my old Blackstar logic analyser to IC47, the 81ls97 as follows…

  • CH0 – _G1_
  • CH1 – A1
  • CH2 – Y1
  • CH3 – A2
  • CH4 – Y2
  • CH5 – A3
  • CH6 – Y3
  • CH7 – A4

It’s set to trigger on CH0 being low. This is what we get.

What I see here is CH0 going low and so the outputs are enabled. Surely that means that CH4 should echo CH3 but as you’ll see, it doesn’t.

Does that mean I have a faulty 81LS97 or a faulty understanding of what’s going on. It seems a coincidence that a friend tells me how he fixed his Nascom and mine has the same fault but the trace above suggests just that.


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