Amiga 500 vs 500 Plus Hardware Compatibility

[Oldsmobile_Mike]

According to http://amiga.resource.cx/search.pl?product=power+pc it doesn't work on NTSC amiga's.

Huh.  They were pretty extensively marketed in the states at the time, can't recall ever hearing that before.  Weird?

[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)...

[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

[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!  

[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.  

[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: