Still confused 8375 Agnus

[AmigaPixel]

Alright,
I think this all falling into place. Correct me if I am wrong (and you know you will!) The Fat Agnus is the DMA controller as well as other functions. It does not hold the actual chip ram but can address 512k, 1mb or 2mb depending on the chip revision.  Hence the need for an adapter on A2000 and A500, the adapter which has the 2mb chip ram on it along with the Agnus 8375 or the socket and the wiring to the Gary chip.

In regards to the A3000, the ones that came with only 1mb chip but had the expansion socket for 2mb, I assume then the Agnus had to be upgraded as well. if so did they require an adapter too or could the chips be plugged right in?

I love learning this stuff, even if it is 20 years later!

[Zac67]

You're pretty much on the right track now - the main point is: there are three different versions/flavors/pinouts of (PLCC) Agnus:
- the one from A500/2000/CDTV - controls 1 MB RAM, older revisions can only use 512 K of that as chipmem, the other 512 K are pseudo fastmem
- the one from A3000 - controls 2 MB chip RAM, A3000s had 1 MB chipmem out of the box, but are easily upgradable
- the one from A500+/A600 - controls 2 MB chip RAM, machines again had 1 MB out of the box but are easily upgradable

That leaves the A500/2000 owners with the wish to upgrade to 2 MB chip. Here come the adapter boards (Mini Megi et al), allowing to fit an 2 MB-Agnus (A3k mostly) on a otherwise not suited mainboard. Which of the 2 MB chips they require depends on their design - the A3k Agnus was first, so it was favoured.

PS: what seriously confuses me is that BBoAH talks about the A500+ as if it has the same chipmem as the A500, even the A501+ is stated to be a 512K expansion. The printing on the board also says so even though it clearly carries 8x 514256 chips = 1 MB! Can someone enlighten me? Is there an A500+ and another A500 Plus and I've been mixing them up all along???

[alexh]

Hence the need for an adapter on A2000 and A500, the adapter which has the 2mb chip ram on it along with the Agnus 8375 or the socket and the wiring to the Gary chip.

1MB not 2MB! The rest of the chip-ram is on the motherboard.

[Zac67]

After a bit of research I found out that I wasn't completely wrong all this time (but not completely right either):
Some early A500+ actually used (old) A500 rev 7 mainboards (1 MB Agnus with 512 KB onboard) but apparently they got the '+' for the 2.04 ROM they featured. 'Real' A500+ have a 2 Meg Agnus and 1 MB onbard, of course.

Again BBoAH isn't too precise here, as it talks about 'OCS' and 'ECS' versions in that respect, neglecting that the 1 Meg Agnus is ECS already (OCS is 512 K chip and 512 K 'ranger').

[AmigaPixel]

@alexh

Ooops! My bad, 1mb chip ram.

[AmigaPixel]

After a bit of research I found out that I wasn't completely wrong all this time (but not completely right either):
Some early A500+ actually used (old) A500 rev 7 mainboards (1 MB Agnus with 512 KB onboard) but apparently they got the '+' for the 2.04 ROM they featured. 'Real' A500+ have a 2 Meg Agnus and 1 MB onbard, of course.

Again BBoAH isn't too precise here, as it talks about 'OCS' and 'ECS' versions in that respect, neglecting that the 1 Meg Agnus is ECS already (OCS is 512 K chip and 512 K 'ranger').

Right, that makes sense. When I had my A500 it only came with 512k, but I was able to upgrade to 1mb with the A501 card or A501+ I can't remember which I had. So it Already had the 1mb Fat Agnus but mine came with AmigaDos 1.3. I guess it would have to since only had 512k installed. I wished I checked to see what rev mobo it had, 5 or 6.

[AmigaPixel]

- the one from A3000 - controls 2 MB chip RAM, A3000s had 1 MB chipmem out of the box, but are easily upgradable....

-That leaves the A500/2000 owners with the wish to upgrade to 2 MB chip. Here come the adapter boards (Mini Megi et al), allowing to fit an 2 MB-Agnus (A3k mostly) on a otherwise not suited mainboard. Which of the 2 MB chips they require depends on their design - the A3k Agnus was first, so it was favoured.

Yes but later A3000's came with 2mb chip already installed. In the those machines was the chip ram soldered onto the mobo or was it upgraded by plugging the extra 1mb into the available socket?

In regards to the Mini Megi and other adapter boards, I would think the 8375 work in those too.

[Zac67]

In regards to the Mini Megi and other adapter boards, I would think the 8375 work in those too.

Which one? 8375 exist with all three different pinouts...

[AmigaPixel]

@Zac67

Well I don't know, Do you know which adapters come with the Agnus chip already installed?

[aaronn]

Hello!  New Amiga fan here in 2023

From the above I could not figure out whether the 8375 / 390544-01 is a 2MB or 1MB chip. Is BBoAH correct with the 2MB spec for this chip?
Most of the times it is referenced as a 2MB chip but in this datasheet (390544-01_8375_Agnus_specification.pdf (amiga.serveftp.net)) (also found on this site) this particular chip is listed as a 1MB chip.

Thank, Aron

[zipper]

I suppose all 3000s had a 2 MB Agnus.

[Castellen]

From the above I could not figure out whether the 8375 / 390544-01 is a 2MB or 1MB chip. Is BBoAH correct with the 2MB spec for this chip?

Most of the times it is referenced as a 2MB chip but in this datasheet (390544-01_8375_Agnus_specification.pdf (amiga.serveftp.net)) (also found on this site) this particular chip is listed as a 1MB chip.

The 390544-01 is definitely a 2MB part.  It was used in later production A500+ and A600, both of which can address up to 2MB chip memory (expansion module fitted).  The datasheet appears to be preliminary information from Commodore.  It also lists various parts that were planned, likely as drop in replacements for earlier 8372 variants, but were never produced.  Only the -01 (PAL) and -02 (NTSC) versions of the 390544 were in production.

It's worth noting that the 390544-0x is only pin compatible with the A500+ and A600, and some third party memory expansions.  It's not pin compatible with A500, A2000, A3000, which uses the slightly different 8372 pinout.  To confuse things further, *some* variants of 8375 (318069-1x) were pin compatible with the 8372 pinout.

For reference, I've found the information on this page to be accurate:
https://bboah.com/index.php?action=artikel&cat=51&id=2174&artlang=en

And here's a photo of a PAL A500+ using the 390544-01:
https://bboah.com/images/download_photos/a500plus_rev8a1_3.jpg