TI-74 RAM Cartridges

[mrvan]

My TI-74S arrived today and reports 40,478 total with the 32 KB memory card install. So fully confirmed, the 32KB memory module does extend memory for both the TI-74 and TI-74S models, but only by an extra 8 KB for the TI-74. 16 KB isn't bad, though. Still represents a doubling of memory.

 

I have one more TI-74S on order. 

 

The Hexbus adapter arrived so will be able to get into programming this bad boy, but the next couple of weeks looks quite busy.

[mrvan]

I also went back to the TI 8 KB memory board that I modified with the 32 KB RAM in the same family. I'd been a little disappointed that it reported only 16 KB total on the TI-74 but it does expand memory on the TI-74S to 40 KB total as well. I described that modification process towards the beginning of this thread. It was much more difficult, primarily due to the need to unsolder the memory from 40 year old pads and they weren't happy about it. I'd hoped to be able to reinstall in an official cartridge case but no way it'll fit with all the jump wires.

 

I definitely recommend @brain's memory board.

[brain]

I still have 4 of the originals, and I bought a small batch of the revised PCB.

[mrvan]

1 hour ago, brain said:

I still have 4 of the originals, and I bought a small batch of the revised PCB.

If you have two of the new to spare, that's my preference. I'd like to encase them--the originals will be a bit tight for that, but beggars can't be choosers. 

 

And I can pay you for them including shipping.

[mrvan]

I wrote a simple program to test the memory. Unsurprisingly the memory passed.

[mrvan]

Another interesting tidbit, maybe not so positive, but maybe can be sorted out here.

 

I have two TI-74S computers where one expands to the full 40KB RAM and the other to only 16KB. I thought these two units would be the same.

 

The serial number for the one that fully expands is 9200754 and probably manufacture date I-10 90.

The other is 0009754 I-0687.

 

I presume the first unit was manufactured 10/90 and the other 06/87. Perhaps there was a small ROM update between these dates.

@brain, what is your TI-74s' serial and mfg date?

 

I don't see any peek operations available so no obvious ways to tell the differences in ROM if they exist.

 

Looking at the ROMs for the unit on data math, the ROM seems to have a -1 revision #, but that one was manufactured 06/87 as well with serial 0009790, only 46 units later than mine. http://www.datamath.org/Graphing/JPEG_TI-74.htm#TI-74S, http://www.datamath.org/Graphing/TI-74S.htm.

 

The board image looks like it'd be relatively easy to unsolder and replace the ROM if that was truly the other difference. Probably not room for a socket. Getting the later ROM image also perhaps challenging. I've not taken apart one of these bad boys to look at the ROM, so will wait to do that a bit until I hear any what to watch for details or do enough research. If that was the solution I wonder if the later ROM will also work in the older TI-74 model. I know I'm jumping pretty far ahead of myself but it'd be cool to "upgrade" the older TI-74 models fully as well.

[brain]

9200962 I-10 90

[mrvan]

5 minutes ago, brain said:

9200962 I-10 90

Although not conclusive, your later date version and mine suggest that perhaps only the later run models support more memory than the older.

[mrvan]

I've managed to find a few other TI-74 S units on ebay. Both are later manufacture dates than the one I have that doesn't fully expand to 40 KB.