A500 trapdoor memory: free design
 

Hi EAB,

A couple of months ago I picked up a cheap A500 from a local ad. It's the first one I've had since the 1990s, so it's quite nostalgic to look at, handle, power up.

When I opened up the trapdoor of this unit, I fully expected to find a battery damaged A501. But, shock, empty! Nothing in there.

So maybe I can find some suitable RAMs from some of my old PC SIMM sticks?
These look pretty good. A quick look at the data sheet shows they are 16-bits wide, perfect for a A500.
http://pibus.info/amiga/a501/pics/sm...7_191044ss.jpg

Break out the heat gun / paint stripper:

http://pibus.info/amiga/a501/pics/sm...7_191200ss.jpg

It comes right off, easy :)

http://pibus.info/amiga/a501/pics/sm...7_191237ss.jpg

Fast forward some playing with a PCB CAD program.
This is all you need:

http://pibus.info/amiga/a501/pics/sm...3_132751ss.jpg

http://pibus.info/amiga/a501/pics/IM...03_134446s.jpg

Done! Ready for a fitment test

http://pibus.info/amiga/a501/pics/sm...3_135153ss.jpg
Damn! Look how old my A500 is, very early serial number.

http://pibus.info/amiga/a501/pics/sm...3_135723ss.jpg

It fits very well. Ready for the smoke test:

http://pibus.info/amiga/a501/pics/sm...3_135818ss.jpg

It works! Never doubted it. Fun to recycle some old scrap RAM chips.

http://pibus.info/amiga/a501/pics/sm...3_140134ss.jpg

Now I have a few spare boards. If somebody wants one for personal use (without parts) I'm happy to send it via letter post for AU$3 in Australia or AU$5 world wide. A small donation above that is welcome but not necessary.
The memory chips are easy to find on old EDO memory sticks, and the capacitor I can include if you don't have it.

EDIT: Demand is too high, so I bumped the price to $5, I need to buy envelopes :)

EDIT: More RAM details: The memory you are searching for is 72-pin, so try that as a search term on Ebay. Technically, the chips are called SOJ42, 1M x 16. You can identify them visually, they are the bigger/longer chips. If you count 21 pins on one side (42 total), it's a winner. They are pretty cheap and abundant, I've seen some advertised for $2 for a few SIMMs that gets you about 8 chips!

So what are those pads for another two memory chips for? Good question! They are untested, but in theory it's for:
- 1.0 MB in A500+ plus to give you a full 2MB chip RAM.
- 1.5 MB in a normal A500 with a gary adapter.

See post #60 about buying a board.

//Pete.

PCB design files
 

Rev B6: Eagle schematic and board: http://pibus.info/amiga/a501/miniA501b6.zip
Gerber files (for production) http://pibus.info/amiga/a501/miniA501b6_gerbers.zip
Changes:
- Added a jumper for RTC enable.
- Added a footprint for SMD RTC chip, so you can populate either one.
- Made jumper drills larger (0.8->0.9mm) per Mathesar's complaint.
- Gary adapter by Mathesar with flipflops for BLIT.
- Corrected JP4 error on gary adapter.
- Improved decoupling caps positioning.

Rev C6: Gerber files (for production) http://pibus.info/amiga/a501/miniA50...40_gerbers.zip
Changes:
- For different memory: SOJ40. Don't consider this board "better" or "newer", it's just for different memory types. Apparently you can find some SOJ40 memory on old AGP video cards.

If you've never had PCBs made, it's dead easy:
- Visit your favourite cheap Chinese board house: Elecrow, iteadstudio, seeedstudio etc e.g. https://www.elecrow.com/pcb-manufacturing.html
- Upload the gerbers ZIP file.
- Select 100x100mm, HASL and 1.2mm thickness.
Done!

Parts List:
- For the main board: http://eab.abime.net/showpost.php?p=...9&postcount=62
- For the real-time-clock: http://eab.abime.net/showpost.php?p=...9&postcount=14

Looks great - I would love to have one. I should have the components available and I would try to make a 1 MB one for a 500+. :)

Wow, that pcb makes my 512k slow ram look.. terrible. I think it was made locally in 1987 or something.. and sure looks the part (i.e. ugly). I would love to have one of the boards and the capacitor. Finding the EDO chips won't be easy over here but I'll try. (Not that I have any idea how to solder the ram chips onto the board, but we do what we must because we can..) Please PM me your paypal details so I can send you the postage cost :)

Out of curiosity, what happens if I make a 1.5MB card and add it to a Rev6 A500?

It will work as a 512k card. You need to have additional logic that selects the bank using that jumper pad next to the RTC battery.

Oh I see.. So no need to bother with more than a single 512k chip :D

Sounds good, I wonder if I got it right for the 500+. Maybe someone with more knowledge of it can enlighten me. As I understand, the 500+ trapdoor header has both XRAS0 and XRAS1 and both are usable (not shared with the motherboard RAM). If that's the case, it should work fine for 1.0MB (two chips). But on a normal A500, XRAS0 is not usable, since the motherboard needs it.

More than 512k would be a lot more difficult, but I made provision for it on the board. I designed the Gary adapter too, but didn't get any made. I'm pretty sure the logic of the adapter would be fine for a 0.5chip A500, but less sure about the 1.0chip A500. You're probably better off getting some fast RAM.

