TI-74 RAM Cartridges

[+acadiel]

On 5/31/2024 at 10:39 PM, 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.

Great job, I'm glad you found another connector that worked!

 

TE scrounged all over for me several years ago, so I'm glad that there's an alternative SKU now.  How's the thickness of it compared to the original?  We will have to tell Steve that we found a working one!

 

[brain]

The board thickness it accepts seems to be OK (seems snug but not overly tight).  As for the physical size of the connector, mrvan needs to micrometer his and you can get the specs on the one I used from the link above to compare.

 

Jim

[brain]

I thought perhaps this might be needed at some point:

 

https://github.com/go4retro/Xtender-TI74

 

[mrvan]

12 hours ago, brain said:

I thought perhaps this might be needed at some point:

 

https://github.com/go4retro/Xtender-TI74

 

Yes, I think so as well. I was a bit challenged inserting my 8 KB modified to 32 KB board into the slot. The use of pliers had the danger of slipping and damaging the board or parts thereon.

[brain]

Updates?

[mrvan]

14 hours ago, brain said:

Updates?

I have the PCB, parts and now smaller soldering station tip and solder. Work has been excessively busy with little free time. Hoping to build the board on Sunday and will provide feedback.

 

once I do that I’ll also write a program to check the memory. 

[mrvan]

On 6/25/2024 at 8:12 PM, brain said:

Updates?

Sadly my built board isn't working. I've visually inspected it multiple times and found nothing obvious.

 

I applied the white-wire fix for memory pin 9 to connector pin 15, lifting the pin off the pad for pin 9. I found no other mods you had to make. Sadly I forgot about this until the memory was full soldered down. 

 

I first tested in my scrappy TI-74 and it indicated no ram present. The cartridge felt loose, so I thought maybe it's not making good contact. I tried again in another unit and it was fairly snug, but reported the same, so I'm confident in the contacts.

 

Attached are pictures of my build, perhaps you may notice something obvious in them. 

 

The biggest challenge aside from dealing with the tiny parts (there's six somewhere on the floor 🙂, thankfully I purchased enough parts to build 5 boards), was the RAM pads. Seems the two sides should be separated apart by 1 mm or so more, so the pads go beyond the chip pins, allowing for easier soldering from the front. I had purchased smaller soldering tips so I was able to get the iron tip between the pads to ensure solder flow. Maybe I could have bent them in a bit to fix that. Didn't think of it at the time.

 

Bending the connector pins was something but under the magnifying glass and using a straight edge and then a scribe I got them bent nicely so the clamped on the PCB well ahead of soldering.

 

 

 

 

 

[mrvan]

@brain, I took some time to inspect the board further.

 

I tested the resisters and diodes and they are ok. I need to get my other meter that checks capacitors. I've assumed they are unpolarized based on their type.

 

I also looked over the connector. The pin spacing from row 1 to 2 is definitely wider, by approximately 50%, than the original connector. Since yours worked that's not likely the problem. I'm wondering if by any chance how I bent the external pins on the connector could have increased the internal spacing, if even just a little bit. Comparing to another unmodified one visually I see no difference, but if the tolerance is tight then just a little might have been enough to not have good connections.

 

How did you solder the parts? Although I used the most fine tip I could find and fine solder, I slight suspicion that I could have inadvertently flowed solder between one or more pads and the large conductive surface area that outlines most of the board traces.  

 

I'd also like to see close up high resolution pics of your board if you don't mind.

 

I'll have more time on the weekend to further diagnose.

[brain]

I am working to get some hi res pics for you.

 

I can't tell from your pics, but did you cut through the trace on that pin with the wire on it (the trace near the RAM IC?)  I can't tell from the pics.  I'll go back and see if I missed telling you that in the email/DM.  But, the trace has to be cut.

[mrvan]

2 minutes ago, brain said:

I am working to get some hi res pics for you.

 

I can't tell from your pics, but did you cut through the trace on that pin with the wire on it (the trace near the RAM IC?)  I can't tell from the pics.  I'll go back and see if I missed telling you that in the email/DM.  But, the trace has to be cut.

I only lifted pin 9 on the memory and ran a jumper to pin 1 on the external connector. I didn't cut any traces. 

[brain]

49 minutes ago, mrvan said:

I only lifted pin 9 on the memory and ran a jumper to pin 1 on the external connector. I didn't cut any traces. 

Ah, OK.  I glossed over that.

 

Well, feel free to ship it back here and I can take a look at it.

 

Will post pics as soon as they upload.

 

 

[mrvan]

11 hours ago, brain said:

Ah, OK.  I glossed over that.

 

Well, feel free to ship it back here and I can take a look at it.

 

Will post pics as soon as they upload.

 

 

I stated I ran the wire from pin 1 on the connector to pin 9 on the memory, I should have said pin 15 on the connector, which is A0. So the change was to move from using A15 to using A0. 

 

I'll look over your pics once you upload them. I have the capacitors still to check.

 

If I can't diagnose, I'll probably take you up on your gracious offer to ship to you to look it over.

 

Thanks, Mark

[brain]

[mrvan]

@brain, only a few things are different in your board vs mine, except that you cut the trace whereas I lifted the pin. I'm noticing that you connected battery wires and I did not. Was the battery powered, and does it need to be?

[brain]

Does not need to be.  I just put it on to check operation.

[mrvan]

16 hours ago, brain said:

Does not need to be.  I just put it on to check operation.

@brain Thanks for your response.

 

I'm going to take you up on your offer and send the board to you for inspection. Likely will ship in the next few days.

[brain]

@mrvan, I should have asked for a shot of the bottom of the board, could have saved some shipping costs... 🙂

 

Notice anything off about pin 5 from the left?

Here's another shot...

 

No solder, and the pin was "hovering" above the pad.

 

Soldered and your unit works.

 

I'll send it back tomorrow.

 

[mrvan]

On 5/31/2024 at 7:39 PM, 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.

Hi Jim,

 

I tried the returned memory board and it reports only a total of 15902 with fre(0) on two of my TI-74 models. I just noticed your picture is that of a TI-74 S, whereas mine are just TI-74 models. I wonder if the ROM is at least a little different between the two that allows for the extra RAM, or maybe you somehow made another change in the system ROM?

 

Thanks for the fix on the board. 

 

And thanks for the response in advance,

Mark

[mrvan]

2 hours ago, mrvan said:

Hi Jim,

 

I tried the returned memory board and it reports only a total of 15902 with fre(0) on two of my TI-74 models. I just noticed your picture is that of a TI-74 S, whereas mine are just TI-74 models. I wonder if the ROM is at least a little different between the two that allows for the extra RAM, or maybe you somehow made another change in the system ROM?

 

Thanks for the fix on the board. 

 

And thanks for the response in advance,

Mark

[mrvan]

@brain OK, I found this link that answers the 8 KB vs 32 KB memory expansion. It states the original TI-74 can only handle an 8 KB RAM cartridge, whereas the TI-74 S can handle both 8KB and 32 KB RAM cartridges. http://www.emmanuel.hp41.eu/ti/ti-74/pc/tiif_3.X/DOC/tiif.txt

 

I found a TI-74 S on ebay and purchased it, so should be on a better path as long as it's in decent enough shape. There was precisely one on ebay so seems a bit more rare than the standard unit.

 

Nonetheless 16 KB total on a TI-74 is still pretty decent, but 40 KB is a lot better :-).

 

This also explains why the 32 KB RAM memory transplant on a genuine TI memory cartridge didn't yield more than 16 KB RAM total.

 

[mrvan]

@brain, also what are your plans for these boards? I think you stated you made updates to the design for the one issue on the proto-PCB. Do you intend to sell the PCBs, completed units (maybe with 3d printed cases), or perhaps sell through Arcade Shopper? 

