Internal 512k expansion?

[jmetal88]

Well, my first try wiring this up was kind of a failure - the PCjr wouldn't boot!

Now I'll have to take the upgrade apart again and do what I probably should have done the first time - connect up one chip at a time so I can track where it's going wrong.

[jmetal88]

Hmm, looks like there's something wrong with my 74ls245. I don't think I have another one. Hmm. Maybe I could stack a couple of 74ls244s?

EDIT: I suppose I could, but I'd need some extra logic to decode the 'direction' line.

[jmetal88]

Well, I just tried replacing the '245 with the one in my Sound Blaster, and it also causes the Jr not to boot. I'm guessing there's some kind of issue where the 8088 can't handle two '245s connected in parallel? Anyone have any ideas for an alternative approach?

[jmetal88]

Also, if anyone could post close-up pictures of the jrHotShot or The One Card, seeing how those guys pulled it off might be helpful!

[Brutman]

I don't think it's going to help you .. Both the jrHotShot and the Legacy Onecard use PALs instead of discrete logic.

Can you point me to the most current schematic?

[jmetal88]

The current schematic is still a bit unreadable.

I only know what connects where because I'm the one who put it together.

Anyway, I tried getting the data lines from the modem slot and cutting out the '245 entirely, and while the Jr did boot, it did not detect the SRAM chip at all.

I'm a little bit worried that the SRAM chip died when things were shorting out before I removed the old version of the upgrade. It could also be that my 74154 or any of my other address decoding chips are bad. I could have made a mistake somewhere (but I checked pretty carefully), or maybe this just can't be done with discrete logic chips (or at least the ones I have on hand).

Anyway, I'm getting kind of tired of working on this. I might pick it up later, but more than likely I'll just start looking for another way to get back to 640k.

[jmetal88]

Oh... Guess I have an iffy solder joint somewhere. Suddenly it's detecting the chip, but it's only counting to 448k, rather than 640k. Um... I'm really not sure what to do next. Guess I'll look it over again.

[jmetal88]

Damn it, I fried something.

The Jr now refuses to boot even without the memory upgrade installed.

*Sigh*

I suspect I killed the on-board 74ls245. I'm not sure I want to mess with it right now, though.

[jmetal88]

Great, now the power supply board died. My PCjr has suddenly turned from the most fun to the most frustrating piece of equipment that I own.

[Vorticon]

I think you need a break from this project What you have been able to achieve with it is truly amazing, but with innovation comes headaches and the ever present Murphy's law...