TI-74 RAM Cartridges

[mrvan]

On 4/29/2024 at 11:00 PM, brain said:

So, one item to validate:

 

Schematic says 1mF and 100mF, but the text that the schematic was linked off of says 1nF and 100nF  Big difference

 

Jim

@brain, the capacitors as read by the DMM are 

C1 = 136 nF

C2 = 3.11 nF

 

So the unit obviously is nF.

[brain]

Good, that makes it easier to place and route, as a 1mF cap is a bigger item.

 

The current design is thus correct.

 

Jim

[mrvan]

What are your thoughts on rolling this out? I could perform an initial build and check unless you have or can obtain a ti-74. I’ve used kicad for layouts and have had boards made before. I’m not familiar with the brd files nor how to go from them to a pcb mfg. Or do you make, sell, etc.?

 

I have no real preferences but that these are available to the community.

[mrvan]

@brain, do you know what the edge connector specs are?

[brain]

On 5/8/2024 at 7:50 PM, mrvan said:

What are your thoughts on rolling this out? I could perform an initial build and check unless you have or can obtain a ti-74. I’ve used kicad for layouts and have had boards made before. I’m not familiar with the brd files nor how to go from them to a pcb mfg. Or do you make, sell, etc.?

 

I have no real preferences but that these are available to the community.

I'm gonna have proto PCBs made, and I'll send out a few (usually I get 5-10).  I'll let you test it out 🙂

[brain]

6 hours ago, mrvan said:

@brain, do you know what the edge connector specs are?

Looks like .5" to me, 15/30 pins.  I'll see if I can spec out the thickness of the PCB that fits into it.

[mrvan]

10 hours ago, brain said:

I'm gonna have proto PCBs made, and I'll send out a few (usually I get 5-10).  I'll let you test it out 🙂

OK, sounds like a plan.

[+acadiel]

On 5/11/2024 at 6:05 PM, mrvan said:

@brain, do you know what the edge connector specs are?

Just as a FYI, I talked to Stephen Reid about the female edge connector on the 74 carts quite a while back - these have a proprietary pitch.  They had custom edge connectors manufactured for them in Japan, and the excess were physically destroyed when the lifecycle of the unit was complete (i.e. custom carts stopped being made, etc.)  New edge connectors would have to be physically manufactured with the proper measured pitch.  The closest thing that even is out there is a TE edge card connector like this:  https://www.te.com/en/product-5650712-1.html - but it's slightly the wrong pitch.

 

[mrvan]

12 hours ago, acadiel said:

Just as a FYI, I talked to Stephen Reid about the female edge connector on the 74 carts quite a while back - these have a proprietary pitch.  They had custom edge connectors manufactured for them in Japan, and the excess were physically destroyed when the lifecycle of the unit was complete (i.e. custom carts stopped being made, etc.)  New edge connectors would have to be physically manufactured with the proper measured pitch.  The closest thing that even is out there is a TE edge card connector like this:  https://www.te.com/en/product-5650712-1.html - but it's slightly the wrong pitch.

 

Well, non-standard parts aren't cool. Is the pitch on this device compatible enough that it can be made to work.

 

I was starting to notice that the PCB connection side of the connector seemed quite rare and I wasn't finding anything out there that seemed to match.

[+OLD CS1]

13 hours ago, acadiel said:

They had custom edge connectors manufactured for them in Japan, and the excess were physically destroyed when the lifecycle of the unit was complete (i.e. custom carts stopped being made, etc.)

Just plain rude.

[mrvan]

1 minute ago, OLD CS1 said:

Just plain rude.

I was thinking similar, and how could TI do something like that. But it was the mid 1980s, and compactness was an issue. 

 

If you look at the PCBs for these memory cards, you can see they are hand-soldered. For the size those that did it did pretty well. Only the best of my solder joints matched their average. So there was a lot packed in.

[Gary from OPA]

There a used one for sale on eBay for the next 3 hours, before bidding ends.

 

https://www.ebay.ca/itm/156196569689

 

[brain]

17 hours ago, acadiel said:

Just as a FYI, I talked to Stephen Reid about the female edge connector on the 74 carts quite a while back - these have a proprietary pitch.  They had custom edge connectors manufactured for them in Japan, and the excess were physically destroyed when the lifecycle of the unit was complete (i.e. custom carts stopped being made, etc.)  New edge connectors would have to be physically manufactured with the proper measured pitch.  The closest thing that even is out there is a TE edge card connector like this:  https://www.te.com/en/product-5650712-1.html - but it's slightly the wrong pitch.

 

I don't want to contradict sreid, but my micrometer measures .699" from start of 1st pin to start of 15th, so .699/14 = .0499" pitch.  I think .05 should work.

[mrvan]

@brain, I think you're right. I ordered several of the connectors. Once received I can also visually check the alignment, etc.

[+acadiel]

3 hours ago, brain said:

I don't want to contradict sreid, but my micrometer measures .699" from start of 1st pin to start of 15th, so .699/14 = .0499" pitch.  I think .05 should work.

Could be slightly off and more compact - I'll have to take pictures of the two connectors next to each other.  The TE connector is larger than the TI one is.  My guess is that the TI one was custom made to fit, and Steve did confirm that it was custom made.

 

Edit:  I measured and remembered one of the issues - the card edge in the 74 is 20.8mm when measured end to end.  I just measured that opening in the TE connector, and it's not wide enough (20mm is a tight fit). I think compared to the TI version, it was thicker as well.  I did correspond with TE about the requirements (this was several years ago), and I need to find that e-mail, but needless to say, that was the *closest* thing they had.  My statement still stands about us needing to either recreate the connectors that can fit that were made for the 74 (or hack the TE one, which will get tedious because they will have to be hand modified).    If one of you finds an easier path, I'm all for it!

