Return of the Blank Screen

Loading tapes had apparently been too much for the poor old PET. Not having solved the original blank screen (seeing as it appeared to fix itself) I wanted to get to the bottom of it this time. I'm still not sure if what I found was the cause of the previous blankness, but it was certainly more concrete.

Whilst probing with the 'scope, I noticed that /IRQ was stuck low at the CPU. The cause seemed to be the 6520 PIA responsible for the keyboard and first tape drive. This would at least explain the tape loading problem and even the random characters I experienced just before the picture went completely. Removing the suspect PIA confirmed the part was bad as the BASIC prompt was restored, although I of course couldn't use the keyboard. The PET however has two PIAs, the other being part of the IEEE-488 interface, so I moved this to the bad chip's socket. Everything worked again (although, of course, if I had a disk drive, that wouldn't have).

While I was under the hood anyway, I thought I'd look into a minor problem I was having with the display. The PET seemed to be stuck in lower case mode, which meant I couldn't get any of those lovely PETSCII graphics. Usually, POKEing to address 59468 will allow you to toggle between the two character sets, but this had no effect. The only thing influencing the GRAPHIC signal, which directly selects the character set via A10 on the character generator ROM, is the 6522 VIA chip. That particular output was stuck high, so I suspect I'll need to add one of those to my parts wish-list too. For the time being, I tied GRAPHIC to ground - I can't switch to lower case any more, but at least I can get the graphics!

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