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.
Copper WireAll wires inserted. They can flop around though.
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.
Aligning the pins while solderingAfter SolderingAnother angle
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.
Installed to SolderAfter soldering and trimmingI was sure to fill the holes.
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.
Time to pull the Socket.Here you see the Kapton tapeIt is a close match.
Below you can see it installed into the PCB. I guess now the Green PCB is fine as they look good together.
It is just barely up over the case. The Top Cover has a hollow in it though so it will still fit.I think it looks great.
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.
I have had the TI 99/4a for awhile now, and posted the Recapping of it. I actually have two TI 99/4a computers as the first one had a bad membrane type keyboard. The second one had a Stackpole type keyboard, which is is working order (although some of the square tubes are split). The Joysticks are awful, and I built up a Joystick Adapter to plug in Atari 2600 compatible joysticks to it. That made things much better for it. I also built up a Pitfall Cartridge for it and 3d printed a shell. Still I haven’t used it much. The Ti 99/4a was the first computer I had, it was old at the time. I had a few cartridges for it, and used BASIC in it at the time. It was quite limited with all the more software I had available.
In anticipation of getting a FinalGROM Cartridge or Backbit or something, I had purchased a 32k Memory Expansion that was for sale from Backbit. After I received it, I designed a 3d printed case for it. It turned out really well and has been sitting with my TI 99/4a since then, but I was unable to use it at the time.
I like the small and clean look of the Backbit 32k.
The FinalGROM Cartridge came in, I made up an SD Card for it. I tested it with some games that did not require the 32k Memory Expansion. Then I plugged in the 32k Memory Expansion and the 99 wouldn’t power up properly, it just made an audio buzz. I checked the contacts, checked the Memory Expansion. I noticed flux residue on the ICs of the 32k Memory board, and cleaned that off just incase it became contaminated. I ended up removing the CPLD from the board, and inspecting it. I soldered back on the CPLD, and the buzz was gone, and I got it working just one time. Then I powered it off and tried another 32k program and it quit working again. I rechecked the soldering on the CPLD, as well as the ram IC and edge connector and touched it all up. It would randomly work, it may pass Jedimatt’s Burning Ram test for a few passes then fail. Then it generally won’t detect the 32k memory until it is powered off a bit and reseated.
Backbit has quit selling the 32k Memory Expansion, and has released on Github as a project to allow you to build your own.
The Backbit 32k is more compact, and should be cheaper to make than Jedimatt’s design if you have all the required equipment to program and assemble it. Jedimatt’s 32k which is available to build by ordering PCBs with the provided Gerber files and standard components. Jedimatt’s 32k requires no programming of components. There are a few issues with building the Backbit 32k. It has a CPLD and that requires programming with a JTAG programmer. The board doesn’t have a JTAG Header. I believe this is simply due to it not being designed as a DIY Project, Backbit is a business and she makes and sells her products. I later found the RAM IC Footprint is wrong, at least for the ram IC that was fitted on mine when I bought it. The CPLD is on the top edge of my ability to solder with the super fine pin pitch. The Backbit 32k has not been around nearly as long as Jeditmatt’s, I can’t be sure that the problem is a defective part. It may also just not like my specific Ti 99/4 for some reason.
Jeditmatt’s 32k design has been around for many years. It is all through hole style components and easier to solder together. Surface mount soldering is not that difficult with some practice. That is at least for the larger parts, the smaller pin pitch like the CPLD gets far more difficult for me.
This left me with a problem. I know I can build Jedimatt’s design, I know it is thoroughly tested. It takes way more parts and time to assemble, and is also more expensive to build. I just love how the Backbit 32k looks. I also currently have no interest in a TIPI which does require Jedimatt’s type of expansion.
I looked around and found a Jedimatt type expansion for a reasonable price with shipping. I would rather build it though. I looked into it. I found all the required parts at Digikey. I also found the Ram and CPLD for the Backbit 32k there.. I also found a more “stylish” case for the Jedimatt 32k board on Thingiverse.
I decided to order the parts for Jedimatt’s board. While I was at it, I did order a replacement ram chip for the Backbit 32k board. I could have picked up the CPLD as well for $3.50 or something. I wasn’t interesting in trying to hand wire that for the JTAG programmer at this point (I may next time I place an order).
