256K RAM Pack

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amenjet
Posts: 179
Joined: Tue Jan 03, 2023 7:54 pm

Re: 256K RAM Pack

Post by amenjet »

Another update. This is really for anyone who is thinking of designing a ram pack or likes the technical aspect.
I now have a 256K ram pack that can be used to store data and can be removed and re-inserted into the organiser. It sounds like a basic feature, and it is, but it was a problem with the first design. I think I know what is going on and have to find a proper fix.

So, technical bit: I am using and active high CE on the RAM chip to power it down. VCC is attached to the CE and when it disappears (slot powered down, although there is a small current still available in that case, or, more importantly, when the datapack is removed) then the CE goes low and the RAM chip ignores all inputs and moves to a very low power mode. That all works fine. When unplugged, all inputs to the chip apart from VCC (I originally had VCC floating which was an error, it's pulled down with a 680K resistor now) which has the battery supply on it to retain the contents of the RAM. The problem is that WE is also low, which is a write, held off by CE being low. When the datapack is inserted, though, it looks like CE goes high before WE goes high and a write occurs. That's not good and the organiser sizes the pack as I think the header is corrupted. A pull up on the WE line stops that happening as a write is not driven when the datapack is inserted. With that arrangement the pack can be removed and re-inserted and no problems.
Unfortunately, the 680K resistor to WE seems to be drawing too much current for the battery and it drains too quickly. I need to revise the circuit to use some other method to hold the WE high or hold off the write. It's useful writing this sort of thing out, as I'm writing this I'm now not sure why the battery drain would be as high as it is. Anyway...

This sort of timing problem is why some hot plug devices have different length connectors as that allows timing to be set up on insertion and removal.


End of technical bit.

Andrew
amenjet
Posts: 179
Joined: Tue Jan 03, 2023 7:54 pm

Re: 256K RAM Pack

Post by amenjet »

After designing a transistor circuit to prevent what I think were spurious writes on insertion, and some losses of data due to what I think was a dodgy battery connection, I have a prototype 256K RAM pack that has held data for several days now, with removal/insertion and reading of data several times a day with no problems. So, it does seem to be there now. More testing needed.
I have sent off a new PCB design which has the transistor on the PCB (the prototype has it hot glued to the board) which will be good for beta testing I think, assuming no more problems crop up.
The battery is held on one of the RAM pack modification PCBs so is removable for replacement and it all fits inside a standard (modified) datapack case.

Andrew
amenjet
Posts: 179
Joined: Tue Jan 03, 2023 7:54 pm

Re: 256K RAM Pack

Post by amenjet »

OK, so after fixing some problems with the circuit, adding a couple of components and fixing two construction problems I think I have a working 256K
RAM pack. I am still testing but no loss of data in several days now. Some pictures attached.
P1150137sc.JPG
As you can see, the battery is removable, and it does fit into a datapack case if you cut away some of the plasti to allow for the connector to fit and also to allow the battery to fit. I think RAM pack case might have a different design to allow the batteries to fit?

Battery life is looking good too, the RAM chip takes about 1uA in standby.

So, more testing but it looks good so far.

Andrew
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PaulK
Posts: 43
Joined: Wed Jan 04, 2023 11:54 am

Re: 256K RAM Pack

Post by PaulK »

Andrew,

That sounds like excellent progress! I would have no idea where to start - you hardware guys are like magicians!

Once you have it working with 256k chips, how easy is it to drop on a bigger chip? Is that a re-design or more straightforward?

Thanks for all your efforts - would happily purchase one or two of these when they get to that point.

Cheers
Paul
amenjet
Posts: 179
Joined: Tue Jan 03, 2023 7:54 pm

Re: 256K RAM Pack

Post by amenjet »

PaulK wrote: Sat May 13, 2023 7:13 pm Andrew,

That sounds like excellent progress! I would have no idea where to start - you hardware guys are like magicians!

Once you have it working with 256k chips, how easy is it to drop on a bigger chip? Is that a re-design or more straightforward?

Thanks for all your efforts - would happily purchase one or two of these when they get to that point.

Cheers
Paul
It would be a redesign, well, a new PCB with a footprint for the new chip, as I doubt I could find one that is larger and has the same pinout. The circuit itself, though, should be fine for any chip up to 2Mbytes, so it's just a matter of adding the extra pins in, or at least it should be.

Regards

Andrew
okto
Posts: 59
Joined: Sun Apr 23, 2023 5:14 am

Re: 256K RAM Pack

Post by okto »

I'd also be interested in purchasing PCBs or shared board layouts, if you're feeling generous.
okto
Posts: 59
Joined: Sun Apr 23, 2023 5:14 am

Re: 256K RAM Pack

Post by okto »

PaulK wrote: Mon Apr 24, 2023 1:46 pm The thought of a 1mb RAM pack is exciting indeed.
Is there even 1MB of OrgII content in the world? I know in the Cambridge Z88 community we laugh about how it's now possible to store every piece of software ever written for the machine AND run them all at once!
thesourcerer
Posts: 55
Joined: Sat Jan 07, 2023 1:00 pm

Re: 256K RAM Pack

Post by thesourcerer »

I have a Widget Psion price list from 1992 which lists a 512k Rampack. It was £586.32 including VAT (which would be over £1500 today)! I think these were the piggy back versions (there is a picture of one that I sold to Jaap on his site). I seem to remember seeing 1Mb and 2Mb versions around the same time (I think by Dynasys or MSE) but I do not know what the format was, and the price was astronomical. Not surprisingly I’ve never seen one!
The piggy back format meant that only one could be used in one slot, though the second slot would accept a 256k Rampack or 512k Flash datapack.
amenjet
Posts: 179
Joined: Tue Jan 03, 2023 7:54 pm

Re: 256K RAM Pack

Post by amenjet »

The 256K ones I have made aren't that expensive, of course.
I've been testing one for a few weeks now and it hasn't lost any data at all. Battery life seems good too.
I have had a report of lost data from a tester, though, which I need to investigate. I haven't managed to re-create the problem, though, so it could be related to the organiser.

Andrew
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Martin
Global Admin
Posts: 202
Joined: Mon Jan 02, 2023 5:18 pm

256K RAM Pack - TEST - PASSED

Post by Martin »

Test Report
As one of the people testing Andrew's 256K RAM pack, I produced the test report (here)

Other Memory Expansion Options
There are details of Andrew's other Memory Expansion options (here)

Sincerely Martin
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