I do like this thread =)
There's some very good advice so far for you cheezmonkey, however I maybe able to flesh out some detail for you
The Amiga OCS/ECS system had many revision of Agnus (Adress Generation UnitS) from the humble 8370 to the 8375, each with subtle differences and bug fixes along the way.
Now, the 8375 AGNUS is pretty much exclusive to 2MB CHIP adaptor boards and the A500+, so a little *light* history -
8370 (Essentially a PLCC package of the original DIP version from the A1000)
These can only assert 512KB CHIP RAM for the custom Chips
Very early revisions of A500's and A2000's came with this 8370 AGNUS
8371
These can assert up to 1MB of CHIP RAM for the custom chips, but will require a modification to the motherboard.
The 8371 AGNUS was almost an immediate update for the 8370, this little puppy could assert 1MB of CHIP RAM for the custom chips to use, however is disabled to 512KB as it requires Address Line 19 to be connected, you can do this by modifying the position of the solder-pad jumper JP2 and JP7A (EXRAM)
It should be noted that for your 1MB Ranger Memory expansion to work it needs to control JP7A (EXRAM) and JP2 - so by modifying your A500 you will probably lose this feature - IMHO 1MB of CHIP is worth more than 1MB of (SLOW) FAST RAM but that comes down to your needs.
8372 / A
Minor updates to the internal circuitry of the AGNUS, still only able to assert 1MB of CHIP RAM for the custom chips
8372AB / 8372B
Last of the 8372 series, this one can assert a full 2MB of CHIP RAM for the custom chips - however while pin compatible with the A500, there are not enough address lines (and logic) available at the AGNUS socket to allow the addition of 2MB of CHIP.
You would require a small break out board to multiplex signals from GARY with Address 19,20 of the AGNUS and EXRAM banking to name a few
8375
I should mention that this is a 1MB version of this on some A2000 motherboards but the most common 8375 is found in the A500+ (Plus), this AGNUS can assert up to 2MB of CHIP RAM for the custom chips, but it is sadly NOT pin compatible with the rest of the Amiga A500 / A2000 / A3000 family.
Okay thats a very brief overview of the AGNUS, my hope is that helps.
There's some very good advice so far for you cheezmonkey, however I maybe able to flesh out some detail for you
The Amiga OCS/ECS system had many revision of Agnus (Adress Generation UnitS) from the humble 8370 to the 8375, each with subtle differences and bug fixes along the way.
Now, the 8375 AGNUS is pretty much exclusive to 2MB CHIP adaptor boards and the A500+, so a little *light* history -
8370 (Essentially a PLCC package of the original DIP version from the A1000)
These can only assert 512KB CHIP RAM for the custom Chips
Very early revisions of A500's and A2000's came with this 8370 AGNUS
8371
These can assert up to 1MB of CHIP RAM for the custom chips, but will require a modification to the motherboard.
The 8371 AGNUS was almost an immediate update for the 8370, this little puppy could assert 1MB of CHIP RAM for the custom chips to use, however is disabled to 512KB as it requires Address Line 19 to be connected, you can do this by modifying the position of the solder-pad jumper JP2 and JP7A (EXRAM)
It should be noted that for your 1MB Ranger Memory expansion to work it needs to control JP7A (EXRAM) and JP2 - so by modifying your A500 you will probably lose this feature - IMHO 1MB of CHIP is worth more than 1MB of (SLOW) FAST RAM but that comes down to your needs.
8372 / A
Minor updates to the internal circuitry of the AGNUS, still only able to assert 1MB of CHIP RAM for the custom chips
8372AB / 8372B
Last of the 8372 series, this one can assert a full 2MB of CHIP RAM for the custom chips - however while pin compatible with the A500, there are not enough address lines (and logic) available at the AGNUS socket to allow the addition of 2MB of CHIP.
You would require a small break out board to multiplex signals from GARY with Address 19,20 of the AGNUS and EXRAM banking to name a few
8375
I should mention that this is a 1MB version of this on some A2000 motherboards but the most common 8375 is found in the A500+ (Plus), this AGNUS can assert up to 2MB of CHIP RAM for the custom chips, but it is sadly NOT pin compatible with the rest of the Amiga A500 / A2000 / A3000 family.
Okay thats a very brief overview of the AGNUS, my hope is that helps.