 

Edit 2:  Added pics.  Note, there almost 1mm larger dimension on the male edge connector than the female edge card.  TE in their email in 2019 mentioned they don’t have anything with a larger opening than 20mm, and they suggested clearance is 19.5mm max, 20mm is very tight fitting.  Nor do they have an edge solder connector similar to the TI one.  Of course, if we want to design something and pay for the design and tooling costs, and commit to a minimum manufacturing run, they are more than happy to partner with us and help us to design a connector.  🙄

 

[+dhe]

Are there any cartridges, that are so common, that they might be sacrificed for the connector?

[mrvan]

11 minutes ago, dhe said:

Are there any cartridges, that are so common, that they might be sacrificed for the connector?

That’s a great question. Occasionally I see ROM cartridges on eBay but most often they are for sale with the computer as well so will command the extra cost. 
 

in a pinch this could work though.

[brain]

Given how little needs to be shaved, a jig and a small drill press might do the trick for a 100 units.  I agree it's somewhat tedious, but not insurmountable.  Most MOQ on custom tooling would be 5K (1K maybe, but it's more rare), and that's a tall order for the TI74.

 

The other option, for those willing to make a tiny alteration, is to shave .5mm off each side of the PCB edge connector.  It's 1/64" on each side. 

[brain]

Boards arrived!

 

Ordered the parts from Digikey, and I ordered this connector to try:

 

https://www.digikey.com/en/products/detail/edac-inc/302-030-221-201/21847474

 

It was more expensive than others, but it allows contact with thinner PCBs, so I thought that might help. 

 

We'l see.  @acadiel's idea of a custom order might be the only option, but i figured it can't hurt to try.  This conn is 20.3mm wide, still too small, but I'll try my drill press to clean out the ends if it does not fit.

 

As well, I'm wondering if someone can help me check the TI 74 schematic for something.  On the TI 74, is Address Bit 0 the lowest address line (like in most systems), or a high one (like on the 99/4a?  It does not matter for this design, but I'd like to make sure my PCB footprint is correct for other projects, like a ROM cart (which would be important)

[mrvan]

21 hours ago, brain said:

Boards arrived!

 

Ordered the parts from Digikey, and I ordered this connector to try:

 

https://www.digikey.com/en/products/detail/edac-inc/302-030-221-201/21847474

 

It was more expensive than others, but it allows contact with thinner PCBs, so I thought that might help. 

 

We'l see.  @acadiel's idea of a custom order might be the only option, but i figured it can't hurt to try.  This conn is 20.3mm wide, still too small, but I'll try my drill press to clean out the ends if it does not fit.

 

As well, I'm wondering if someone can help me check the TI 74 schematic for something.  On the TI 74, is Address Bit 0 the lowest address line (like in most systems), or a high one (like on the 99/4a?  It does not matter for this design, but I'd like to make sure my PCB footprint is correct for other projects, like a ROM cart (which would be important)

That's great! My order arrived with the other suggested connector. I'll look it over on the weekend to see how comparable it is with the original, and how adaptable.

 

I have the memory and maybe a connector that can be made to work. 

 

 

[brain]

PM me an address and I'll send a board to you.

 

Obviously, I make no promises yet, as I've not tested it here.

[brain]

Success!  32kB additional RAM

 

Notes:

 

• 15/30 .05" connector seems to work fine.  There's no side to side play, but it's not tight, so I think the choice I made works
• connector has riser tabs on the edges that keep the PCB from sitting flush.  Modified the PCB to adapt
• I forgot to order a 3K3 resistor.  A through hole one from my stash was used.
• I was too cute for my own good, thinking that TI did the "TI address/data numbering" on the TI74.  They actually did not, but since I did, when I first tried it, it could not find the memory.  I looked at the original schematic, noticed how the ROM was connected, and realized my error.  It's not a huge deal on RAM, but I had connected A15 and not A0.  One bodge wire later and it works.

Battery backup appears to work as well, though if you pop out the RAM and boot the machine, it appears to reset.  Not sure how that works, exactly.

 

I might need to shorten the PCB just a bit ti allow cases.  I'll see when someone tries out a case design for it.

[mrvan]

1 hour ago, brain said:

Success!  32kB additional RAM

 

Notes:

 

• 15/30 .05" connector seems to work fine.  There's no side to side play, but it's not tight, so I think the choice I made works
• connector has riser tabs on the edges that keep the PCB from sitting flush.  Modified the PCB to adapt
• I forgot to order a 3K3 resistor.  A through hole one from my stash was used.
• I was too cute for my own good, thinking that TI did the "TI address/data numbering" on the TI74.  They actually did not, but since I did, when I first tried it, it could not find the memory.  I looked at the original schematic, noticed how the ROM was connected, and realized my error.  It's not a huge deal on RAM, but I had connected A15 and not A0.  One bodge wire later and it works.

Battery backup appears to work as well, though if you pop out the RAM and boot the machine, it appears to reset.  Not sure how that works, exactly.

 

I might need to shorten the PCB just a bit ti allow cases.  I'll see when someone tries out a case design for it.

That is awesome!

 

As to the case, I'm hoping that the dummy cartridge cases can be used. I believe they are the same as those with actual RAM or ROM.

 

I've read there's two use cases for the RAM. One is as a storage and the other is a RAM expansion. In the latter case if you boot without the RAM it breaks the expansion. If I have that right then the battery backup isn't terribly useful in my opinion. I've only used the RAM as an expansion. 

[mrvan]

@brain, this really opens up the usability of the TI-74 and likely the TI-95 as well. Paired with the HEXTIr this thing is going to rock.