Unknown memory expansion for Amiga 500

[flaviosr]

Dear All,
I have looked all over the Amiga Hardware Database but I have not found the picture of a memory card I have found in a "new" Amiga 500!
I am almost sure it is there (since I have always found everything) but I haven't found it.
The Italian sentence only mean "exact side for connection".
There is also a 3 way switch.
Can somebody help?
Thank you

[psxphill]

Dear All,
I have looked all over the Amiga Hardware Database but I have not found the picture of a memory card I have found in a "new" Amiga 500!
I am almost sure it is there (since I have always found everything) but I haven't found it.
The Italian sentence only mean "exact side for connection".
There is also a 3 way switch.
Can somebody help?
Thank you

There are 4 256k x 4bit memory chips on it, so it's a 1 mb memory card with a single pole, double throw (on/off) switch.

It's for the a500+, to give you 2mb of chip ram. There were lots of clone cards, you won't find them all documented.

[flaviosr]

There are 4 256k x 4bit memory chips on it, so it's a 1 mb memory card with a single pole, double throw (on/off) switch.

It's for the a500+, to give you 2mb of chip ram. There were lots of clone cards, you won't find them all documented.

Thank you for the help.
I do not have an A500+ to test. Could it be that the switch is used to select 512kb or 1Mb?
mmm...  4 x 256kbit x 4bit is 512 kbyte, right?

[magnetic]

back in the old days there were many 3rd party ram cards that are rare

[BLTCON0]

@flaviosr
I think the intended meaning/translation is "soldering side" (vs the "components side").

Anyway, as already said it's a generic 3rd party ("TECNI" if I read it well) trapdoor expansion, one of many such expansions that flooded the market since ~1989 (when 256Kx4 chips became cheap enough).

What almost all of these expansions (including this one) failed to acknowledge was that the existing 8370/71 Agnus couldn't properly refresh 256Kx4 chips without helper circuitry, so this expansion should better only be used on an A500 with at least the 8372A Agnus. With the 8370/71 it's prone to frequent Guru's.

Yes, 4 chips each 256K x4 form a 256K x16 RAM bank, so its size is

262144 (the 256K part) words of 16 bits each (the x16 part), equals 262144 * 2 bytes = 512 KB.

The switch in these cards is typically a disable/enable switch, so 2 positions are enough. I don't know why they used a 3 position switch but it doesn't hurt.

The way such a switch usually works is by grounding (or not) Gary's _EXRAM signal, which is available on the trapdoor pins. When _EXRAM is grounded, Gary knows that 512 KB of RAM exist at $C00000 (slow/ranger RAM). When not, Gary doesn't acknowledge the presence of any RAM there.
(however the RAM is still there at $C00000 and fully active... it's just not acknowledged i.e. Gary will invalidate any $C00000 access).

So it's not actually an enable/disable switch but rather an acknowledge/disregard switch with the RAM staying active nonetheless.

This has an interesting implication when performing the 1 MB chip RAM mod on a rev5 A500 motherboard.
Rev5 has the JP2 jumper but lacks the JP7A one. Well, it's sufficient to change JP2 and set the expansion to "disable" via the switch and voila, 1 MB chip RAM instantly without motherboard modifications or Gary pin lifting :-)

[On rev3 boards, JP2 has a completely unrelated function. There's no other equivalent jumper as well,  so it can't be done without motherboard modifications].

[flaviosr]

@BLTCON0
Thank you! How many informations. :-)
I take the occasion to ask two question I have ever had.
Since the Amiga 500 with this memory expansion is still sealed, never opened...
1. Is it possible to check the motherboard revision from software?
2. Similarly, is it possible to know chips revision from software?
I could put the same memory expansion in a well known and opened Amiga 500, but now I am curious...
Thank you

[mechy]

@BLTCON0
Thank you! How many informations. :-)
I take the occasion to ask two question I have ever had.
Since the Amiga 500 with this memory expansion is still sealed, never opened...
1. Is it possible to check the motherboard revision from software?
2. Similarly, is it possible to know chips revision from software?
I could put the same memory expansion in a well known and opened Amiga 500, but now I am curious...
Thank you

1> no you must open the a500.
2>generally no,but you can run whichamiga which might show agnus ver. or sysinfo.
3>the ram expansion is for a500,so yes u can install it. with proper agnus you get 1MB chip mem with it

[BLTCON0]

@BLTCON0
Thank you! How many informations. :-)
I take the occasion to ask two question I have ever had.
Since the Amiga 500 with this memory expansion is still sealed, never opened...
1. Is it possible to check the motherboard revision from software?

No, but it can be done without opening the case.
Open the side expansion slot cover and observe the expansion connector, top row of pins, left to right.
In rev3 and rev5 pins 1 and 2 are joined together into a wide pin.
In rev6A and rev8A pins 1 and 2 are discrete.

(See the photos carefully and you'll see what I mean. There are also solder holes exactly behind joint pins 1+2 on rev3/5 boards which don't exist behind disjoint pins 1 and 2 on rev6A/8A boards).

So if it's a rev3 or rev5, how can you further tell which one it is?
Just use some software that turns the audio filter on/off. On rev3 motherboards turning the filter off is not possible, so if filter on/off does nothing to the sound, it's a rev3. If sound changes with filter off, it's a rev5.

If it's a rev8A (A500+ motherboard) configured as a simple A500 (no real time clock, only 512 KB, RAM expansion appears as slow RAM at $C00000) it will still have the 8375 ECS Agnus. Also, those A500s usually have the wide name sticker with full name and not the small square one with just the logo.

If it's a rev6A the only way it could have the 8375 is if it has a Megachip installed, but since it's unopened... it will have either the 8372A (most likely) or in some cases the older 8371 (PAL).
If it has the 8371, it will also have the refresh correction circuitry factory installed (take a look at the rev6A photo, near the trapdoor expansion connector, and you'll see a small upside-down board which normally shouldn't be there).

2. Similarly, is it possible to know chips revision from software?
I could put the same memory expansion in a well known and opened Amiga 500, but now I am curious...
Thank you

Yes, software can tell you about the Agnus and Denise, e.g. Sysinfo, whichAmiga.

[psxphill]

Thank you for the help.
I do not have an A500+ to test. Could it be that the switch is used to select 512kb or 1Mb?
mmm...  4 x 256kbit x 4bit is 512 kbyte, right?

oops yeah, my mental arithmetic is not what it was.

[flaviosr]

Wow! Thanx a lot. The terrible situation is that I'm not at home for one more week but now I'm very curious!
I will check immediately!