24 May 2019, 20:57 | #1 |
Registered User
Join Date: Apr 2011
Location: Brighton, UK
Age: 42
Posts: 114
|
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 |
24 May 2019, 21:43 | #2 |
Registered User
Join Date: Jan 2004
Location: Yorkshire
Posts: 744
|
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. |
25 May 2019, 11:44 | #3 |
Registered User
Join Date: Apr 2011
Location: Brighton, UK
Age: 42
Posts: 114
|
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. |
25 May 2019, 15:14 | #4 |
Registered User
Join Date: Oct 2016
Location: Norway
Posts: 174
|
Sweet, where can I buy one?
|
25 May 2019, 16:24 | #5 |
Registered User
Join Date: Sep 2013
Location: Beeston, Nottinghamshire, UK
Posts: 240
|
Wow great idea and nice implementations Kai!
|
25 May 2019, 20:06 | #6 |
Registered User
Join Date: Sep 2018
Location: Nottingham
Posts: 340
|
I would love 2mb chip on my rev 6a....never going to happen though.
|
25 May 2019, 20:18 | #7 |
Used Register
Join Date: Jul 2018
Location: Liverpool
Age: 41
Posts: 437
|
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 |
25 May 2019, 20:40 | #8 |
Mighty Pirate
Join Date: Dec 2017
Location: On the borderline
Age: 44
Posts: 177
|
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... |
25 May 2019, 20:43 | #9 |
Registered User
Join Date: Apr 2011
Location: Brighton, UK
Age: 42
Posts: 114
|
|
25 May 2019, 21:21 | #10 |
MI clan prevails
Join Date: Jul 2010
Location: Belgrade, Serbia
Posts: 1,443
|
Wow, you really came in guns blazing
So glad we have you here! |
25 May 2019, 23:23 | #11 |
Registered User
Join Date: Apr 2011
Location: Brighton, UK
Age: 42
Posts: 114
|
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?
|
25 May 2019, 23:47 | #12 |
French in Australia
|
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? |
25 May 2019, 23:51 | #13 |
Registered User
Join Date: Apr 2011
Location: Brighton, UK
Age: 42
Posts: 114
|
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.
|
26 May 2019, 00:14 | #14 |
Used Register
Join Date: Jul 2018
Location: Liverpool
Age: 41
Posts: 437
|
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. |
26 May 2019, 10:46 | #15 | |
Registered User
Join Date: Sep 2018
Location: Nottingham
Posts: 340
|
Quote:
Would that bee like a mini mega chip....or ace2...? |
|
26 May 2019, 12:53 | #16 | |
Unregistered User
Join Date: Sep 2012
Location: Copenhagen / DK
Age: 44
Posts: 4,190
|
Quote:
http://www.amibay.com/showthread.php...project-Poland) 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. |
|
26 May 2019, 14:42 | #17 | |
Retro maniac
Join Date: Feb 2017
Location: near Munich / Germany
Posts: 486
|
Quote:
|
|
26 May 2019, 17:11 | #18 |
Registered User
Join Date: Aug 2004
Location: www.amigakit.com
Posts: 2,051
|
@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. |
26 May 2019, 17:53 | #19 |
Used Register
Join Date: Jul 2018
Location: Liverpool
Age: 41
Posts: 437
|
As I understand it, this is more of a replacement of the onboard memory, in case parts become unavailable.
|
26 May 2019, 23:18 | #20 |
Registered User
Join Date: Apr 2011
Location: Brighton, UK
Age: 42
Posts: 114
|
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 |
Currently Active Users Viewing This Thread: 1 (0 members and 1 guests) | |
Thread Tools | |
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
2MB A500/A500+ trapdoor RAM | cmorley | Hardware mods | 19 | 16 October 2018 15:08 |
A500 + A3000 Agnus 2MB chip RAM via trapdoor | cmorley | Hardware mods | 13 | 04 September 2018 01:24 |
WANTED: A500 Plus with 2mb chip from a UK seller. | BarryB | MarketPlace | 1 | 21 January 2014 16:26 |
2mb chip plus other upgrades on my A500 Rev 6A? | BarryB | support.Hardware | 12 | 31 December 2013 00:42 |
For Sale: 2mb RAM Expansion for A500/A500+ | PowerPie5000 | MarketPlace | 1 | 22 November 2010 01:10 |
|
|