Agnus variants
Agnus is one of the custom chips used in all OCS/ECS versions of the Amiga. It was made in a number of versions over the lifetime of the Amiga, usually with improvements with the amount of Chip RAM it could address. A number of other changes were also made over the years, mostly as a result of changes of chip production technology. Unfortunately, Commodore really messed up the part numbering system for Agnus, especially with later versions causing a great deal of confusion. This document is intended to attempt to clarify this situation, although definitive information has been hard to find.
Above is an example of an 8372AB chip, and is an example of the sometimes messy state of labelling and identifying Agnus chips. The 'B' appears to have been added later and is in an odd position. The suffix on the part number appears to have been removed and then reprinted so that it now reads '318069-039'. This chip was probably originally incorrectly labelled as a 1MB 8372A, part number 318069-02. The labelling was then altered to be in line with the actual 2MB Agnus contained inside. I do not know if there are any important differences between the -039 and the -03 variants.
Below is a table of some common Agnus versions.
| Unofficial name | Chip No. | Part No. | Memory size | Chipset | Video system | Amiga models | Package | Note |
|---|---|---|---|---|---|---|---|---|
| Thin Agnus | 8361 | 252125-01 | 512k | OCS | NTSC only | A1000, A2000A | DIL | Probably an early version that was not common. |
| Thin Agnus | 8361 R3 | 252125-03? | 512k | OCS | NTSC only | A1000, A2000A | DIL | |
| Thin Agnus | 8367 R0 | 252362-01 | 512k | OCS | PAL only | A1000, A2000A | DIL | |
| Fat Agnus | 8370 | 318070-01 | 512k | OCS | NTSC only | A500, B2000 | PLCC | |
| Fat Agnus | 8371 | 318071-01 | 512k | OCS | PAL only | A500, B2000 | PLCC | |
| Fatter Agnus | 8372A | 318069-02 | 1MB | ECS | Switchable | A500, B2000 | PLCC | May sometimes have -029 suffix. |
| Obese Agnus | 8372AB | 318069-03 | 2MB | ECS | Switchable | A3000(T) | PLCC | May sometimes have -039 suffix. |
| Obese Agnus | 8372B | 318069-03 | 2MB | ECS | Switchable | A3000(T) | PLCC | Same chip as the 8372AB |
| Obese Agnus | 8375 | 318069-10 | 2MB | ECS | PAL default | A500+, A600 | PLCC | Different pinout to other Agnus versions. |
| Obese Agnus | 8375 | 318069-11 | 2MB | ECS | NTSC default | A500+, A600 | PLCC | Different pinout to other Agnus versions. |
| Fatter Agnus | 8375 | 318069-16 | 1MB | ECS | PAL default | A500, B2000 | PLCC | Replaces 318069-02. VBB modification required. |
| Fatter Agnus | 8375 | 318069-17 | 1MB | ECS | NTSC default | A500, B2000 | PLCC | Replaces 318069-02. VBB modification required. |
| Obese Agnus | 8375 | 318069-18 | 2MB | ECS | PAL default | A3000(T) | PLCC | Replaces 318069-03. VBB modification required. |
| Obese Agnus | 8375 | 318069-19 | 2MB | ECS | NTSC default | A3000(T) | PLCC | Replaces 318069-03. VBB modification required. |
| Obese Agnus | 8375 | 390544-01 | 2MB | ECS | PAL default | A500+, A600 | PLCC | Different pinout to other Agnus versions |
| Obese Agnus | 8375 | 390544-02 | 2MB | ECS | NTSC default | A500+, A600 | PLCC | Different pinout to other Agnus versions |
Notes
As can be seen from the table, using '8375' to specify what kind of Agnus you need is rather problematic as it refers to a whole range of different versions. Some of them are 1MB, others are 2MB. Some of them even have different pinouts and are completely incompatible with each other. So when referring to any Agnus from the 8375 series, it is strongly advised to use the far more descriptive part number, ie: 390544-01.
Although the table above specifies that some Agnus variants are limited to certain variants of the Amiga, eg: the 8372B was intended for use in an A3000, it is possible to use an 8372B in an A500 or A2000 by modifying the motherboard to take 2MB of RAM. It is also possible to incorporate a 2MB Agnus of any version as well as 2MB of Chip RAM into a small PCB that plugs directly into the Agnus PLCC socket. The manufacturers of these expansions may have used a 2MB Agnus with a completely different pinout like the 39044-01 as it may have been plentful and easy to obtain at the time.
About VBB
The 8375 series of Agnus was made using a different process compared to the earlier versions. This required a connection to the substrate of the chip be taken out to a pin for connection to a bypass capacitor. Unfortunately there were no spare unused pins on the Agnus, so Commodore decided to sacrifice the PAL/NTSC selection pin. This meant that it was no longer possible to select either PAL or NTSC by a hardware jumper, so two different versions of Agnus were made. A PAL one and an NTSC one. It was still possible to select any supported video format in Workbench prefs, so in that regard the 8375 is still more flexible than the old PAL or NTSC only versions. Chips with VBB were marked as such. An equivalent VBB chip can be used to replace an 8372A or 8372B, however the PAL/NTSC select pin needs to be left floating, and a 0.1 uF capacitor to ground needs to be installed.
Introduced 29 January 2024. Updated 29 January 2024. Version 1.0