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HP48GX RAM card design
06-10-2017, 11:06 PM
Post: #1
HP48GX RAM card design
Hello All,

For anyone interested in making their own RAM cards for HP48GX calculators, I have just uploaded a KiCAD design on which I've recently been working. The design files are hosted in a git repository at:-

https://bitbucket.org/cloudycat/rc48-512/overview

along with a small amount of documentation. As you can see from the README, I have built two of the cards myself and have been using them for the past few weeks with no major problems so far.

The files are released under a CC-BY-SA licence which essentially means that any use can be made of them as long as modifications are also released under the same licence.

Obviously I cannot give any guarantees as to the correctness of the design or fitness for purpose (particularly as the information I found on the net about this was incomplete and sometimes inconsistent). Nevertheless, I had enough faith in the design to consider it worth the risk with my one and only HP48GX calculator!

Any comments welcome,
Paul
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06-12-2017, 01:08 AM
Post: #2
RE: HP48GX RAM card design
I was going to ask about SX but see you mentioned it. I was thinking the SX can't address that much so I'd need to use a smaller ram chip.
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06-12-2017, 12:29 PM
Post: #3
RE: HP48GX RAM card design
(06-12-2017 01:08 AM)EugeneNine Wrote:  I was going to ask about SX but see you mentioned it. I was thinking the SX can't address that much so I'd need to use a smaller ram chip.

Yes, I hoped that the addition of the A17 and A18 switches would allow operation in an SX as well as operation in either slot of a GX but, as I described in the README, something I don't understand is going on there, so now I can't be confident of it working in an SX.

When I have some more time I'll write a bit of software and look at the signals on a scope to try to figure it out (unless someone out there knows what is happening and can educate us first!).

Paul
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06-21-2017, 09:11 PM
Post: #4
RE: HP48GX RAM card design
(06-12-2017 12:29 PM)pdo Wrote:  Yes, I hoped that the addition of the A17 and A18 switches would allow operation in an SX as well as operation in either slot of a GX but, as I described in the README, something I don't understand is going on there, so now I can't be confident of it working in an SX.

Further testing shows that there is an intermittent problem with one of the two boards I have assembled: it can randomly cause bad card data errors. I need to investigate further, but I suspect a duff solder joint as my soldering is not the best!

However, the other card works fine. It can be used in slot 1, switching between 4 different port 1 configurations via SW17/18's '0' & '1' settings, or in slot 2, mapping the 4 banks to ports 2 to 5 with SW17/18 set to 'A'.

Given this turn of events, I feel more confident that the card should also work in an SX too. I will feel even more confident when I have fixed the problem with the other card!

Paul
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06-22-2017, 12:15 AM
Post: #5
RE: HP48GX RAM card design
Looks like a bit of empty space on the board, maybe move everything toward the connector a bit and then sw3 could move over where the battery is leaving space between it and sw2 to make a larger pull hole.

The ram looms to be only $5 so I'll order a couple next time I make a digikey order and once I have them make a board and try it in my SX.
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06-22-2017, 03:04 AM
Post: #6
RE: HP48GX RAM card design
(06-22-2017 12:15 AM)EugeneNine Wrote:  The ram looms to be only $5 so I'll order a couple next time I make a digikey order and once I have them make a board and try it in my SX.

I'd love to see if this design would work in an SX! I pulled mine from mothballs a while back, along with a TDS memory card. It had a CR2016 holder that allowed the coin cell to be replaced while the card was in the SX so that RAM content was not lost. Perhaps in lieu of programmatic bank switching, the address switches could be moved to the interior to make room for the battery?

~Mark

Who decides?
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06-22-2017, 12:54 PM
Post: #7
RE: HP48GX RAM card design
It just may be a while before my next order, I recently got an order of parts and don't have much time to work with electronics now a days so I don't make orders too often.
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06-22-2017, 08:44 PM
Post: #8
RE: HP48GX RAM card design
(06-22-2017 03:04 AM)mfleming Wrote:  I'd love to see if this design would work in an SX! I pulled mine from mothballs a while back, along with a TDS memory card. It had a CR2016 holder that allowed the coin cell to be replaced while the card was in the SX so that RAM content was not lost. Perhaps in lieu of programmatic bank switching, the address switches could be moved to the interior to make room for the battery?

Well, it may not be as much of a problem as you might think. I realized that my testing of the C1 & C4 battery change capacitors was done with the faulty board, and that the fault was probably invalidating my battery change tests. So today I retouched some of the solder joints on the misbehaving board and this evening was able to confirm that the board seems to behave correctly now :-)

I then redid my battery change tests:-
1. Take the board out of the calculator.
2. Remove the battery from the board, hold for 10 seconds, then reinsert battery.
3. Put the board back in the calculator and check objects in all 4 ports are still okay.

I did this three times and am happy to report that the test passed each time. Not exhaustive I know, but encouraging non-the-less.

I now feel an edit of the README coming on...

Paul
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