Agnus 8375

[mrknight]

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.

[mikrucio]

YEAH Mate

just do it, works a treat, no need for a switch tho. its more of a permanent mod.
1mb chip is great. but you really need some fast ram for your Amiga 500.
to complement.

[Zac67]

If you relocate the slow RAM make sure you cut _EXRAM as well (otherwise the RAM will appear in two different locations and the system will crash). The 2 MB chip RAM expansion requires this mod as well.

2. The Gary chip lines are for providing the extra addressing required. The Agnus socket as is can only address 1 MB.
3. Agnus pinouts can be found in the service manuals. I've added a comparison chart to Wikipedia a while ago. The inner workings are not so easy, there are several somewhat complex state engines. You should take a look at the Hardware Reference Manuals.
4. Yes, Agnus is doing chip/slow RAM refresh as well.

[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 ..

[psxphill]

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 ..

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+.

[mrknight]

just do it, works a treat, no need for a switch tho. but you really need some fast ram for your Amiga 500.
to complement.

If I'm to do it, I might as well connect it to a switch for compatibility reasons. Although I don't expect to use it. I have been looking at the 8MB FAST RAM board by Kipper2k. It is on my want/need list:razz:

I have bought a Kickstart 37.350 and trying to figure out how to get a CF card "HDD" mod. Not sure how to do it. Any ideas?

If you relocate the slow RAM make sure you cut _EXRAM as well (otherwise the RAM will appear in two different locations and the system will crash). The 2 MB chip RAM expansion requires this mod as well.

The _EXRAM signal is set by JP7A if I remember correctly. Just cut it open and all will be well. And JP2 need to be changed to the other position. This is what I have read about the 1MB CHIP RAM mod. Hope I haven't forgot anything.

2. The Gary chip lines are for providing the extra addressing required. The Agnus socket as is can only address 1 MB.
3. Agnus pinouts can be found in the service manuals. I've added a comparison chart to Wikipedia a while ago. The inner workings are not so easy, there are several somewhat complex state engines. You should take a look at the Hardware Reference Manuals.

That meanst that Gary is having address line for up to 2MB CHIP RAM but these lines are unused?
I've seen this pinout table. Thanks! Unfortunately, it's not mentioned which specific chip is it. There are some incompatibilities, even within the same four number series what I've heard.

[Zac67]

The _EXRAM signal is set by JP7A if I remember correctly. Just cut it open and all will be well. And JP2 need to be changed to the other position. This is what I have read about the 1MB CHIP RAM mod. Hope I haven't forgot anything.

Yes, that's it.

That meanst that Gary is having address line for up to 2MB CHIP RAM but these lines are unused?

Gary has access to all upper address pins.

I've seen this pinout table. Thanks! Unfortunately, it's not mentioned which specific chip is it. There are some incompatibilities, even within the same four number series what I've heard.

Some time ago, alexh made a pretty good survey on how to determine the Agnus version... Here it is: http://eab.abime.net/support-hardware/21961-help-update-amiga-hardware-agnus.html

[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 ..

[psxphill]

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 ..

I believe one of the changes to the a500 revisions might be the agnus orientation.

[Oldsmobile_Mike]

I believe one of the changes to the a500 revisions might be the agnus orientation.

Correct.  See here:

http://amiga.resource.cx/manual/MegAChip.pdf

[mrknight]

I will do the 1MB chip RAM mod tomorrow after work. Shouldn't take too long since my A500 motherboard already is on the desk.

It seems that my original question number one will be an issue. A preliminary check shows that a PLCC-84 plug cost between $40-$60. I asked one supplier and I could get them for around $25 if I buy 100 units. Are these adaptors really this expensive? Sounds like I might as well but a real MegaChip...