A great way to diagnose a signals problem is using a logic analyzer. They can provide a graphical representation of data on several inputs. What we have here is a 4 input logic analyzer developed by Vassilis Serasidis. The circuit can support up to 4 inputs being sampled at 100Khz. On the website, he provides the schematics and breadboard layouts so you can make your own.
This logic analyzer makes me want to revisit the portable game console project. I could include the same functionality and add capture/playback type features.
This is a great debugging tool to have in your arsenal. If you’re in the market for a logic analyzer and don’t want to go the DIY route check out the Saleae Logic. You can pick one up for only $150.
Image from serasidis.gr