Mike's PCjr Page

Does anybody know if the Drive Two memory expansion card (w/ 384K) can be hacked in order to boost RAM to 640K or even 736K? I'm thinking about something like piggybacking 4164 chips or replacing them with 41256 ones. I already have a bunch a 4164 chips lying around.

Thanks,

Christian

You just can't piggy back chips .. that doesn't work without some design changes. (The chips on top are slightly different than the chips on the bottom, which allows the piggybacking to work.)

You might be able to change to use 256K chips, but you'll need to look at the onboard DRAM controller first to see if it can handle it. And you may have to make some modifications to the board too. It's not a terribly difficult project to analyze the board and design the modification, but if you haven't done any board design before it's not for you.

Take a picture of what you have and write down the chip part numbers and we'll see what we can do ...

A long time ago on the previous forum we had a member who replaced the memory board with one of his own design. It was a 512KB SRAM and some address decode logic, which was great - no DRAM controller and no lost memory cycles for refreshing the DRAMs. (Read: It was faster than usual.) And the reduced part count improved reliability and limited power consumption. If there are enough of us interested it would be worth reviving that design and having real memory boards made up. (I have a few Racore memory boards that are dead that need replacements.)

-Mike

The DRAM controller is a TMS4500A. I'd rather use the 4164 chips I already have. I know it would involve a little more than just piggybacking new chips on top of existing ones. I would have to bend some pins and run jumper wires. I've seen home computers upgraded that way. Well, I'll look into that. I just wanted to know if anybody had experience upgrading the exact same board that I have.

Thanks,

Christian

Ok - you know what you are talking about. Some people come in with less skill and just assume that piggybacking the same chip on top of an existing chip works, and that's a no-no.

The data sheet that I found says the TMS4500A supports up to 256KB of RAM for that controller. Yet my memory board has just one TMS4500A, but 512KB total of RAM. I'm confused.

I found a datasheet too, but it's an abridged version. Yes, it "directly adresses 256KB without external devices", but this card has a few other chips on it, so I guess the designers managed to address 384K (added to the 128KB already installed in your Junior, you get 512KB). The datasheet also says that it supports chips up to 64Kbits in capacity, so 41256 chips might not be an option for upgrading.

As regards your dead memory boards, have you tried replacing the DRAM chips? Although 4164's are pretty reliable (much more reliable than the 4116's were), they can die.

Christian

My version of the Racore memory board has 2 banks of 256K chips. They are Micro 1259s - clearly 256Kbit DRAMs. Something isn't quite right with the memory controller datasheet, because these should not be supported.

I'm not going to debug the dead boards unless I'm desperate. I'd rather spend the debug time designing a new board that uses static RAM instead for the reliability and performance reasons.

It looks like we don't have the same board. Mine has 6 banks of 64 KB, each bank being comprised of eight 64 kbit chips. It could be that 256 kbit chips didn't exist yet at the time this datasheet was written.

However, I think I found the information I was looking for in the "PCjr Club Library" archive, which I downloaded from this board -- see the "Eugene PCjr Club Library Collection" topic in the "PCjr Software" forum.

For those who would be interested, see:

PCjr_Club_Library.zip\PCjr Club Library\U-612 PCjr Tool Box\RACOR640.DOC
PCjr_Club_Library.zip\PCjr Club Library\U-612 PCjr Tool Box\RACOR736.DOC

Christian

I'm resurrecting this old thread to inform you that I finally took the time to perform the 640 kB upgrade described in the aforementioned RACOR640.DOC document. I confirm that it works. It took me a few hours to complete. It takes good skills at soldering.

Cheers,

Christian