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504K MEMEX


Ksarul
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Here's one for fun. I've been playing with the MEMEX board a bit over the last few weeks (inspired by the repair work @InsaneMultitasker was doing to repair one that was on his bench). I realized that there are a lot less memory cards out there to expand the memory of a Geneve than there are people who could actually use one. With some additional help from @9640News, I was able to suss out the final components and layout traces needed to actually build some of the 504K boards. These boards will work fine as 2M boards for those who have an existing GENMOD, but so far as I know, no one has taken the time to thoroughly work out the operation of the two PALs on the GENMOD board (MAME has them emulated, IIRC, but I don't know if it decodes all of what they were supposed to do). I added spots to solder (or socket) the SIP resistors needed for use in GENMOD zero wait state mode and I added a switch to allow users to turn zero wait state mode on or off on the card without having to bend out pin 12 of U6. Some MEMEX boards had this switch mod using a pair of flying wires (one of which was connected to the aforementioned pin 12), now the mod is part of the layout and zero wait state can be activated if needed. Do note that zero wait state modes only work with GENMOD though, so it will be in the off position for most users. The board will be ready to expand if we decode the necessary GALs though, so I figured baking the capability in might be a good idea.

 

Now I have to do a schematic to match the original boards and the updated ones mentioned here--and go through my entire layout and validate the connections a few times to make sure everything is where it belongs. I've already caught a few miswired pins. . .and I want to make sure there aren't any more of them before I commit to making a prototype. Some of the chips used were mighty specific--right down to a manufacturer or two that met the original specifications. Not all of them are still in business either, so I'll have to do some experimentation to see what currently available chips can substitute for the ones that are too uncommon to use now. I was able to get exactly two of one of the Philips chips I needed, which was enough for one board. Lots of places manufacture the chip--but back then, apparently only the Philips chip met Don's reliability specs. Hopefully, it was just the first brand of chip that met his spec and he stopped looking there, as opposed to being the only brand that met it. Luckily, I've been able to find enough NOS chips of the other brand-specific chips to build at least 10 of the boards (and for some of them, I have enough to build more than 50). It will be an interesting experiment. . .

MEMEX-II-ComponentView.png

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This is cloning a board, with a dynamic RAM refresh controller built from jelly beans…

 

DRAM made economic sense then. 74F maybe, until the 90s.

 

Still, kudos for bringing it back onto the market, so someone could buy it. 
 

But… making a half-meg memory board would take 4xTC551001-70 static RAMs, DIP-32 are common.   Plus buffers and the address decoders. Fewer chips than SAMS. 

 

 (This is the BOM of the working 512K card in Geneve2020. Works OK at 3MHz.) 

 

 

 

 


 

 

 

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  • 2 months later...
14 hours ago, SkyPilot said:

MEMEX looks interesting, but with so many people having fully expanded standard TI systems, isn't time for a new Geneve board even if it's FPGA based?

This one is primarily to fill the need of the existing Geneve owners, as compatible memory expansion boards for it are exceedingly difficult to find--and generally for an unreasonable cost. These boards will resolve both parts of that problem.

 

As to new Geneve boards, that is a project @FarmerPotato has been working on for quite some time. It is not a trivial task, especially with the limited knowledge available on the inner workings of the gate array.

 

When my memory chips arrive, I will be able to finish the build of my test board, as I already had all of the other necessary parts on hand. It took me a while to find a source of compatible chips for a reasonable price, which is always the most complicated part of anything using old-school parts. I do that on purpose too, as just about anyone who knows how to solder can build one of my boards, which would not be the case if I switched over to surface mount components.

 

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  • 2 weeks later...
  • 2 months later...

I have the next two weeks away from work, so I should get some hobby time again--and finishing several MEMEX boards for test is on that list, Dan. I haven't been able to touch my soldering iron since early November. . .it has been that crazy at work and at home. Too many tasks and too little time, LOL.

 

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4 hours ago, Ksarul said:

Too many tasks and too little time, LOL.

 

I get it!!!

Went in to the kitchen as I was standing there an overhead light went out, opened the cabinet and a shelf full of food started to dump on my head - two plastic clips used to hold the shelf up gave way.

 

As I'm putting the food from the shelf to the counter, my wife asks if I can ultrasonically clean some jewelry she wants to give as Christmas gifts, o'yea and the cat chewed the cable on the iron in half, can I get that fixed because she needs to iron for her work holiday lunch. That's just home, then there is work.... 😃 Thank you for the update!

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  • 1 month later...

I have finally gotten the time I needed to finish populating test board #0001. I will be sending it off for testing soon, as my Geneve isn't accessible at the moment. . .note, the empty sockets are for the additional RAM that can be used by folks with GENMOD installed on their Geneve.

MEMEX-0001.JPG

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3 hours ago, Ksarul said:

have finally gotten the time I needed to finish populating test board #0001

Very nice looking test board!!  Looks great populated with chips and sockets.  Best of luck for test card #0001.

 

I was staring at a few of my cards this past weekend, and I wondered if there would be any merit to adding a few through holes (one on each side of the regulator?) that could be used to strap down a DC-to-DC converter.  Glue and other adhesives can be quite messy and leaving the converters 'loose' can result in them getting easily twisted or torn from the board.  A second set of through holes for an extra capacitor on the input and/or output might also be beneficial.  Anyway, just a few random thoughts. 

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37 minutes ago, InsaneMultitasker said:

Very nice looking test board!!  Looks great populated with chips and sockets.  Best of luck for test card #0001.

 

I was staring at a few of my cards this past weekend, and I wondered if there would be any merit to adding a few through holes (one on each side of the regulator?) that could be used to strap down a DC-to-DC converter.  Glue and other adhesives can be quite messy and leaving the converters 'loose' can result in them getting easily twisted or torn from the board.  A second set of through holes for an extra capacitor on the input and/or output might also be beneficial.  Anyway, just a few random thoughts. 

I'll add the extra holes for the DC-to-DC converter, as that seems to be a popular board mod (and I may just add that to any board with enough room to do so as a forward-looking mod). On the caps, that 100uF cap is already on the output side along with the .1uF connected there. The other .1uF is alone on the input side. There might be enough room to slide a larger cap into the area above it without eliminating the possibility of the tie-down holes for the converter. If I ever need to make a second run of these, I will definitely make those changes.

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13 hours ago, Ksarul said:

I have finally gotten the time I needed to finish populating test board #0001. I will be sending it off for testing soon, as my Geneve isn't accessible at the moment. . .note, the empty sockets are for the additional RAM that can be used by folks with GENMOD installed on their Geneve.

MEMEX-0001.JPG

Looks like good progress!!!

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