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Designing a portable RAM card

Posted by: bigmessowires on 2014-10-17 09:01:40 I just want to say this is very cool. I was going to offer some suggestions, but everything already looks great to me.

Posted by: techknight on 2014-10-17 09:35:55 Your input would be appreciated. This is my first ever CPLD project.

Posted by: NJRoadfan on 2014-10-17 11:42:55 You could always use a i8042 to control the A20 line. 😉 Is there any benefit to mapping the full 9MB? Isn't the 68000 limited to addressing 8MB max with the typical Macintosh memory mapping?

Posted by: techknight on 2014-10-17 16:39:11 the portable is an oddball. All RAM is mapped up until you run into ROM. Rom doesnt begin at 40 0000 as on the SE/Plus,etc. it begins at A0 0000 or 90 0000. Something like that.  Why would I use an i8042? thats a keyboard controller.

Posted by: NJRoadfan on 2014-10-18 09:18:04 Why would I use an i8042? thats a keyboard controller.  Ask IBM, seemed to work for them 😛   (It was a joke) That means the Portable can do something no other 24-bit addressing Mac can do, access more than 8MB of RAM. Neat.

Posted by: Paralel on 2014-10-18 16:00:24 You think Apple meant for it to address that much memory, or it's just a fluke/oversight/they didn't care that it could be made to address that much?

Posted by: techknight on 2014-10-18 16:11:59 the GLU allows addressing up to 5MB. a PDS card can address it all up to 9MB.

Posted by: CC_333 on 2014-10-19 12:58:41 But, if I understand correctly, your card will allow more RAM in the RAM slot than the 5 MB the GLU limits it to, correct? c

Posted by: techknight on 2014-10-19 14:06:12 i hope so.

Posted by: CC_333 on 2014-10-19 14:49:54 By way of some special pass through connector in the ROM slot that breaks out the needed extra address lines? How will that be done, exactly? Or did you solve that by doing all the encoding/decoding/etc. on the card? I will definitely be interested in one when it's ready. ...And a new back lit ribbon cable for the LCD (or a decent repair of the current one). c

Posted by: techknight on 2014-10-19 18:06:14 Youll see.... the address decoding and chip select is all done on the CPLD thats on the RAM card.  I carried a tap for DTACK, and address strobe/AS. that way, I can just bring those 2 wires over to the adjacent slot to get full decode.

Posted by: trag on 2014-10-20 12:28:49 Eagerly awaiting the test results -- and I don't even have a Portable.  I just have pent up project pressure (PUPP).  I had this project in mind for years.   It's wonderful to see it get done even if I'm not involved (other than pulling a box out of the attic).

Posted by: techknight on 2014-10-20 16:49:12 Well, i am just waiting on PCBs.  Plus waiting to refill my expendable budget to get parts from digikey.

Posted by: aplmak on 2014-10-21 09:31:53 Techknight....  I don't know if this helps but I just found an 8MB PDS RAM card in my collection!!!!!!!! This gives you the full 8MB.. It's plugged into a 5126.. I don't know if there is more if you plug it in a 5120 but I will try... Look at the pics... It says Sunland Micro Systems Matt

Posted by: aplmak on 2014-10-21 10:58:23 Same total ram on 5126 and 5120... But curious... Could I put in the PDS ram and a regular RAM card?? Or will it just see the max of 8MB?? Matt

Posted by: Paralel on 2014-10-21 11:46:19 It's one or the other, if I'm not mistaken. The max is 9 MB, not 8, according to the legwork that TN has done.

Posted by: techknight on 2014-10-21 17:02:06 if you look, that is an 8MB card, but only 7MB is populated.

Posted by: aplmak on 2014-10-21 19:30:27 Yeah I had forgotten that.. duhh.. 1mb on board.. 🙂 Could make it a 9MB I suppose.. but will only work on 5120 like that no?? Did u want me to send it to you so you can take a peek at it??

Posted by: Paralel on 2014-10-22 10:42:55 It does make me wonder why they didn't popular that last set of pads. Maybe they just thought 9 MB was a strange number?

Posted by: techknight on 2014-10-22 16:30:47 Well I did screw up. on one thing... The boards did ship, and they shipped DHL. Unfortunately, they uncovered some big scandal at the ShenZhen dock, which is where my package is stuck at.  Something about counterfeit money and other stuffs, causing packages to be delayed up to a month or so before it leaves the dock via DHL.  I wish i would have known this prior to ordering and shipping via DHL...

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