It is relatively simple to convert this to an A500+ 1MB expansion, but you will need to solder a few jump wires to hook up the RAS + CAS signals to the other mem bank and to the motherboard connector. See elsewhere in this subforum for information on how to do this, someone converted a 512k A500 expansion to 1M some months ago.

Ah well, here you go:

http://eab.abime.net/showthread.php?t=83121

It doesn't spoon feed you, but it has the info you need.

Congrats PeteAU on a nice design, I like how you made it configurable. Did you check http://eab.abime.net/showthread.php?t=49965 for info on the gary adapter logic? I'm at work, so I didn't take a look at how you did yours yet. :-D

Could any of you fine gents let me know what kind of SIMM I should hunt for chips to solder onto this? Do I want 512Kx16-bit? Would 1Mx16-bit work?

Yeah, I just looked at the 500+ schematic, I think it won't need any wires for 1MB, just the right jumper settings.

Jope, did you already make a similar board?

For the Gary adaptor, I tried to cater for two scenarios, the first one is easy, the second one more challenging with just NAND gates:

Scenario#1: 0.5chip 1.5slow - only 2 wires from Gary to trapdoor's 74139.
Chip 000000 - 07FFFF (internal, RAS0)
Slow C00000 - C7FFFF (trapdoor, RAS1, A20:A19 is 00)
Slow C80000 - CFFFFF (trapdoor, RAS1, A20:A19 is 01)
Slow D00000 - D7FFFF (trapdoor, RAS1, A20:A19 is 10)

Scenario#2: 1.0chip 0.5/1.0 slow - this requires an extra wire to JP2 (Agnus A19) to force RAS1 at $C0. I have chosen to use A23 instead.
Chip 000000 - 07FFFF (internal, RAS0)
Chip 080000 - 0FFFFF (trapdoor, RAS1, A23:A19 is 01)
Slow C00000 - C7FFFF (trapdoor, RAS faulty, A23:A19 is 10)
Slow C80000 - CFFFFF (trapdoor, RAS1, A23:A19 is 11)

My suspicion is, in scenario #2, the A500 will normally assert a RAS0 for $C0, which needs to be overridden. But I'm not sure, I can't see Agnus's guts/logic. Does Agnus use A19 (pin 59) to decide which RAS?

Besides that, the adaptor also changes Gary's (or actually Agnus's) RGAEN and RAMEN signals in this way: http://pibus.info/amiga/a501/1m5_gary2.png

I think I achieved _all_ of the above with just two 74F00s. Maybe.

Thanks for the links Jope, I'll have a read of those threads later.

Anyway... I tried to buy a A500+ motherboard on ebay.co.uk a few times, but you Brits don't want to ship them to AU for some reason. Are you hording them?

If there are any left, I'd like a board or two.

Looks great, been on the lookout for an expansion for my A500 plus. Sent a message.

hi have you got a revision B schematic for the 1.5mb version?

I populated the real time clock and with a bit of minimal testing, it appears to work :) The clock is completely optional of course, you don't need to bother with it if you don't want one.

The parts list for RTC:
U5 or U6: RTC62421/RTC72421/RTC62423/RTC72423
C10 or R7: 47K-100K 0805 resistor (yes a resistor!)
C12: 100nF 0805 capacitor
C13 or C19: >=2.2μF >=10V 0805 or 1206 tantalum
R1, R2, R3: 10K 0805 resistor
R4, R5: 220-470 0805 resistor
D1: BAT721C or BAT54C diode
Battery Holder: Some cheap ebay one.

I don't really want to post the whole thing, in case it's still wrong and people start making them. The Rev B main board mainboard is unchanged, only the Gary adapter is tacked onto the side, so you'd have to cut it out with a saw, but save a few bucks by getting both produced in one hit.

Here's a alpha/beta/notready. I'm sure it's fine for 0.5meg chip ram system, but you tell me if it'd work on a 1meg.

Sorry for quoting myself, but I'm having trouble finding 512Kx16 chips for this board, would it work fine as a 512K slow ram upgrade to an A500 with a 1MBx16 chip soldered on? (EDIT: Nevermind saw the updated OP :) 1Mx16 it is)

EDIT2: Ended up buying some Alliance AS4C1M16E5-60JC from Aliexpress.. 1Mx16 5V DRAM EDO chips.. Hope I'm doing it right guys..

Thanks, sent money through paypal.

I know its un-tested, but in theory what jumpers do I need to join for 1mb chip expansion on an A500+? I guess i'll try it as 512kb first, one step at a time.

Thanks again

This should work. EDIT: Added note about ENABLE jumper. This image is only for the A500+ "plus", not a standard A500.

From the A500+ service manual (talking about the 8375)
"In the 1 meg configuration, A19 is still used to determine the RAS line to be asserted. If A19 is low RAS0* is active and if high RAS1* is active"
I suspect the predecessors of 8375 work the same way.

In (unmodified) R5 and R6A A500s CPU's A23 is routed to Agnus' A19 via JP2. In (unmodified) R8a A500+s CPU's A19 is routed to Agnus' A19 via JP2.

can you tell me where i may find this memory test tool/module?
http://pibus.info/amiga/a501/pics/sm...3_135818ss.jpg