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Old 15 June 2016, 19:52   #1
avinitlarge
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A500 plus 2MB mod, what am I doing wrong?

Im trying to add a extra chips to a 512k board to bring it up to 2mb. I bought 8 new chips, fitted 4 and it shows the extra ram (512k), I piggy backed the other 4, Still 512k. Pin 17 the left 2 chips goes to Pin 11 of U32, Pin 17 the right 2 chips goes to Pin 5 of U32. I would have thought that this would have worked or have I missed something?



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Old 15 June 2016, 20:51   #2
Zetr0
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@avinitlarge

I like your style sir - you will need to ensure that the CE (Chip Enable) pin of top 512k bank is wired to the correct RAS line - do you have the schematics for the A500/A500+ if so it has all you need in there =)
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Old 15 June 2016, 21:04   #3
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You're almost there. The crucial problem here is, that now both sets of RAM chips are still connected to the same memory bank.

There are CAS and RAS signals going to the DRAM chips.

In the original setup, the RAS signals are all shared among the chips and the two separate blocks of RAM have individual CAS signals.

In an A501+, we have two RAS signals and two CAS signals.

The A500's trapdoor connector has the RAS signals on pins 37 and 38. Check RAM chip's pin 4 on the expansion, and see whether they're connected to pin 37 or 38 on the trapdoor connector. Seems by the schematics that it should be pin 38 (XRAS1)

What you need to do is bend up pin 4 of the top chips and connect them all together, then connect that RAS bank to the unused RAS pin on the RAM expansion connector Should be pin 37 (XRAS0).

Double check the schematics for the A500 rev 6, A501 rev 6 and the A500+ rev 8 and A501+ to be sure.

Good luck!

Haha, this actually makes me want to make some ghetto A501+s too. :-)

Last edited by Jope; 15 June 2016 at 21:13.
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Old 15 June 2016, 21:57   #4
avinitlarge
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Ha ah, Yes, just seen it on the schematic after you said, Silly me, I studied it for ages too.
Works now Thank you
Just needs to be tidied up a little and the clock parts pulling out

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Old 15 June 2016, 23:34   #5
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Old 16 June 2016, 01:43   #6
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Can you do this with any other Amiga..?
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Old 16 June 2016, 06:23   #7
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Good show! This only works for the A500 plus.
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Old 16 June 2016, 09:40   #8
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Or Rev. 8 A500s if you retrofit the missing logic to upgrade it to a full A500+ board.
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Old 16 June 2016, 15:47   #9
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This setup definitely has a kinky feel to it.
You should probably shorten the long yellow wire though or tape it on the board so it does not float, it seems like it would be very easy to tear it apart by mistake along with a track or two.

The blue gunk on the top traces does tease my curiosity: w,as this due to a battery leak?
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Old 16 June 2016, 15:59   #10
avinitlarge
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I've shortened the wire and tidied it all up. Took the clock components out and fitted them to another board.
The blue stuff is nail varnish, previous battery leak as you guessed ��
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Old 16 June 2016, 18:30   #11
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Quote:
Originally Posted by Jope View Post
Good show! This only works for the A500 plus.
I thought you could piggyback videocard RAM chips on an A600 for 2MB Chip, too.
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Old 16 June 2016, 18:56   #12
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Quote:
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I thought you could piggyback videocard RAM chips on an A600 for 2MB Chip, too.
Sure, but those go on the motherboard. :-)
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Old 17 June 2016, 20:40   #13
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Can you do this on the A1200 to give it 4 meg..or is it just the A500 and A600..?

Sent from my GT-I9505 using Tapatalk
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Old 17 June 2016, 20:42   #14
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Can you do this on the A1200 to give it 4 meg..or is it just the A500 and A600..?
The modification you see is for Chip RAM. You cant't do that with Fast RAM.
And no you can't have 4MB of Chip RAM.
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Old 18 June 2016, 08:39   #15
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The modification you see is for Chip RAM. You cant't do that with Fast RAM.
And no you can't have 4MB of Chip RAM.
Actually, with a very simple hardware extension consisting only of a few logic gates it would be relatively simple to implement bank swapping for Chip RAM. This way one could use way more than the original 2MB to store graphics and the likes (but one could not access it all at the same time, only 2MB of this memory would be visible at any moment).

Obviously, no existing software would be able to take advantage of it so that would be useful only for a modern developer but who knows? I definitely like the idea.
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Old 18 June 2016, 09:58   #16
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It's easier than that. Just move the chipram jumper into the 8MB position
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