Retro Challenge – Finishing Post Within Sight

Despite a late start today, things have gone well so I actually feel like I’m ahead of the game right now.  Certainly not finished, but with most of the major hurdles now behind me, the only thing left is writing a bit of Z80 assembler code.  And even that is starting to look manageable.

I started off by throwing a load of capacitors at the matrix board.  If it was a power issue, then this would likely help… and, would you believe it, it did!  I redid the test that was giving me odd results yesterday with 3 matrices and got exactly what I expected to see!  (Well, actually, I got what I should have seen. I actually expected it to still behave oddly).

So, I added the other 2 matrices and they worked too.  Then they didn’t.  Then they did.  The ‘not working’ had all the hallmarks of a short circuit between one of the X axis and one of the Y axis.  So, I pulled out all the displays and had a poke about…


Can you spot a short circuit in there?

I poked about and generally freed up all the wires, put the displays back testing them one by one, and it seems to work fine.  So, as long as I don’t breath too hard, I think that gremlin will be kept at bay.

Using BASIC I ran a few more tests, and it would appear that I did get the X and Y axis wired up wrongly.  Oh well, it just means I will have to write my name as






instead.  Yeah, sure, I could go through and rotate all the ASCII character encodings by 90 degrees, but do you really think I’ll bother to do that?  Well, we’ll see…

I also fired up my EPROM programmer and did a test on a 64k EEPROM that I designed one of the other boards for. [This needs a computer with a real parallel port, so my Toshiba Satellite Pro 4600 running Windows 2000 keeps me well within the Retro Challenge guidelines!]


The 64k is divided up in to 8 banks of 8k by 3 jumpers.  These jumpers set the A13, A14 & A15 lines either high or low.  I have tested this by burning a copy of MS BASIC to the EEPROM at an offset of 0x2000 and then again at 0x4000, and these work fine with the appropriate jumpers connected.  And there’s still 6 more slots left on this chip before I need to erase it!

So, tomorrow, I’ve got no more excuses, it’s time to crack on with the Z80 assembler code.  It should be reasonably straight forward… but I just know there’s going to be something to catch me out.