Jedimatt’s 32k Parts List. I have provided links for more specific parts. The most generic parts can be purchased easily. I included a link to the type of Round Female Socket Pins that I used, Digikey does have them as well if you are willing to look for them.
Jedimatt’s PCB Design. Download his Gerber files and order from your favorite PCB manufacturer.
I downloaded the Gerber files from Jedimatt’s site. Then I went over to JLCPCB and ordered the boards as that is where I order my PCBs from. I managed to order 5 of them and have them shipped to me for a total of $4.11 ($2.00 for the boards and $2.11 for shipping and taxes). I did kind of mess up, I didn’t want them in Green, but forgot to change it. The other colors did list they would take 2 more days to ship though.
I placed my order with Digikey for the parts I required. Between the PCBs and the Digikey order, I paid less than I would have for a completed 32k Memory Module on Ebay. I did have the 74HCT138 already, as well as the Resistors, Capacitors, 2.54mm Pin Headers, 2.54mm Jumper, DC Power jack, LED. I am also 3d printing the case. I ordered 5 of each of the 245s and 21s as well as two of the ram ics (and a replacement ram ic for the Backbit 32k incase that was the fault there).
First the Backbit 32k. I replaced the ram ic. The one I ordered was the exact part number that had been on it. I had the same results. Initially I couldn’t get it to work at all, once it made the buzz on power up, on removing it and trying it a few more times, it just failed every time I ran the RAM Test on it. It then worked for a little one time.. So it is on the shelf incase I want to buy another CPLD for it at some point.. I am wondering if it may be the edge connector is for some reason not making good contact. I am thinking I may get out the other TI 99/4a to see if it works on it. That one has no keyboard in it though, and I can’t start the ram test without one. The keyboard for it may be just working well enough to do that though if i reinstall it..
Next I moved on to the Jedimatt 32k build.
The boards came and looked great as usual.
The part that needed the most consideration was the Edge Connector. The edge connector that I purchased is of the same series as the one I purchased to replace the Cartridge Connector on my Commodore 128. Being that the connectors are the same 44pin, the difference between is the one for the Commodore 128 was a Right Angle connector. Being how it worked with the Commodore 128 I figured it should work for the TI 99/4a. It needed modification though, to remove the “mounting hole ears” on the side just like it had with the Commodore 128. This was a bigger deal here due to fitting in the 3d Printed case, the opening is very tight, and I needed to file it down a bit smaller than I thought I would.
There is a difference between the Edge connector between the Jedimatt and Backbit 32k. Jedimatt recommends some “individual pins” to use as extenders on the Edge Connector. It doesn’t seem they are available. I used Round Pin Female Socket strips. This is required because the “legs” of the Edge connectors are too short to reach deep enough into the side of the Ti 99/4a, or into the pass through socket of other Sidecars like the Speech Synthesizer.
I printed out the case and fitted the board to check how far the connector came out. With no pin headers in between it was way to short. I then tried with 1 pin header to see how that fit. It was still to short, compared to the Backbit 32k. The Backbit 32k didn’t use pin headers to extend the connector, it was cheated a bit by not putting the edge connector pins the whole way through through the PCB. The 3d printed case I made for the Backbit 32k is much thinner than the cases designed for the Jedimatt 32k (or at least the one I am using). I could rework the 3d printed case for the Jedimatt 32k, and maybe I could have managed then with a single pin header to extend it. That was not something I wanted to do though, it would have taken a good bit of work on the model.
That meant I needed to stack 2 of the Round Pin Female header pins for each of the 44pins of the edge connector. Which required stripping out 88 of the Round Pin Female header pins. The next step was to insert them into stacks of two each. The stacked pins were then soldered together, holding them with my Helping Hands. I went to stacking 5 pairs at a time and standing them up in holes in the PCB by the time I was done. While it was a bit awkward being they could spin around and even be picked up by the solder iron, that was quicker than trying to get the alligator clips in the Helping Hands to hold them properly.
The pin header strips before removing, then the individual pins, a pair of stacked pins and in the middle a pair soldered.Starting to put the pairs of pin headers on the connector.The connector installed.
