2megs ChipRAM on rev.6a Amiga 500 Motherboard

Metalguy66

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A while back, a guy named Wild Cobra on Amiga.org brought the following to my attention: As evidenced by the service manual schematics, with the right jumper settings, the rev 6a Amiga 500 is actually designed to accept an A3000 Agnus chip and a full 2 megs of ChipRAM on the motherboard!

This does require DRAM of a higher density (1meg x 4) than stock, but the additional address lines are already present on the motherboard both on the DRAM side as well as the CPU side of Agnus.

All that has to be done is to replace the Agnus chip with the A3000 part, remove the factory 256k x 4 DRAM chips, install four 1meg x 4 DRAM chips, and make the appropriate jumper changes to enable it.

Recently, I was able to verify that this does in fact work on real physical hardware in front of me. All of the "docs" I've seen on Aminet and elsewhere in the past have seemed to reflect that you need to build a quite involved hardware project in order to accomplish this. They are wrong.

So, for the benefit of all who may wish to have these instructions, please have a look at this:


A500_REV6A_2MEG_CHIP.jpg

note: Only an actual A3000 Agnus (one of the two part numbers listed in the diagram) will work. 2meg Agnus chips made for the A500+ or A600 will not work as they have a vastly different pin arrangement. I have done my absolute best to include all needed instructions/details in this single .jpg image. Enjoy.
 
A while back, a guy named Wild Cobra on Amiga.org brought the following to my attention: As evidenced by the service manual schematics, with the right jumper settings, the rev 6a Amiga 500 is actually designed to accept an A3000 Agnus chip and a full 2 megs of ChipRAM on the motherboard!

This does require DRAM of a higher density (1meg x 4) than stock, but the additional address lines are already present on the motherboard both on the DRAM side as well as the CPU side of Agnus.

All that has to be done is to replace the Agnus chip with the A3000 part, remove the factory 256k x 4 DRAM chips, install four 1meg x 4 DRAM chips, and make the appropriate jumper changes to enable it.

Recently, I was able to verify that this does in fact work on real physical hardware in front of me. All of the "docs" I've seen on Aminet and elsewhere in the past have seemed to reflect that you need to build a quite involved hardware project in order to accomplish this. They are wrong.

So, for the benefit of all who may wish to have these instructions, please have a look at this:


A500_REV6A_2MEG_CHIP.jpg

note: Only an actual A3000 Agnus (one of the two part numbers listed in the diagram) will work. 2meg Agnus chips made for the A500+ or A600 will not work as they have a vastly different pin arrangement. I have done my absolute best to include all needed instructions/details in this single .jpg image. Enjoy.
Thanks for this info. So if you do not find required Ram chips to be soldered, say a rev 5 to achieve 2Mb chipmem, one can still acquire a DKB megachip or mini megi still right?
Correct me if I am wrong..
 
This has nothing to do with a rev. 5. Yes there are many rare/expensive adaptor boards out there that can be used to upgrade other machines and versions.

The DRAM shouldn't be too hard to find. Do a Google search for 514400 DIP.

I would guess that it would be much harder to source an A3000 Agnus chip, but ask around in the normal Amiga hardware channels and I'm sure someone has one they would part with for a reasonable price.
 
This is great news! Now to track down the right agnus...

Are we limited to the two part numbers on the image, or would other models of Agnus work?
 
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***ONLY THE TWO PART NUMBERS LISTED IN THE DIAGRAM WORK***
These are the two specific Agnus models used in the A3000.

The other variations of the 8375 and 8372B are either not 2meg models, or have a vastly different pin arrangement and will not work. Agnus chips designed for the A600 or A500+ will NOT work.
 
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Thanks Metalguy, I actually have a spare 8372AB lying around so I've ordered some ram chips and will upgrade my Rev 6a A500 shortly! :cool:
 
Guys PLEASE READ my entire first post..

The model number of the agnus (8372B,8372AB, 8375) is not important.. It is the PART NUMBER!

Only the two part numbers listed on diagram I posted will work.. It has to be an A3000 agnus.. The 8375 from the A500+ or A600 does not work. Over 20 pins are different.
 
2megs ChipRAM on rev.6a Amiga 500 Motherboard

Guys PLEASE READ my entire first post..

The model number of the agnus (8372B,8372AB, 8375) is not important.. It is the PART NUMBER!

Only the two part numbers listed on diagram I posted will work.. It has to be an A3000 agnus.. The 8375 from the A500+ or A600 does not work. Over 20 pins are different.

Lucky for me I have the correct spare agnus lying around in my spare Amiga chips box.

49bdb300efd898fc699def148e7ca72c.jpg


Ignore the apparent remarking, it's the real deal.
 
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Yep.. That's a PAL A3000 Agnus. That one will work. If you have problems sourcing the 514400 (1meg x 4) DIP package DRAM chips, let me know. I may be able to point you in the right direction.
 
Yep.. That's a PAL A3000 Agnus. That one will work. If you have problems sourcing the 514400 (1meg x 4) DIP package DRAM chips, let me know. I may be able to point you in the right direction.

They were easy to find but thanks for the offer. :)
 
I was finally able to get a 318069-03 for my PAL A500 Rev 6a from ebay (I jumped as soon as I saw it listed ;) ) and ordered the RAM as well. Very much looking forward to doing this.

Does anyone know the compatibility with other Memory expansions? I have an AdRam board waiting to go into my machine as well.
 
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Fast ram boards should be fine, only chip ram and trapdoor expansions will be incompatible.
 
Since the AdRam 540 uses both Trapdoor and Gary socket, I assume that means a no? I noticed one of the things that the Amiga Database mentions is: "the adapter board connects to Gary's socket - if 1 MB Chip RAM is present, a wire has to be soldered between the adaptor and the motherboard" I guess since 2MB Chip on-board isn't normal, they don't mention it specifically. Are Fast Ram boards done via the Expansion Slot on the side only?
 
Since the AdRam 540 uses both Trapdoor and Gary socket, I assume that means a no? I noticed one of the things that the Amiga Database mentions is: "the adapter board connects to Gary's socket - if 1 MB Chip RAM is present, a wire has to be soldered between the adaptor and the motherboard" I guess since 2MB Chip on-board isn't normal, they don't mention it specifically. Are Fast Ram boards done via the Expansion Slot on the side only?

I'd be very surprised if that board worked with 2 mb chip ram installed. The fast ram expansions I was talking about, that would work, are the CPU based expansions.


I doubt there is any harm trying it tho.
 
True, I am going to do everything via jumpers so they can be undone without damage, so good point. Worst case, someone can buy my fully stocked AdRam 540 as I would rather enjoy the 2MB Chip Ram.
 
The Adram 540 is a pile of ****. You dont want that on your machine in any case. No "fastram" board that goes in the trap door slot is actually fastram. Its "slow ram".. In otherwords, ram that can not be used by the custom chips, (and thus gets allocated by the OS as fastram) but is on the chip-bus side of agnus, so it suffers from bus cotention whenever the custom chip DMA is in use.. WORST OF BOTH WORLDS..

Back in the day, when ANY RAM above 512k was preferable to none, it KIND OF made sense.. Now.. No.. You are slowing your system down by having it.
Fastram should only ever be installed on the side-car bus, or on an accelerator.. All other ram should either be chipram or leave it off.. Otherwise, you are causing a needless performance issue..
 
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