Use 1MB Expansion Card in A500 rev6a

[Sim085]

Out of the box the A500 rev6a comes with 512KB of ChipRAM. An expansion card for the A500 rev6a would come with another 512KB of RAM recognized as SlowRAM. I know there is a motherboard hack that can be done so that this would be recognised as ChipRAM so that one can have a total of 1MB ChipRAM. The MegaChip provides another 1MB ChipRAM, so if used with an A500 rev6a with motherboard hack one would have a total of 2MB ChipRAM.

However with the above setup, the expansion card would still be only contributing only 512KB of ChipRAM from the total of 2MB ChipRAM, unlike for example in the case of an A500+ where the expansion card would contribute 1MB from the maximum 2MB ChipRAM.

Is there a motherboard hack for the A500 rev6a motherboard such that the system would get a total of 1MB of extra ChipRAM from the expansion card and another 1MB; either from motherboard (from such hack) or through something like the MegaChip?

In summary I would like to get 1MB ChipRAM from the expansion card on a rev6a motherboard.

[Jope]

The reason those megachip cards exist is, because the 2MB Agnus is not compatible with the 512/1MB Agnus pinouts. So you would basically have to reimplement the megachip yourself in order to make a rev 8 motherboard out of the rev 6.

It is possible, it takes a lot of work, it will probably be a rat's nest of wires, in the end you're better off with the megachip + 1MB combo.

[Sim085]

Originally Posted by Jope

It is possible, it takes a lot of work, it will probably be a rat's nest of wires, in the end you're better off with the megachip + 1MB combo.

Definitly don't want a rat's nest of wires

I understand that a new Agnus will be required because the one that comes with the rev6a has the 1MB limit. However, apart from this, I was hoping there was some hack which would at allow 1MB from the expansion card.

[Jope]

Keep the existing 512k on the motherboard and get one of those 1.5MB or 2MB ranger fast boards that supplies the other 512k for Chip RAM and then the rest is C00000 RAM..

[xbox123]

You can buy a 2MB RAM expansion for your A500 Rev 6A.
This will give you a 512KB on board and 1.5-2MB from the expansion.
I got mine from AmigaStore
It comes with all you need (GARY adapter) to have total 2MB without soldering on it or 2.5MB with one wire mod.

Its very easy to install and comes with manual. I ve got mine for my Rev6a.

[jbilander]

1 MB Chip RAM takes you far, especially if you have some Fast RAM (slow-fast or real) available at boot time that Workbench can load into automatically. This will leave you with almost the whole 1 MB of Chip RAM free and available when using WB. The Fast RAM is by default being considered higher priority RAM than Chip RAM (is it prio 0 vs -10?). Having some Slow-fast RAM at C00000 can also help with running some old games from floppy that were poorly written (hardcoded) to check for ram at C00000 location in order to run. Ofc if you install an accelerator with real fast ram onboard, let's say 8 MB, there isn't much need for Slow-fast at C00000 because then you probably run all games through WHDLoad from harddisk anyway, and the games are adapted to work.

[Sim085]

The thing is that I have an A500 with 1MB ChipRAM and an A500+ with 2MB ChipRAM using standrad expansions for these machines. What I want for this particular A500 rev6a is to hack it into an A500+. I have bought an A500+ Denise and replaced this. Next I would have liked to get it to work with 1MB expansion. However it looks like the hacks out there either get 2MB from motherboard making the bottom expansion slot useless (need A3000 Agnus), or else get 512KB from motherboard, 512KB from expansion and 1MB from megachip, or else use one of these boards which give 512KB ChipRAM and 1.5 SlowRAM.

[Jope]

Perhaps it's time to condition yourself to look elsewhere for adding extra RAM then. Perhaps the 68k socket or the AmigaBUS connector.

[Sim085]

Originally Posted by Jope

Perhaps it's time to condition yourself to look elsewhere for adding extra RAM then. Perhaps the 68k socket or the AmigaBUS connector.

Have 8MB FastRAM connected to the 68K socket (I believe that is the maximum) + IDE. To my knowledge (from what I have read) ChipRAM cannot be added from socket or side expansion.

It is just this 2MB ChipRAM mod which I am missing. Originally I did not mind having a MegaChip (which would bring the machine to 2MB ChipRAM) but before going that direction I wanted to see if it was possible to get 1MB from the bottom expansion card and 1MB from motherboard / megachip hack.

