Amiga 500 vs 500 Plus Hardware Compatibility

[Oldsmobile_Mike]

I recently picked up a memory expansion unit (1MB) that was designed for the a500+ (Gigatron A500 Plus: http://amiga.resource.cx/exp/gigatron500plus) When plugged into the Amiga 500, it does not recognise the expansion.

Does anyone know whether this is a hardware incompatibility issue or a fault with the expansion card?

Haha, I like how people do things almost as if it's a test.  To intentionally see if they can break things, LOL.  

At a guess I'd say it's adding the memory into a different address range than where the A500 is looking.  It recognizes zero additional memory?  An easy test for this would be to plug it into an A500+ and see if it works correctly in that system.  I don't see much info on that card on the web, but as an alternative try to track down what address range the A500 uses (this is easy to find), what address range the A500+ uses (this might be a little harder to find), and what range that card uses (this will probably be the hardest bit of info to find).  Obviously you will need some overlap there for the card to be detected properly.

It's a stock A500, right?  Only 512K on the motherboard, no other expansions anywhere?  What version Agnus (probably won't make a difference, even OCS would see it as slow-fast ram, but still)...

[Tenacious]

What version Agnus (probably won't make a difference, even OCS would see it as slow-fast ram, but still)...

I've always thought that Agnus provides the refresh for CHIP and SLOW/FAST memory.  The limitation is the amount of memory the various Agnus versions will refresh, the last version (as used in the A3000, A600, and A500+) refreshing up to 2 MB.  True FAST ram (like Supra's 500RX, etc) is accessed by the CPU alone and these expansions include their own refresh circuitry.

AS a side note, my A500 Rev 5 has empty sockets for another bank of 512k on the motherboard.  If the original chips are pulled and (1M x 1? chips) are installed in all positions, one can have 2MB of chip on the motherboard.  It also requires the 2MB Agnus be installed.  I did this about 20 years ago, but, now I can't find the docs I used for the mod.

[Oldsmobile_Mike]

What I'm thinking is, if he only has an 8370 Agnus or something, that's only able to address 512K of chip memory, the best he'd be able to get out of this - if he's able to get it to work at all - is an extra 512K of slow-fast memory.  Like an A500 that hasn't had the 1MB hack.

Re: side note, the later Rev. A500's can be expanded to 1MB on the motherboard (not 2MB).  He'll need to add the chips (and capacitators, too... or was it resistors?  Instructions are on the net), and an 8372 or similar-model Agnus.  That can address 1MB.  This memory goes in the same address range as the "belly slot", so if he adds the chips to the motherboard he cannot also add a trapdoor expansion.  It's one or the other.  I'm always surprised at people who do this mod, since a trapdoor expansion is a lot easier to add, but I can't fault people for wanting to do a little hardware hacking.  ;-)

Here you go, read this thread and the links:  http://www.lemonamiga.com/forum/viewtopic.php?t=9808&sid=973d466e609d0b1e96db05a5bdc8cb14

[ElPolloDiabl]

Here is a hack to give 2 Megabyte of chip ram to an Amiga 2000. It must apply to an A500 as well.

Dave's Amiga page:
http://members.iinet.net.au/~davem2/overclock/A2000-2MB-chip.html

[Lurch]

As I said further up, 1MB expansion on a standard A500 will show as 512... I have done this a few times in the past and when my A500 plus started playing up and I switched back to a rev6 board this was also the case.

[Oldsmobile_Mike]

Here is a hack to give 2 Megabyte of chip ram to an Amiga 2000. It must apply to an A500 as well.

Dave's Amiga page:
http://members.iinet.net.au/~davem2/overclock/A2000-2MB-chip.html

Nice hack!  A500 obviously doesn't have an "upper row", different density of ram chips in most models... might still be do-able.  Interesting project, not for the faint of heart though!  

[Tenacious]

Nice hack!  A500 obviously doesn't have an "upper row", different density of ram chips in most models... might still be do-able.  Interesting project, not for the faint of heart though!  

This is very similar to the modification I referred to above.  You are right about the trap door memory, mine had to be disabled except for the RTC.  This machine is still working great with 2 MB of CHIP on the motherboard.  I will look some more for the docs I used (Some one else wrote them, I simply benefitted ).

I haven't done it since because the 1 MB CHIP ram modification is adequate for my needs and that money is better spent on true FAST ram (even more memory and better performance gains).

