ACA500plus A500 rev 6a 1MB chipram

nizce · Jan 22nd 2023

Hi,

First I want to say that the ACA500plus is a great product!

Just got back into my Amigas some months ago and loving it. Being away from it almost 30 years makes you forget and the ACA500plus makes it so easy to get back into.

But I do have a problem and wondering if someone knows what might be wrong.

I remembered that I back in the day thought that the all the different memory types in the Amiga was confusing(not as much as a PC though haha) So I might be missing something obvious here.

Well here we go.

First some info about my setup:

- I have a ACA500plus that I use with my Amiga 500 rev 6A.

- I use the Amiga 500 original PSU(I use the same one with my A600 and A1200 both with accelerators without issue)

- I also have a M501-s trapdoor expansion that has 512kb and RTC https://bigbookofamigahardware.com/bboah/product.aspx?id=1033

- My 6A It has a Agnus 8372A and from what I've managed to google it should be able to address 1MB chipram.

The Problem:

When I boot into Workbench 3.2.1 it seems to only seems to show 512Kb chip ram.

In the ACA500plus menu at boot, if I press F10 and go into expert menu, the option "1M Chip RAM expansion" states "unavailable".

Toggling the RTC setting to trapdoor gives me the correct time, so that leads me to "think" that at least the m501-s works(partly?).

Am I correct that with the above I should be able to get 1MB chipram without having to solder any jumpers on the Amiga?

If so what could be the problem, is the m501-s broken, someway to try/verify that?

thebajaguy · Jan 22nd 2023

OS 3.2.1 - there is a ShowConfig tool. You can see the memory ranges the OS knows about.

See if there is a 512K Fast memory space showing at $00C0.0000 - that is where the trapdoor memory appears if the 1M ChipRAM modification has not been done. When done, it extends the 512K ChipRAM from $0000.0000-007F.FFFF to $0000.0000-00FF.FFFF. (The '.' is added here for readability, and won't be in the actual output).

The modification is a pad gets opened, and another 3-pin position moves to the 'other position'. Online information has the detail. Best done by someone with some experience in soldering.

nizce · Jan 22nd 2023

Hi!

thanks!
I solved it.

The problem was that I had removed the on/off switch from the expansion card when moving it to my new/working A500, and didn’t put the switch back.

didn’t know that removing the switch puts it in “off” position by default. So added a small wire between the two pads labeled on on the expansion card and now it works

Didn’t have to do any mods to my a500 montherboard

thebajaguy · Jan 22nd 2023

The motherboard was likely already modified to support 1MB ChipRAM setting.

Some 3rd party trap door expansions have that jumper/switch for compatibility with some games which didn't follow the rules, and may expect a different memory footprint.

Jens · Jan 23rd 2023

Quote from thebajaguy

The motherboard was likely already modified to support 1MB ChipRAM setting.

No - the ACA500plus can turn the internal trapdoor expansion into chipram without any modifications. Thanks for helping out here, but everything you've written would only apply to a computer that does not have the ACA500plus. This will let you software-switch the trapdoor expansion to chip ram and still provide $00c0.0000 memory that has cycle-exact speed at 7MHz CPU speed.

You may tend to say "that's impossible without hardware modifications" - and you're almost right. The trick is to make real-time alterations to the bus signals while the access is in progress. I'm particularly proud of that, and thought that it would have been common knowledge after almost ten years of the feature being available on my A500 cards (ACA500 was the first to introduce that feature).

nizce · Feb 6th 2023

A very nice feature indeed since I don’t want to open it up and cutting traces and such with the risk of destroying it