Anyone build a DRAM memory expansion card for the Apple I or Mimeo

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I was going to do the 20k or 32k mod on one of my boards, but the though of cutting some of the traces on Mike Willegal's beautiful Mimeo work would pain me Wink, so I want to build a DRAM expansion card that can plug into the side. I have a bunch of mk4116 16k chips from an old Apple clone. Has anyone done this before? I could come up with my own schematics by studying the Apple I guide and the data sheets but if someone has it already would save me some work. I know I could use a static ram chip like the Replica I pretty easily, but actually want to do something that would be more period correct in design since I know the 4116 chips were released in 76-77.

Thanks,
Corey

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Re: Anyone build a DRAM memory expansion card for the Apple ...

hello Corey,
i hope you bear several traps in mind that come up with that idea.....
4116 need a defined refresh-cycle.....
1.
in all old apple-computers the refresh was performed by a trick reading-out the video-area....
but because the video-area in the apple 1 was defined rather small a refresh from the apple 1
itself probably might not recover the entire issue and the refresh probably might only serve a smaller part of the (expanded)-memory....
2.
due to the fact that you of course don´t want to change content of ROM to stay original - it therefor might become neccessary to "preload" some 6502-machine-code as a TSR ( terminate and stay ready ) programm that executes the refresh in the background....
3.
I propose to first take a look at the solutions provided in those days to the timex or sinclair computers like the ZX80 or the ZX81.... the third-party solutios of the days offered a wide range of 16k, 32k and 64 k RAM-expansion-boxes that were pluged to the computer at the backside... and some of them used 4116 chips together with some additional counter and decoder-chips to provide a hardware-refresh solution...
I do have several circuit-plans of that kind of solution from those old ZX81 days...
and just to be correct: in those days the 4216 static chip also started in market - but it just wasn´t used normally because the price was fourtimes of the 4116... - i have an magazine from 1978 that describes the expansion by the 4216 for the PET )....
at least i just wanted to remind to this aspects...
in case of further questions to that topic give me an pm...
regards speedyG

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Re: Anyone build a DRAM memory expansion card for the Apple ...

hello corey,
just a further explanation to the zx81 above....
in fact the zx81 and the zx80 have been made and sold with static RAM 2114 and therefor that computers didn´t have any kind of refresh routine in the ROM. That forced the third-part companies, that offered a RAMexpansion with the 4116 and later with the 4164 to keep care that a kind of "pseudo-refresh" was delivered by the hardware itself.....
exactly this specific point leads up to the point that it might be rather better solution to exactly examine the design of those RAMexpansions and the related circuitplans.... up to my knowledge the problem itself ( the missing refresh-routine ) and the solution ( to execute a simple "hardware-based-refresh-routine" ) wasn´t treated with any other kind of computer in those days....
the remaining question to this point is: how much knowledge do you have to approach this problem ? Is it enough to send you the circuitplan of such a RAMexpansion and you do unterstand the point to look for and modify it to the Apple 1 or is it more usefull to get linked with pm´s and me to assist you with the creation of a solution.... on my side the task is not to tricky due to the fact that i studied at university here in Germany electronic engineering of microprocessors and i´m rather familiar with the needed knowledge... but i can´t take a guess on your level of expertise...
regards....
speedyG

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the second part includes less friends but a lot more joy on life....

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Re: Anyone build a DRAM memory expansion card for the Apple ...

A 4116 DRAM board should be fairly easy. The Apple 1 has enough refresh cycles to handle the additional refresh addresses without a problem and all but one multiplexed line is already there. The additional address pin needs to be multiplexed and the /CAS line has to be gated to perform the Enable function instead of the Enable pin that is used on the 4K DRAMs. The easiest address mapping to use is the new 32K at 0000:7FFF and 4K of the on-board RAM at 8000:8FFF and 4K at F000:FFFF.

A few cautions, use plenty of decoupling on the supplies, especially the 12V, and allow for small value (47 ohm of less) series resistors in all the address lines to the DRAMs as the input capacitance is quite large and with the extra line length to get to the boards severe undershoots can occur. The resistors damp the lines nicely.

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Re: Anyone build a DRAM memory expansion card for the Apple ...

hello wsander,
i just enjoyed the fantastic photo of your apple ! this is really great in the details....
so of course i asume that you exactly know what i mentioned and assume that your detailed knowledge is the best recommendation to the facts.... i just mentioned up to my knowledge about the difference in both ROMs and the slightly changes in both versions of basic. Up to my knowledge the apple II supports the graphics and the refresh is just also related to the refresh of the expanded area of videomemory allocated to the graphicspages too ... at the other hand the apple I has to my knowledge no area allocated to graphics but only textdisplay and therefor the area allocated to the videodisplay is much smaller.... i´d guess that WOZ didn´t figure out to make a refresh to the entire possible 64 k of adressspace but instead to only use smaller refresh of the availiable videoarea with the thought to reduce cpu-time used for refresh and to speed up the computer.... that is the reason that i mentioned the possibility of the "trap" -
but i guess that as result to your own experience with the apple1 my reminder to that possible trap is recognized by you to be wrong...
I allready made contact to mike and hopefully will be within the next release of mimeoboards too.... so i´m glad to recognize apple1 owners that show up with technical knowledge to the topic beyond only using the apple1....
sincerly
speedyG

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Re: Anyone build a DRAM memory expansion card for the Apple ...

hello AF-members,
in another thread i´ve now scanned the documentpages of the ElComp Slotrepeater including the PCB-docs and layout and
i´ve published it in my Applebox-pages at:

http://www.harrowalsh.de/Elektronik/APPLEBOX/slotrepeater.htm

so you can download the PDF-File from there and view the document.

As explained in earlier posting i suggest this to be the preliminary publishing that lead up to the development of the Mountain Expansion Box....

the reason to post this also here within the apple I topics, is that this board has another remarkable point:
with few modifications this PCB can be modified also for use with the apple 1 and that would permit the development
of apple 1 addon cards standing besides away from the apple 1 and with perfect access to developed own PCBs.....

happy downloading and happy building...

speedyG

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....