[AdelaideAmiga]

@Oldsmobile_Mike & Lurch
- Yes, it recognises zero additional memory.  I.e. the same memory is recognised with or without the ram card.  By the way this information is being accessed from the main workbench screen where the bytes are registered on the top centre of the screen.
- The machine is a completely stock 500, with a gotek system.  Agnus chip is 8371.
- There are no other expansions

Thanks for your help so far everyone!

Much appreciated

George

[Tenacious]

I found some notes, and, it appears that the OCS Agnus 8371 (8370 for NTSC) was introduced with the early A500s.  It would address only 512kB of CHIP and 512kB of SLOW/FAST trapdoor memory.  It would produce PAL video modes.  

There was an earlier DIP design for the A1000, 8361 for NTSC and 8367 for PAL.

When Commodore introduced the ECS Agnus (8372A, 8375-PAL, 8375-NTSC), even though these would address 1 MB of CHIP, they still chose to map the trapdoor (512kB only) memory as SLOW/FAST on all A500s (not +).  This is where the pad-jumpering mod came in to remap the trapdoor to CHIP.  IIRC, Commodore actually recommended this mod for users of the A570.

This is where my notes get confusing.  The 2 MB Agnuses (8372B for the A3000, and the confusing part, 8375 for the A500+ and A600) were introduced and all the memory (motherboard and trapdoor, if present) was mapped as CHIP only.  Motherboard memory and trapdoor memory jumped from 512kB to 1 MB each at this time for the new A500+ and A600.  

The 8375 is listed as a 1 MB and as a 2 MB Agnus.  Would it do 2 MB all along?  Did it come along significantly later than the 8372A?  Perhaps it was the first 2 MB design?

Corrections?

EDIT:  Wikipedia has this to say, though it seems to not cover a number of exceptions.  http://en.wikipedia.org/wiki/MOS_Technology_Agnus

[Oldsmobile_Mike]

Long story short, there are more versions & revisions of Agnus than I have hairs on my head.  

If it were me doing this, and I'm just saying, if it were me, and you have a Rev. 6 or similar model A500 board, I'd look for an 8372A Agnus and pop that in, then do the motherboard mod (the couple jumpers/solder pads) to change the address of the trapdoor memory to chip, then get yourself a normal 512K A500 memory expansion, don't try to do anything crazy with an A500+ memory board.  This will give you 1MB chip memory (512K on the MB and 512K on the trapdoor).  Obviously once you make the mod I wouldn't try running the system without that memory in-place.  Then add some fast memory with something like a Kipper2K board, or an ACA, or a sidecar, or a processor expansion.  And if you want to later bump up to the full 2MB of chip, find something like a MegaChip.  They pop up pretty often on ebay for around $100.  While you're at it add a 3.1 ROM and the ECS Denise for full ECS compatibility.  You can find those on Ebay all the time for under $20 each, or AmigaKit sells the ROM's new, still.

Or you could do the motherboard mod for 2MB, like detailed above.  A lot more complicated, but pretty sweet deal.  

[Tenacious]

Long story short, there are more versions & revisions of Agnus than I have hairs on my head.  

Does that green helmet in your avatar hide a profusion of revision, err hair?  

[Oldsmobile_Mike]

Does that green helmet in your avatar hide a profusion of revision, err hair?  

LOL!  :lol:  Nah, I have lots of hair still (by some miracle).  But there's even more revisions of Agnus!  (Agnuses?  Agnii?)

http://www.amiga.org/forums/showpost.php?p=788182&postcount=35

[Oldsmobile_Mike]

Hey, did you ever get this figured out?  I glanced at "that other Amiga forum" a minute ago, and right off the bat I saw this link:

http://en.wikipedia.org/wiki/Amiga_500#Memory_map

This is what I was trying to suggest you look for, earlier.

If I were to hazard a guess, and with the disclaimer that "Hey, it's 3:00am where I am right now", but I would say that your A500+ RAM board is set up to map memory into:

0x0010 0000     1024.0     Extended Chip RAM for ECS/AGA

Whereas your normal A500 is looking for slow-fast ram at:

0x00C0 0000     1536.0     Internal expansion memory (pseudo-fast, Slow RAM on Amiga 500)

Anyhow, study that chart, something to think over while I get some sleep.  G'nite!  :lol: