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Old 24 May 2019, 20:57   #1
Kai
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Lightbulb 2MB CHIP RAM replacement PCB's for A500+ and upcoming A500++ boards

So something that irritates me about the retro scene these days is the scarcity of some parts, namely 256 x 4 DRAM's because the only source seems to be dodgy sellers on eBay or Aliexpress, almost all from China - and you have no idea if it'll work at all, and 50% of the time they arrive in a plastic bag with broken pins in my experience.

So, to mitigate this - time to make a replacement board that fits in the place of the original RAM chips.

The first board i spun up was just to adapt some DRAM chips i bought off eBay for £10 - newer TSOPII 44/40 EDO ones, as a test:

https://github.com/kr239/A500-DRAM-Board



That board should give 2MB of CHIP RAM in an A500+, so no more trapdoor expansion possible (not that you'd need any) - use a FastRAM board for more.

Second board though, I decided to eliminate the whole sourcing issue, by going for NEW parts that are available from mouser and digikey - so, using SRAM instead of DRAM, i present this:

https://github.com/kr239/A500-SRAM-Board



Again - these are designed for the A500+, however, the schematics should be adaptable to the regular A500.

However, that only gives 1MB CHIP RAM. And wanting to flex my KiCad skills, I went for broke, and with the help of Niklas over on the RMC Discord, and some pointers from ElectronAsh - I present this:

https://github.com/kr239/A500-SRAM-B...chematics/v1.2



The pinnacle! 2MB chip ram, uses BDRA9 from U34 to provide the needed A9 signal. It uses all two 8Mbit Cypress 5v SRAM chips and discrete 74ABT logic - no faffing with CPLDS, no programming, just need a decent soldering iron (I use a £40 Yihua 937D), a K-Tip on the iron, a boatload of flux and some decent solder!

Take these for what they're intended though - rev0, beta, experiments - i hope they're worth something to the community as a whole, and if you want to fork the project, change stuff, submit suggestions etc - go nuts for coconuts, they're released under GNU GPL v2
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Old 24 May 2019, 21:43   #2
Mick
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Very nice mate I love to see things like this, such ingenuity. If an A500 PCB ever gets released open source it would be ideal to have something like this integrated as well.

Last edited by Mick; 24 May 2019 at 21:51.
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Old 25 May 2019, 11:44   #3
Kai
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v1.3 of the 2MB SRAM board released - realised that feeding A18 with /BRAS0 is probably not enough - so a single 74AHCT1G79 is added, feeding /BRAS0 and /CAS in, to get A18 out.

https://github.com/kr239/A500-SRAM-B...chematics/v1.3


Last edited by Kai; 27 May 2019 at 01:38.
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Old 25 May 2019, 15:14   #4
talybont
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Sweet, where can I buy one?
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Old 25 May 2019, 16:24   #5
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Wow great idea and nice implementations Kai!
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Old 25 May 2019, 20:06   #6
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I would love 2mb chip on my rev 6a....never going to happen though.
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Old 25 May 2019, 20:18   #7
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Is the 8375 being a different pinout to the 8372 the only obstacle to putting it into a 500?

I'm wondering if a card soldered in place of the PLCC agnus socket could provide whatever was missing? I'm assuming not if it hasn't already been done - seems like an obvious upgrade if it were possible.

John
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Old 25 May 2019, 20:40   #8
SukkoPera
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It has been done, it's called "MegaChip". Icomp made their version, called ACE2.

But don't worry about those, something new is coming soon...
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Old 25 May 2019, 20:43   #9
Kai
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Quote:
Originally Posted by talybont View Post
Sweet, where can I buy one?
Well, for the 2MB SRAM, i'm going to generate the v1.3 gerbers and upload them to GitHub, but i'll do a board run of 10 or so, to get started, for those not wanting to deal with board houses
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Old 25 May 2019, 21:21   #10
Lord Aga
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Wow, you really came in guns blazing
So glad we have you here!
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Old 25 May 2019, 23:23   #11
Kai
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Okay so as the 2MB SRAM board is 4-layer - each board looks to be around £11 +P&P if i'm to break even - would that sound fair?
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Old 25 May 2019, 23:47   #12
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Interesting thread. I may ask a silly question showing I don’t know the A500+ very well but how do you connect your board to the A500+??
And is there another gain with this replacement? Fastest memory access?
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Old 25 May 2019, 23:51   #13
Kai
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Its just a like for like replacement - instead of the DRAMS, you plug it into the sockets the original DRAM chips would normally fit in U34. It might be faster, but the intention here is to avoid old out of production parts.
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Old 26 May 2019, 00:14   #14
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Yeah potentially in the future if a 500+ PCB is made opensource then people will be able to print off their own boards, and with mods like this the only real requirements will be the original chipsets.

Stuff like this keeps the Amiga flame going.
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Old 26 May 2019, 10:46   #15
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Quote:
Originally Posted by project23 View Post
Is the 8375 being a different pinout to the 8372 the only obstacle to putting it into a 500?

I'm wondering if a card soldered in place of the PLCC agnus socket could provide whatever was missing? I'm assuming not if it hasn't already been done - seems like an obvious upgrade if it were possible.

John

Would that bee like a mini mega chip....or ace2...?
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Old 26 May 2019, 12:53   #16
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Quote:
Originally Posted by project23 View Post
Is the 8375 being a different pinout to the 8372 the only obstacle to putting it into a 500?

I'm wondering if a card soldered in place of the PLCC agnus socket could provide whatever was missing? I'm assuming not if it hasn't already been done - seems like an obvious upgrade if it were possible.
This board is exactly what you're describing:
http://www.amibay.com/showthread.php...project-Poland)

Quote:
Originally Posted by Kai View Post
Okay so as the 2MB SRAM board is 4-layer - each board looks to be around £11 +P&P if i'm to break even - would that sound fair?
Doesn't sound too bad to me. I assume that all the components are readily available and thus also relatively affordable? If a complete DIY kit could be sold for £25-30, then I think many people could potentially be interested.
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Old 26 May 2019, 14:42   #17
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This board is exactly what you're describing:
http://www.amibay.com/showthread.php...project-Poland)
We have updated our design, it is Genlock compatible now
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Old 26 May 2019, 17:11   #18
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@Kai

Good start to EAB life- I like your project and contribution.

We also recently made a low cost trapdoor expansion for the Amiga 500 Plus here which upgrades to maximum 2MB Chip:

http://amigakit.amiga.store/product_...roducts_id=502

We also have the A500 512KB expansion based on the same board:

http://amigakit.amiga.store/product_...roducts_id=767

Our aim was to make a very low cost memory expansion that is readily available to keep our hobby going. We were concerned with quality/price issues of products from Ebay sellers and part time "resellers" in the Amiga world.

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Old 26 May 2019, 17:53   #19
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As I understand it, this is more of a replacement of the onboard memory, in case parts become unavailable.
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Old 26 May 2019, 23:18   #20
Kai
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To be fair - i've been lurking here for about a decade or so - not my first post

Yes, the idea was not to do the n-th million trapdoor expansion, but for the CHIP ram on the motherboard itself - which does fail, and as these machines get older, will fail more often, and with no source of new parts, may result in a stack of dead machines, which is obviously no good.

As for the total cost of the 2MB SRAM board - probably closer to £40, as each SRAM chip is £9 from mouser, and you need two, plus the SIL headers, PCB and the other SMT parts.

I'll do a full BOM later, but it's my birthday tomorrow so it'll have to wait
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