Advice: ACA500Plus Card Vs Virgo Development 2MB RAM Expansion

[lemonje]

I’m the proud owner of an Amiga A500/R6 that I’ve had for 25 years and still use (mostly at Christmas) to play and attempt to finish some of my childhood games.
My Amiga has a RAM expansion via the trapdoor. This is a Virgo Development card I also bought 25 years ago, and having recently checked it after seeing videos of ram card batteries leaking and eating circuit boards, I’m impressed at the build quality and durability of this expansion board.

I seem to remember buying the Virgo Development 2MB RAM Expansion because you didn’t need to hack or jumper the motherboard. You slide the expansion board into the trapdoor and then fit the socket adapter that’s tethered to the board to the GARRY chip via a blank socket. I believe the GARRY adapter effects some of the PINS so that the memory is visible to the machine. Anyway this has been working fine for years.

Recently I decided to pimp my Amiga after seeing videos on the ACA500 Plus. Why not, a faster machine with 1MB chip ram, 7MB fast ram for 100 Euros, Yes please.

I ordered the ACA500plus last week, it hasn’t arrived yet, but yesterday my heart sank when I read the following at another forum.

“unfortunately the memory expansion currently in the trapdoor is not compatible with the Chip mem function on the ACA500plus, due to the way the Gary adapter interferes with the addressing. From Jens, the creator of the card: It’s a hardware limitation of the expansion itself: The expansion uses an adapter on the Gary-chip. It generates signals for the trapdoor expansion, which in turn selects one out of the four 512k-blocks. Since this selection is purely based on the CPU address, but not on the DMA address that Agnus generates, the switching signals are plain wrong at times.​

You can try to remove the Gary-adapter and just operate the trapdoor expansion without it. This will most likely turn it into a simple 512k unit. You may need to add a pullup-resistor to the open signals.”​

Has anybody had any success with this combination. I would like to get my machine to 1MB chip ram and use the ACA500plus accelerator, but I don’t think I can do it with the current board type. I’m open to buying an alternative expansion card just so look as it’s not one thatrequires the motherboard to be hack, just in case I wish to use the Virgo adapter again in the future.

What are my options? I would like to use the ACA500plus and get my 1MB chip ram... is this possible?
Many thanks

Steve

 

[Sardine]

Hi,

better hardware guru's will follow im sure.

I also have a rev 6 A500,, to get 1mb chip ram (was 512k + 512k slow ram in trapdoor) I simply cut a trace and joined another. hey presto 1mb chip ram which works no issues as my son has my rev6 with an ACA500+

The adaptor you mention is this for 2mb chip ram ?? no gary adaptor is required for 1mb unless you have the wrong Agnus chip ??

look here.
https://www.amibay.com/showthread.p...ation-Caution-dialup-users-excessive-H-W-pr0n!

 

[lemonje]

Thanks for posting back.

Yes my A500/R6 has the Fat Agnus 8372A which I believe can only address 1MB chip ram, so I cannot exceed this. The ACA500plus card which I hope to have soon states that it can turn the trapdoor memory from slow ram to chip ram without modifications i.e. just plug in the aca500plus via the hdd port and instant 1MB chip ram. However, as I understand it my old trapdoor memory card which has a GARRY adaptor won't let this happen.

Unless there is an alternative way of getting this to work (since the aca500 was launched), I think the only alternative is to seek another trapdoor 515KB ram card that the ACA500 can convert to chip ram and remove the 2mb Virgo card. As the aca500 has extra onboard ram I won't miss the memory I'm removing. The only problem I can see is that I don't want cut and solder the motherboard, as this means I could never unplug the new aca500 and put the virgo 2mb card back in without undoing those mods. I'm trying to get an advantage without physically altering the machine permanently. Does that make sense.

Are there memory cards that can fit in the trapdoor that do not need board mods to work, or was a board mod always required if you didn't have a GARRY adapter based ram expansion.
Cheers

Steve

 

[Sardine]

from what i believe

if you remove the 2mb ram adaptor and replace with a 512k (nice and cheap,, keep the 2mb board as you might regret selling it and you never know it might come in handy for an A2000 or if you get an A500+ you can fit back into you rev6)

the ACA500 will convert the 512k slow ram to chip ram for you without the need of cutting traces,, the cutting is only required if you dont have an ACA500 card.


also quite a bit more expensive the new V4 Vampire 500 i do believe will support 4mb chip ram... i know,, wow,,,

 

[RichyV]

512k boards are readily available for little money. And, as Sardine says, keep your card for the future.... Seller's regret and all!

 

[BLTCON0]

@lemonje
As said, you'll need to install a plain 512 KB trapdoor RAM card.


As far as the 8372A Agnus is concerned, the 512 KB maximum that is allowed on the trapdoor is already chip RAM.
The Agnus can't see 512 KB at $0 and another 512 KB at $C00000, it just sees its own 1 MB address space (first 512 KB blocked accessed by RAS0, second 512 KB block - at the trapdoor- accessed by RAS1).


So the trickery used by those "illegal" 2 MB trapdoor cards would fail the 8372A Agnus, as those 2 MB consist of a 4x repetition of a 512 KB block, each one however still refreshed and addressed through the Agnus second RAS line (RAS1).
The Gary trickery would map
at $C00000 the 1st 512 KB block
at $C80000 the 2nd 512 KB block
at $D00000 the 3rd 512 KB block
at $D80000 the 4th 512 KB block (which would not be fully usable with an RTC as it sits at $DC0000)


Now suppose you get the ACA and configure the $C00000 512 KB (1st block) as chip RAM, i.e. causing it to appear at $80000 instead.
Now suppose the CPU accesses something in the $C80000 block. The Gary trickery does its job and enables this block to "appear" in the trapdoor.
Now suppose next thing is an Agnus DMA access in the $80000 512 KB block. CPU/Gary doesn't get in the way of things here, so the card is left with the 2nd block enabled on the trapdoor.
Since the same selection line is used (RAS1) Agnus will access the trapdoor, and either read bogus data (from somewhere in block 2 instead of block 1) or will write bogus data (to block 2 instead of block 1).


Perhaps this could be solved if those 2 MB cards also took into account whether it's a DMA instead of a CPU cycle, and always reverted to block 1 in case of DMA cycles. But apparently none was designed that way. Plus, they're not true Fast RAM, so apart from very specific purposes (e.g. chipset-timed code) they're better off replaced by true Fast RAM for general performance purposes.

 

[lemonje]

Many thanks everyone for the advice, it's fantastic to get this much help and technical advice for a machine built 27 years ago.
Thanks @BLTCONO for the technical analysis of what is going on under the hood. I've not got the documentation for the Virgo Card anymore, but the boot instructions injected into my workbench disk seem to agree with your address analysis mentioned.

What I'm going to do now, is purchase a nice clean simple 512KB ram expansion from one of the amiga kit sites to go with my new ACA500+.
When it arrives I'll remove the 2MB Virgo card and GARRY adaptor. That way with the aca500+ plugged in I can get the 1MB chip ram without the instability of the GARRY slight of hand address remapping.

Cheers
Steve