So is this a typo on the page? Is this really a 1MB Agnus from the Enhanced Chip Set?
Yes that page is incorrect, the specific 8375 you have is 1mb. Commodore really caused confusion by marking them as 8375 even though they have different pinouts.
http://www.amiga.org/forums/archive/index.php/t-35082.html Internally they are the same, but the 8375 that fit in a rev6 a500 are only 1mb.
I've used Amigas for a couple of decades now and never knew that the A500 could "easily" be upgraded to 2MB Chip RAM this way.
I'm not sure I'd class it as "easily", first you would need to get an 8372B. They were only fitted to late A3000's. I have no idea why they switched pinout and then switched it back. I'd probably sell the megachip and buy an a500+
Here is what you'd need to do:
"Ok. Heres what is kewl:
According to the service manuals, the A3000 (8372B 318069-03) AGNUS is pin compatable with the rev 6a A500! All of the signals are in the right place. Even the one (pin 41)that controls PAL/NTSC on bootup. According to the information in the Rev 6a a500 schematics, and the A3000 schematics, you should be able to drop an 8372B 318069-03 AGNUS into an A500, add a 74LS139 DEMUX chip to create the 4 separaet CAS signals, add the extra DRAM, and it should work!
http://home.earthlink.net/~kjones66/images/Compare.gif SO heres what you do to put 2megs chipram on the motherboard in a rev 6a A500:
First, replace the 8372A agnus with an 8372B 318069-03 AGNUS.
Next, get out your soldering iron and the following materials:
Eight 256k x 4 DRAMs
One 74LS139 demux
Some insulated hookup wire (I use 40ga Kynar)
Add A20 Address line:
1)On the back of the motherboard, Hook a wire from pin 35 of the agnus socket to pin 48 of the 68000 scoket.
Demux original CAS signals with DRA9:
2)bend up pins 7 and 12 of U35.
3)Take the 74LS139 and bend all the pins out horizontal and clip them to about 2mm (1/8" length).
4)hook a wire from pin 8 of U35 to pin 3 of the 74LS139.
5)hook a wire from pin 13 of U35 to pin 13 of the 74LS139.
6)hook a wire from pin 56 of your AGNUS socket to pins 2 and 14 of the 74LS139.
four separate CAS signals:
7)hook a wire from pin 4 of the 74LS139 to the pad on the motherboard where pin 12 of U35 used to be connected.
8)hook a wire from pin 12 of the 74LS139 to the pad on the motherboard where pin 7 of U35 used to be connected.
9)Piggyback 8 more 256k x4 DRAMS on top of the existing 8, bend up pin 17 on all 8 of the added chips. Solder all other pins to the pins on the chips below them.
10.)hook all the pin 17s together on the left 4 (U18,U19,U22, and U23)and also to pin 11 of the 74LS139.
11.)hook all the pin 17s together on the right 4 (U16,U17,U20,and U21) and also to pin 5 of the 74LS139.
clock enable:
12.)hook pins 1 and 15 of the 74LS139 to pin 3 of U33.
Hot, ground, & mounting the extra IC:
13.)hook pin 8 of the 74LS139 to pin 10 of U35.
14.)hook pin 16 of the 74LS139 to pin 20 of U35.
15.)Use hot-glue or double sided tape to secure the 74LS139 on top of some existing IC..
Getting A19, A20, XCLK, and _XCLKEN straight:
16.)Make sure that JP2 and JP5 are set as follows:
http://home.earthlink.net/~kjones66/images/jumpers.JPG DOUBLE CHECK EVERYTHING and make sure nothing is shorted/grounded etc..
NOTES:
Ok.. its possible that DRD9 and CCK might each need about a 10ns additional propogation delay.. I doubt the timing is THAT critical, but I have not built this mod, so I dont know. I will try it as soon as I can get my hands on an A3000 Agnus.
Also, these instructions are assuming that you are starting off with a rev. 6a A500 motherboard that has already been fully populated with 1 meg (eight 256k x 4 DRAMs) of Chipram."
