A2048

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Use in A500+ computer

The A2048 can be used in several different configurations, but is mainly meant for use on an A500 board revision 8. If that board is fully equipped as A500+ mother board, you can use the A2048 as a +1MB chipram expansion with this jumper setting:

Use as 2MByte chip ram in A500 Rev.8

The most likely reason why you've come to this page is that you own an A500 Rev.8, which is not a full A500+. The beauty if this board revision is that it is equipped with a 2MByte chip ram Agnus, and the A2048 lets you take advantage of that. A few simple steps in modification are required, and they include cutting traces and soldering. Do not attempt this modification if you're not comfortable with such modifications or if you don't have the required practise in modifications like these. While the end result will be a very cheap way to get your A500 to 2MBytes of chip ram, a possible outcome may be that you destroy the computer beyond repair if something goes seriously wrong. We therefore emphasize that you and only you alone are responsible for executing this modification. We've done everything possible to make these instructions simple, but can't be held responsible for any damage that you do to your gear.

Step 1: Test in 512k config

First of all, you should make sure that your computer works with a trapdoor memory expansion at all. This will test all signals towards the connector and might give you some peace of mind. This jumper setting will downgrade the A2048 to a simple 512k expansion:

Step 2: Route address line A19 to Agnus

By default, jumper JP2 of the Rev.8 main board is connected "top-middle". This routes address line A23 to the Agnus chip. For 2M Chip ram operation, you will need address line A19 in that place. Open the connection from "top to middle" and close the connection from "bottom to middle" as in this picture:

Step 3: Route DRA9 signal to EXRAM pin

By default, the 56-pin connector of the A500 has a signal that identifies a 512k memory expansion. This signal is called "EXRAM". We will not need it any more, so the pin is free for routing the additional signal that is required to address more memory. Luckily, the Rev.8 board has jumper JP7 to (de-)activate the EXRAM function. We are abusing this jumper to route the DRA9 signal to the pin. Separate all three pins of the jumper and solder a wire to the rightmost pin of the jumper as shown in the pictures below. The wire should be 9cm long (about 3.5 inch). The other end of the wire must be soldered to pin#2 of the unpopulated U32 chip - that's where the DRA9 signal is easily available.

Step 4: Deactivate on-board chip ram

The local 512k of chip ram is not required any more and must be deactivated. You do this by pulling the CAS lines high (connect to 5V). These CAS lines are routed through jumpers under the unpopulated U32 chip: Separate all pads of these jumpers first, then make new connections as in the pictures below. The "link 1" and "link 2" wires to to pin #16 (top right) of U32, which remains unpopulated. Further, close the "middle-bottom" connection of the two jumpers to route the Agnus-side CAS signals to the trapdoor connector.

Step 5: Verify A2048 jumper setting

The A2048 must be jumpered properly to make use of the DRA9 line on the EXRAM pin. Here's a picture of the factory default setting that we use to test the A2048 prior to packing/shipping:

Step 6: Insert and test!

Now insert the A2048 expansion and launch your computer. You will notice a slightly longer startup time, as the computer needs a tiny bit more time to test all the memory. Remember to put the shield back on when re-assembling the computer!

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