[jbilander]

If you really want 2 MB ChipRAM in your Rev 6A then I think your best bet would be to do the mod shown in picture below.

http://www.rasterline.com/images/ami..._2MEG_CHIP.jpg

Step 1 in the description doesn't list all part no. of the Agnuses you can use for this mod (it is missing 318069-18 VBB PAL), here they are, and they are all quite rare and hard to source these days:

Agnus (2MB)     8372AB  318069-03       NTSC/PAL
Agnus (2MB)     8372B   318069-03       NTSC/PAL
Agnus (2MB)     8375    318069-18 VBB   PAL
Agnus (2MB)     8375    318069-19 VBB   NTSC

The 318069-18 and -19 were substitutes for the -03 version and have Vbb (Voltage Back Bias) connected to pin 41 no longer switching between PAL and NTSC.
Source: http://www.amigawiki.de/lib/exe/fetc...us_reworks.pdf

Another alternative is the ACE 2, if you can source one second hand. From the picture it seems to use an Agnus (2MB) 318069-11 VBB NTSC and the description says switchable between PAL and NTSC video mode by software.
https://icomp.de/shop-icomp/en/produ...duct/ace2.html

On this page you can find a nice complete (?) list with all the different Fat Agnuses made:
http://translate.google.fr/translate...hl=fr&ie=UTF-8

Hope this helps, happy hunting

[AmigaHope]

I'm surprised nobody's made a drop-in FPGA 2M Agnus replacement board yet. All code already exists, a tiny FPGA has enough logic elements to do it, and the onboard memory would easily hold the second 1M so no separate memory chips would be required. The board could just have the FPGA, passive components, and a header to flash the firmware.

[Sim085]

Originally Posted by AmigaHope

I'm surprised nobody's made a drop-in FPGA 2M Agnus replacement board yet. All code already exists, a tiny FPGA has enough logic elements to do it, and the onboard memory would easily hold the second 1M so no separate memory chips would be required. The board could just have the FPGA, passive components, and a header to flash the firmware.

+1 to this.

[Sim085]

Originally Posted by jbilander

Hope this helps, happy hunting

Thank you for this.

Yes, I have been looking at this for a long time. The problem is that this mod makes the bottom expansion slot useless as all 2MB of ChipRAM come from the new DRAM chips. I do wonder if maybe changing the jumper settings would achieve what I want 1MB on motherboard and 1MB from bottom expansion card and maybe the author of this mod just wasn't after this. Unfortunatly I would need to do the mod before I can try it and an A3000 Agnus is not very easy to find.

Reading from another thread here it looks possible to force an A500 rev6a to ingore the 512KB on motherboard and take 1MB from the bottom expansion card. I wonder if this + some mega chip would give me what I want.

[cmorley]

Originally Posted by AmigaHope

I'm surprised nobody's made a drop-in FPGA 2M Agnus replacement board yet. All code already exists, a tiny FPGA has enough logic elements to do it, and the onboard memory would easily hold the second 1M so no separate memory chips would be required. The board could just have the FPGA, passive components, and a header to flash the firmware.

Tiny FPGAs have a few kilobits of memory not megabytes. They are typically not 5v tollerant either. I doubt if the code exists anywhere... mister and the like only need to emulate what Agnus does not implement it at a pin compatible level.


So you'd need level shift buffers, RAM and the FPGA.



This would be a lot of work and they'd realistically cost £40-50 or more. Even more if you use a plug in adapter (they are expensive) otherwise the buyer would need to desolder the Agnus socket - not a beginner level thing.

[jbilander]

Originally Posted by Sim085

I do wonder if maybe changing the jumper settings would achieve what I want 1MB on motherboard and 1MB from bottom expansion card and maybe the author of this mod just wasn't after this.

If it were that simple it would've been done long ago and you would have a guide to follow already. The problem is on 8372B they had to remove the 2nd RAS line due to limited pinout, and replaced it by an extra mux address pin, requiring extra circuitry on the Amiga 3000 to bring back the 2nd RAS line. You can read about it here, the last paragraph:
http://theamigamuseum.com/the-hardwa...chipset/agnus/

And the pinout change, see pin 56:
https://en.wikipedia.org/wiki/MOS_Technology_Agnus

And the A3000 Schematics, check the circuitry after DRA9, pin 56 and _RAS pin 57 from Fat Agnus:
https://www.amigawiki.org/doku.php?i...ice:schematics