TI 99/4a 32k Memory Sidecar Alternate Memory IC Adapter

I recently built a Jedimatt 32k Memory Sidecar for my Ti 99/4a computer. https://hobbytronics.home.blog/2024/05/04/ti-99-4a-32k-memory-sidecar-build/
This was after having a Backbit 32k Memory Sidecar module that was not working properly.

When buying the parts for the Jedimatt 32k build, I purchased a new SRAM IC for the Backbit 32k unit. I replaced the ram IC on the Backbit 32k unit and it still wouldn’t work, reliably. That indicates some other fault on it, either the CPLD or somehow the Edge Connector on it not making proper connection. It will “randomly” work for a short time then quit again.

The Static Ram I purchased for the Jedimatt 32k unit uses a Low Power CMOS 28pin 256-Kbit 32k x 8 Static RAM that is TTL Compatible that runs at 2.7V to 5.5V. The Backbit 32k unit uses a CMOS 28pin 256-Kbit 32k x 8 Static RAM that is TTL Compatible that runs at 4.5V to 5.5V.

I looked at the Datasheets for the two of the Static RAM ics. With one being branded Alliance Memory and the other Cypress, although both list Alliance Memory Inc as the manufacturer. I looked over the pinouts, and they are pin compatible, short of the Address inputs are numbered differently. The primary difference is the footprint. The AS6C62256-55PCN that I purchased for the Jedimatt 32k build is a 28PIN 0.6″/15.24mm Width DIP Package. The CY62256NLL-55SNXIT that is on the Backbit 32k is a 28PIN 0.295″/7.5mm Width SOIC Package.

I purchased some SSOIC to DIP Adapter PCBs. I had assumed they would be sized to adapt to the 28PIN 0.6″/15.25mm Width. I was wrong, they are wider. They are also the narrower SOIC Footprint, which happens to be the same footprint on the Backbit 32k PCB. That meant the CY62256NLL already had the legs bent to go on the narrower footprint, which by the way, is no fun doing. I guess it would have been to easy to just solder the CY62256NLL-55SNXIT to the Adapter and put on the Round Pin headers.

I started with soldering the CY62256NLL-55SNXIT to the Adapter PCB. I then installed a row of Round Pin Headers in Pins 15-28. This is because of where it needs to fit on the Jedimatt 32k PCB. I next need to install a row of offset Round Pin for Pins 1-14.

I went back to the same Round Turn Pin Sockets that I had used on the Jedimatt 32k Edge Connector. I installed the Round Turn Pin Socket row into a Round 28Pin IC Socket. This is why I used the Round 28Pin IC Socket instead of the standard dual wipe 28Pin IC socket that I specified in the parts list for that build. I then inserted the adapter board above to find the spacing I needed to get everything aligned.

Once I had it held in the spare 28Pin Round Pin IC Socket, I took some Solid Copper Wire (a strand from some Cat 6 Solid Core Copper Network Cable) and stripped the insulation from it. I shaped it to fit into the Round Pin Socket. I made up a total of 14 pieces.

Above you see the little copper wires as fitted into the Round Pin Header. I then placed the PCB onto the wires to Hold them in proper alignment. Once they were held in place, I soldered them into the Round Pin Header Socket.

Once the copper wires were soldered, I installed the Adapter PCB into the spare IC Socket and soldered the wires into the PCB to get the proper alignment for everything.

With it upside down, I then made sure to have good soldering on the bottom to help the Copper Wires keep secure. The bottom of the PCB had Kapton Tape on it to keep the Copper wires from wearing into the solder mask and shorting over time. Now the adapter board is all soldered up. It was time to see if it will fit.

Below you can see it installed into the PCB. I guess now the Green PCB is fine as they look good together.

It was time to power it up and see if it works.

It seems to be stable and working properly. I am going to leave this CY62256NLL-55SNXIT on the adapter PCB installed into my Jedimatt 32k Memory Sidecar. I will put the 28PIN DIP Static Ram IC back in my spare parts. I have 4 more Jedimatt 32k PCBs that I could build up. I may still try to revive the Backbit 32k Memory Sidecar, but if I don’t, I could at least salvage the other CY62256NLL-55SNXIT and the Edge Connector to build another of the Jedimatt PCBs.

8/31/25 A little update on the Backbit 32k Memory Sidecar. I did buy a new CPLD for it a while ago, but before installing it, I decided to strip down the whole board. I removed the 5 Caps, the Edge Connector, the ram and the cpld. I cleaned the pcb. I reinstalled the Edge Connector flipped 180 degrees, I felt it potentially was a contact issue, and flipping the connector may make a difference. With the edge connector “pins” being flush with back holes on the pcb, it may have had a bad solder joint in one of the pins where the solder didn’t flow properly to the pin? I then soldered the Ram IC, CPLD and finally the capacitors back on. With practice I have been doing much better with soldering the CPLD. Also with good flux, and a new microscope for inspection. The primary issue with the board was solder bridges, some very small bridges kept hiding between the CPLD pins, but the new microscope made them much easier to see. After cleaning and reinspecting the pcb, it is working in the TI 99/4a just fine. I tested a couple 32k programs and they started fine, I have been running Jedimatt’s Expansion Memory Test Burnin as seen above for over 20 passes on it. I’m glad it is finally working reliably. This does mean I now have two 32k sidecars, for now I think I’ll use the Backbit one. I also have a spare CPLD, the primary reason I reinstalled the original one is I really don’t know how to program the new one, as to if I can use the same method I have used for the ZX Nuvo 128 board, or if I would need a different programmer. The board also does not have any traces going from the JTAG pins, meaning soldering on very small wires to very small pins to even attempt it. There may have been a bad solder connection or bridge, or it may be the edge connector in some way. I did find that the Andonstar 246S is a great upgrade over the little lcd microscope I had previously. It is much clearer and I could see the solder points and solder bridges that needed to be cleared. It has enough clearance to the microscope that I can use it to view the screen while soldering.