Agnus 8375

[AAACHIPSET]

I have a rev. 6A A500 with an Agnus 8372A 318069-02. I'm thinking of doing the two jumper hack on the mother board to change the RAM expansion from slow RAM to chip RAM. I will install a switch that I can reach from the expansion port so I can easily change the RAM type for the expansion easily. Has anyone done this?


I have also bought an Agnus 8375 390544-01 from Amibay and expecting delivery very soon. I bought these after seeing a board called mega chip or similar. You replace the existing 8372 with this board and you plug a 8375 into this expansion board. The board has 1MB or RAM and with this solution you will have a total of 2MB chip RAM. Cool!

Since I'm learning how to design PCB I thought it would be a good project to do something similar. I do have a few questions.

1. What do you call these PLCC chip replacement you plug into a PLCC socket? I need one of those with either pins of feet so I can attach it to the expansion board. And where can I get those?

2. It seems on the mega chip one or two pins are being connected to the Gary chip. Why is this so?

3. Is there any detailed data sheets for both the Agnus revisions I've mentioned (8372A 318069-02 and 8375 390544-01)? I need this if I am to figure out the pins as well as the detailed function of the chip.

4. Is Agnus responsible for doing RAM refresh?


I hope you can help me with these questions. Hopefully I will feel confident enough to start with the project but there is no guarantees.

i just put some  replacement chips from my old a500 to another board ..including the gary chip  an the 2 meg fat agnus ..at first it didnt work ..then i realized that the agnus chip on the other a500 was orientated differently to my original board ..is this common??  ..i thought a a500 board would be the same  an chips  same  as each other ..

[AAACHIPSET]

I wouldn't expect an a500 board to have a 2meg agnus at all, if it was really a 2meg agnus then it sounds like an a500+ board. Unless someone fitted a 2meg agnus later. However even then the a500 board can't provide 2mb of chip ram, you need an a500+ board. Some people think the ram is inside the agnus, but it's not. The only difference between them is a different number of address lines so they can access more/less memory. Up to rev 5 you could only have 0.5mb on the motherboard, rev 6 added the ability for 1mb on the motherboard but I don't believe any machines shipped like that. If you only had 0.5mb on the motherboard then you need to add 0.5mb trapdoor memory to get 1mb chip ram. Rev 8 was the first to ship with 1mb on the motherboard and support 1mb in the trapdoor. Megachip allows you to have 2mb but that is bypassing the motherboard limitations by using another pcb.
 
Are you sure it was a 2mb agnus? An 8375 may either be 1mb or 2mb as commodore re-used the same number.
 
http://en.wikipedia.org/wiki/MOS_Technology_Agnus
 
There were multiple revisions of the mother board, rev 6 or earlier is a500 and 8 is a500+.

yeah my original a500  ..bought from kmart ...kickstart 1.3...just before 2.04  came out..then i upgraded it to 2 meg chip with the superfatagnus  board ..which also connects to the gary chip ..an put 2.04 kickstart in..
was at least a year before the a500+ came out ..never did get the hires denise chip though ..