That was a lot of time consuming work. Stripping the 88pins out of the strips without loosing or damaging them. Then stacking and soldering them together in pairs without making an awful mess of it. Then installing them onto the modified Edge connector that I had taken the wings/ears off and sanded down a bit more to fit through the 3d printed case.
To get the alignment correct, I fitted the pins and edge connector into the PCB. I then soldered the 4 corner pins into the PCB. I then aligned the edge connector and soldered the 4 corners of the edge connector to the pins. This let me align it all nicely, and after that it was easy soldering the rest of the now captive pins to the Edge Connector and PCB. Doing the soldering that way worked very well. It is very solid, it doesn’t look too bad overall once it was finished. I had tried to find what other people were using to extend the Edge Connector, it looked like one example was using the same Round Pin Sockets stacked up.
Above you can see the Jedimatt 32k Edge Connector does extend just a little bit higher than the Backbit 32k Edge Connector when they are in the cases. That was the closest I could get it with using the Round Pins to extend the Edge Connector. That does make it have a bit more gap between it and the Speech Synth Sidecar and or the TI 99/4a. I am thinking of that “heatsink looking” interface bit on the side of the Speech Synth Sidecar where it meets up to the TI 99/4a. I could make one of those and stick it on the Jedimatt 32k to support the gap in a very similar way.
The Backbit 32k has a serious time advantage due to not needing to extend the Edge Connector pins. You can see below the BackBit 32k just solders in the Edge Connector, keeping the pins as barely in as possible to extend it out as far as possible. You also see it has far less components overall. (Update 8/31/25 the Backbit 32k is working now, I striped it to be the bare board, flipped the edge connector 180 degrees and reinstalled the ics and capacitors, and with a new microscope I could inspect the work properly to clear any solder bridges and see that all the pins were soldered properly)
Backbit 32k
The rest of the Jedimatt 32k build is strait forward through hole soldering. Keep in mind that you have to install the Jumper for the Power Selection. To the front it uses the 5V from the Ti 99/4a Expansion port (requires a mod to the Speech Synth Sidecar to use), and toward the back of the TI 99/4a it uses the Power Jack and you need to connect up a 5V Power supply. Be sure to only use at 5V Power supply. I am using the Ti 99/4a to power it from the Edge connector with my modified Speech Synth Sidecar. Also install all the ICs on the board in the correct orientation, Pin 1 toward the Capacitor.
The case design intends the LED to tight to the board. There is a “clear” lens bit to print. I am not sure how that is fitted and decided to mount the LED pointing out through the hole.
As modeledImpacts the Screw here.Removed a bit so the screw fits.
I had taken the top out of that circle so the LED would clear, but on trying to close it still didn’t close. On pressing it a bit, and then opening the case, I saw that the screw in the corner hit that one spot around the LED housing. On removing a bit of the plastic there, the case then closed properly. I printed it in a Silver and Black theme to fit the TI 99/4a color scheme. The 4 screws are not specified, I used four 2mm by 10mm long screws and nuts (or 2.5mm, but I believe 2mm), a little shorter screw may have cleared the LED housing..
I connected it up and ran the Ram Burn in test on it. It passed all the tests for 12 cycles, then found an error on one of the banks. I powered it off and back on, and ran the test again, it was working again. I am not sure the issue, or if it was a random failure. I did do some more testing with it today, and it worked properly. I tried out a few 32k Games and Programs all of them ran fine.
The case printed reasonably well, but has room for improvement. The Silver is a sliver PLA, the 99 4 and A had little nearly “floating bits” that are missing. They were all partly there, but due to a defect on one and the poor strength of those bits, I removed them. I am thinking of possibly reprinting it as a 2 color print with a black insert filling in the letters to give it strength to retain the floating bits. I didn’t print the “inner blocking piece” You can see the PCB and parts inside through the openings. The back is printed in Black PETG, the case itself is a snap/friction fit top. The screws hold the PCB to the Bottom, but do not hold the “top” on.
Jedimatt 32kBackbit 32k
The Speech Synthesizer doesn’t pass 5V through it without a modification as shown on Jedimatt’s website. You can alternately use the external 5V Power Jack to power the 32k Memory expansion. If you have built either of the 32k Memory Expansion Sidecars, it should be no problem to add the wire to connect the 5V Pin in the Speech Synthesizer.
Muldrf's Hobbytronic 