Internal 512k expansion?

[jmetal88]

I think I might order the parts to try this out tomorrow.

I need to order new RAM for a broken TI-99/4a, and since that'll also come from Jameco, I'm thinking I should order these parts at the same time.

[jmetal88]

I decided to go ahead and wire together the chips I already have.



I've been checking my connections with a multimeter as I go, which turned out to be a good idea, as I discovered a short in my 7420 chip and I had to pull another one out of the bin. Actually the new 7420 is marked a little bit confusingly, as it has two numbers, S7431 and N7420, stamped on it. I'm pretty sure it's a 7420 though, as I looked through my bin and found another chip that looks to be from the same manufacturer that's marked S76-something-or-other and N74-something-or-other, which leads me to believe the N number is the type of chip and the S number is probably some kind of (partial?) serial number.

There are also multiple other chips in the bin that have identical N7420 stamps, but each has a unique S code.

[Vorticon]

Looking forward to seeing how this project turns out. If nothing else it's a great exercise in digital logic. I unfortunately don't have that kind of expertise, so I stuck with modifying a 128K sidecar instead by replacing the RAM chips (huge hassle)

[jmetal88]

I unfortunately don't have that kind of expertise,

Neither do I.

Seriously though, I'm kind of trying to figure this out as I go. The thing that kind of makes it easier on me I guess is I have a binful of ICs at my disposal - one of my dad's friends gave them to me when he decided he'd never use them.

I finally received my PCjr keyboard today, and the SRAM chip should be arriving tomorrow. Hopefully my idea will work, as a lot of the stuff I've tried running today gives me errors about not having enough RAM. I've got an old copy of Sharedata's Wheel of Fortune I'd love to play on this.

[jmetal88]

Got the parts in today! AWESOME!

I'm gonna finish up the work on my TI-99/4a before I get started on this though. I really need to get that out of the way, and it'll probably take less time (even with more solder joints to worry about).

[jmetal88]

PCjr PCB with sockets piggybacked:

[jmetal88]

Testing...

[jmetal88]

AWESOME!

[jmetal88]

*Whew!* I am super glad that mess of wiring fits underneath the floppy drive. It doesn't jam up the motor either, I just booted my DOS 3.3 disk to prove it.

Now I've gotta make a disk with the RAM driver on it so I can actually use this, haha.

[Brutman]

Very nice! Write it up nice, and we'll get it posted. I'd love to do a technical review of it ..

The SRAM is a nice solution - no DRAM controller needed. I'm glad it worked.

Mike