 

I'm interested in more of the unpopulated boards, perhaps waiting for the fixed design. I have enough parts still to make another nine boards, save some of the diodes, capacitors and resisters that launched from my tweezers and remain lost on my garage floor. 

[brain]

6 hours ago, mrvan said:

Hi Jim,

 

I tried the returned memory board and it reports only a total of 15902 with fre(0) on two of my TI-74 models. I just noticed your picture is that of a TI-74 S, whereas mine are just TI-74 models. I wonder if the ROM is at least a little different between the two that allows for the extra RAM, or maybe you somehow made another change in the system ROM?

 

The code or the hardware design must be the issue,

 

3 hours ago, mrvan said:

@brain OK, I found this link that answers the 8 KB vs 32 KB memory expansion. It states the original TI-74 can only handle an 8 KB RAM cartridge, whereas the TI-74 S can handle both 8KB and 32 KB RAM cartridges. http://www.emmanuel.hp41.eu/ti/ti-74/pc/tiif_3.X/DOC/tiif.txt

 

Nonetheless 16 KB total on a TI-74 is still pretty decent, but 40 KB is a lot better :-).

 

This also explains why the 32 KB RAM memory transplant on a genuine TI memory cartridge didn't yield more than 16 KB RAM total.

 

I did not know that.  My TI74 was obtained through Stephen Reid's generosity, to test HEXTir, so I am a bit lacking in knowledge on it.

Edited August 3 by brain

[brain]

3 hours ago, mrvan said:

@brain, also what are your plans for these boards? I think you stated you made updates to the design for the one issue on the proto-PCB. Do you intend to sell the PCBs, completed units (maybe with 3d printed cases), or perhaps sell through Arcade Shopper? 

 

I'm interested in more of the unpopulated boards, perhaps waiting for the fixed design. I have enough parts still to make another nine boards, save some of the diodes, capacitors and resisters that launched from my tweezers and remain lost on my garage floor. 

To be honest, I did not have a plan yet, but am open to any/all options.  I can 3d print, but someone would need to design a case.  I am happy to make a small run, if folks express interest.  Greg may also want to build these, since he's done some of the other designs.

 

I have a few more of the original PCBs.  I should see if anyone else wants a board or two, but I can send you a few more of the original design.  I can also run the updated PCB, but I think it'd be best to make sure it fits a 3d printed cart case before I run a larger batch

[mrvan]

3 minutes ago, brain said:

To be honest, I did not have a plan yet, but am open to any/all options.  I can 3d print, but someone would need to design a case.  I am happy to make a small run, if folks express interest.  Greg may also want to build these, since he's done some of the other designs.

 

I have a few more of the original PCBs.  I should see if anyone else wants a board or two, but I can send you a few more of the original design.  I can also run the updated PCB, but I think it'd be best to make sure it fits a 3d printed cart case before I run a larger batch

Looking at my board installed in the TI-74 in its carrying case there's a decent amount of room. It does seem that if the board can be made a little shorter lengthwise that would help a future enclosure fit flush with the edge of the computer itself. I've personally not gotten into the 3d printing yet. My son has done some, probably more out of interest than need. I might be able to get some of his help during the end of this summer break.

 

If you're willing to provide me one or two more that'd be great. I'll make use of them. I'll soon have 4 TI-74s/95s. Once I have the HEXTIr adapter I ordered from your site I'll be able to start writing some software. My development pipeline has common TI basic code for TI-99, CC-40, and should support TI-74 and TI-95.

 

In terms of determining interest, maybe create a new post that indicates success and two are now working? This post may be stale for the group, maybe not paying much attention to it anymore. Not sure.

[brain]

 

I had already shortened the board a bit, so that should be OK. 

 

I have a Multi ROM design I can finish, but it really needs someone who can write a bit of menu code to pick the ROM to run (and an app to program the thing).  Let me know if there's interest and someone who can help with code.

 

Jim

Edited August 3 by brain
fixed picture