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Author Topic: 2MB Chip RAM directly on A500 rev. 6 motherboard?  (Read 14490 times)

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Offline Metalguy66

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Re: 2MB Chip RAM directly on A500 rev. 6 motherboard?
« Reply #44 from previous page: November 10, 2008, 10:03:09 PM »
Ok. Im bumping this thread back up, because I like talking about A500 stuff...

Converting a rev 6c A501 into an A501+

So you got a rev 8a A500+ motherboard.. Its got a nice 8375 2meg AGNUS chip, but only 1 meg of chipram on the motherboard. If you want 2 megs of chipram, you generally have 2 options.

1)piggynack 8 more 256k x 4 DRAMs on top of the existing ones on the motherboard.

2)Buy an A501+ (or aftermarket equivelant) that puts an extra meg of chip ram in the trap door expansion slot.

Both of these are fine alternatives.. But if you have an old A501 (512k expansion) laying around that you arent using, you can easily upgrade it to a full meg, providing a third alternative for gaining 2 megs of chipram.
Heres how:

NOTE: This will only work on the rev 6c A501 board. If your A501 is one of the earlier revisions that has 16 DRAM chips, this mod will not work!

1) Find jumers JP2 and JP3 on the A501 card. These are set to enable the "refresh kludge" from the factory. We dont need a "refresh kludge" because our our 8375 generates refresh signals to handle the full 8-bits of row adressing needed by our 256k x 4 DRAMs. So get a razor blade or X-acto knife, and carefully cut the tiny trace that goes between the center and upper pad of JP3. Then cut the tiny trace that goes between the center and lower pad of JP2. Now put a blob of solder to connect the center and lower pad of JP3. Add a second blob of solder to connect the center and upper pad of JP2. CHECK YOUR WORK WITH AN OHM METER!

2) Now get 4 extra DRAM chips. These should be 20 pin DIP package, 256k x 4, 120ns or faster. Bend up pin 4 on each of these DRAMs.

3) Piggyback each DRAM on top of one of the 4 DRAMs on the A501 board. Solder all pins to the chip below, except pin 4 (which should be bent up.)

4) Solder a wire connecting all of the bent up pin 4s, and also connecting to the solder pad directly above JP9. (see picture)
This creates the additional RAS signal we need for the extra 4 DRAMs.


Thats it! Stick it in your A500+, boot it up, and you should have 2 megs of Chip Ram.

What about the Realtime Clock you ask?

By default, your Rev 8a A500+ motherboard will use it's on-board RealTime Clock, and ignore the one on the A501 card. If you would like it to use the Realtime Clock on the A501 card, then close jumper JP9 on the A500+ motherboard.
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Offline fiat1100d

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Re: 2MB Chip RAM directly on A500 rev. 6 motherboard?
« Reply #45 on: March 26, 2009, 10:15:55 AM »
Quote

alexh wrote:
A600 use AGNUS part numbers 390544 only on v1.0 motherboard. They use 318069-10(pal) and 318069-11(ntsc) on motherboards v1.3, 1.5 and 2D. No idea what the pinout differences are (if any).


As far as I know, 390544-01 and 318069-10/11 are pin compatible, since I once bought a MegAChip which had the NTSC 318069-11 and then bought on eBay a small lot of PAL 8375s. Mostly were 318069-10, but there was one 390544-01 too, and they all worked on the MegAChip. Also tried 318069-10 on A500+ and it works. Of course I had checked schematics of A600 (with 318069-10) and A500+ (with 390544-01) first :-)
 

Offline Wild Cobra

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Re: 2MB Chip RAM directly on A500 rev. 6 motherboard?
« Reply #46 on: September 10, 2010, 02:41:51 AM »
Quote from: czarek;422974
Hi, I know it is possible as I saw few people did it. My question is - how?
I have already soldered whole megabyte onto motherboard and re-soldered jumpers so it's visible as 1MB Chip RAM. The possible problem may be that it's pre A500+ motherboard. It had space for 1MB ram and 8372 Agnus, but doesn't have space for RTC circuit and when I put 8375 Agnus it just gives me black screen. Was no problem with replacing Denise to full ECS though.
So all of you A500 experts - please share your secrets :-)
Thanks in advance!
I posted the instructions back in '92 on a BBS, but I doubt we can find that now.



 

Offline Wild Cobra

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Re: 2MB Chip RAM directly on A500 rev. 6 motherboard?
« Reply #47 on: September 10, 2010, 02:48:48 AM »
Double check a few things first. I reconstructed this modification mostly from memory without an actual rev 6A in front of me. I found the photo that I cropped and modified on line, along with schematics. When I did this years ago, I carefully verified actual connections. I'm pretty sure I'm right, but cannot guarantee it.

Verify that the top right pad of the 2x4 pads goes to pin 35 of Agnus and the top left goes to pin 48 of the 68000 (Address A20). Pin 35 is the external clock for the "A" version, but A20 for the "B" version. It's hard to read the online schematic I found, but Agnus pin 56(?) is the /RAS1 for the 2 bank memory, 8 256k x 4 chips for 1 meg, and /RAS1 isn't used for just 1 bank of 4 chips. /RAS0 is. With the (B) Agnus, it is DRAM address 9 (DRA9). This should go to pin 2 of U35 if I read the schematic right.

You see, you can simply add four 256k x 4 memory to it and have 1 meg chip memory. Only /RAS0 is needed for 1 bank.

The 2x2 jumper changes should reroute /RAS0 (after latching though the 74F244 U35, pin 15 to 5) from the occupied bank of 256kx4 pin 4 to the unoccupied bank pin 4 where the 1 meg x 4 will be placed. What was /RAS1 (8372A) is A20 (8372B), latches through the 74F244 U35, 2 to 18, rerouted to pin 5 of U16 to U23. In the first bank, U16 to U19, pin 4 and 5 are connected, so the 1meg x 4 memory cannot be used there. The 256k x 4 memory chips pin 5 is NC, but A9 for the 1 meg x 4.

A500 rev 6a and rev 7 schematic

JDEC memory pinout goto PDF page 77

A500 board photos
« Last Edit: September 10, 2010, 02:57:52 AM by Wild Cobra »