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techknight

Designing a portable RAM card

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...(send 2-self addressed mailed planes to yourself and open one up to see it and keep the other sealed until a "judge needs to see it," and keep both together)...

 

I can say, with true and absolute certainty, that this hasn't been accepted as evidence in patent court for at least 40 years, if not longer.

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Here is my CPLD code in a nutshell: 

 

Chip Select/Bank Select Signals.
A20, A21, A22, A23 Address line decode. The chip is hooked up between A0 and A18, or A1 to A19 if you look at the portable bus. 

 

Base RAM:
0000 = No chip selected, all lines off = 0

 

Extended RAM:
0001 - Chip 1 Select ON, Lower Bank ON = 1
0010 = Chip 1 Select ON, Upper Bank ON = 2

 

0011 = Chip 2 Select ON, Lower Bank ON = 3

0100 = Chip 2 Select ON, Upper Bank ON = 4

 

0101 = Chip 3 Select ON, Lower Bank ON = 5

0110 = Chip 3 Select ON, Upper Bank ON = 6

 

0111 = Chip 4 Select ON, Lower Bank ON = 7
1000 = Chip 4 Select ON, Upper Bank ON = 8

 

When I say Upper/Lower bank, I mean just toggling A20 high or low. Since the macintosh portable has base RAM at 512K-Word, or 1MB, I decided to break the expansion down into 1 meg chunks as well. Since the toshiba chips are 1M-Word, or 2Mbytes, just deciding where A20 is determines if I am working with the first 512kword, or the second 512kword. 

Edited by techknight

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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?

Edited by NJRoadfan

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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. 

Edited by techknight

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Why would I use an i8042? thats a keyboard controller. 

Ask IBM, seemed to work for them :p  (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.

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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?

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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

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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

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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. 

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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).

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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

 

post-2150-0-17984100-1413909084_thumb.jpg

 

post-2150-0-47469500-1413909089_thumb.jpg

 

post-2150-0-20909500-1413909095_thumb.jpg

 

 

Matt

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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

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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.

Edited by Paralel

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