Retro Challenge 2020/04

The Retro Challenge is a one-month long excuse to do cool stuff with old computers and blog about it (vaguely wrapped up as a competition).

(This year the challenge is being hosted by Mark Sherman at  I started my challenge on 1 April without knowing for sure if it would be running this month. but I wanted to do it anyway!)

In 2018 I tried to restore two Osborne 1 portable computers.  One restoration was completely successful, but with the other I couldn’t narrow down the problem in the disk controller as I lacked sufficient test equipment.

I now have an Oscilloscope and a Logic Analyser so hope to be able to complete the repairs.

My updates will appear here:

  • Day 1: I eliminated the Double Density board as the source of the problem: Twitter link
  • Day 2: I used the oscilloscope to prove that the floppy controller is talking to the floppy drive (probably correctly, although I’m not yet sure): Twitter link
  • Day 3: I connected up a Logic Analyser to the floppy controller and started to look at its communication to the Z80 CPU: Twitter link
  • Day 4: I copied some LA signals to my desktop computer, and have started learning how to write a protocol decoder: Twitter link
  • Day 5: I looked into the exact timings listed in the floppy controller datasheet in preparation to writing a protocol decoder.  However the logic analyser data doesn’t match! Smoking gun? Too early to say. Twitter link