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Jumpers on PCB 1.1
On A314 PCB version 1.1 there are two jumpers, JMP2 and JMP3. Here's information about how to set them.
JMP2 is related to the 1 MB chip RAM mod for Amiga 500 motherboard rev 6.
In order to understand how to set jumper JMP2 you should understand the two steps of the 1 MB chip mod (as described in this image):
- Cut JP2 on the motherboard and re-solder in the opposite direction.
- Cut JP7A on the motherboard.
The reason for cutting JP7A in step 2 is to disconnect the trapdoor EXRAM pin.
When JMP2 on A314 is closed, the EXRAM pin is connected to ground, and when JMP2 is open the EXRAM pin is disconnected. That means that by leaving JMP2 open, step 2 of the 1 MB chip RAM mod is not needed. Step 1 of the mod is still needed.
On motherboards that do not use the 1 MB chip mod, JMP2 should be closed.
JMP3 is used to support both Amiga 500 and Amiga 500+. A trapdoor memory expansion for an A500 provides 512 kB RAM while a trapdoor expansion for A500+ provides 1 MB RAM.
The trapdoor expansion port has two RAS pins, RAS0 and RAS1, that signal the start of a memory access.
- In an A500, RAS0 indicates an access to the 512 kB memory on the motherboard, and RAS1 indicates an access to the 512 kB of trapdoor memory.
- In an A500+, RAS0 indicates an access to the lower 512 kB of trapdoor memory, and RAS1 indicates an access to the upper 512 kB of trapdoor memory.
JMP3 controls if RAS0 is connected (closed) or disconnected (open) from A314. Hence:
- In an A500 JMP3 should be open.
- In an A500+ JMP3 should be closed.
If JMP3 is closed in an A500 then A314 will respond to accesses that are intended for the motherboard RAM, which will not work well. It is possible, however, to disable the RAM that sits on the motherboard. By cutting jumper JP3 on the motherboard, and setting JMP3 on A314 to closed, then all memory accesses will be served by A314.