Jumpers (Amiga 500)

the amiga 500 has various jumpers on the board below is the explanation as to what each does jp2 the jp2 is located to the top right of the bios it controls if the upper the jp2 jumper middle pin connects to the angus a19 (pin 59, 6 pins in from bottom left) the bottom pin goes to a23 (of gary and 68k), and the top pin goes to a19 (of gary and 68k) by default, the middle pin has a trace connecting to the bottom, configuring the angus a19 to the gary/68k a23 if you cut the bottom to middle trace, and solder the middle to top, this maps angus a19 to the gary/68k a19 jp2 pin 1 2 (default) = upper ram and expansion ram all mapped to chip ram jp2 pin 2 3 = upper ram or expansion ram mapped to 512kb max of slow ram (if both upper ram and expansion ram installed, console won't boot) jp7a this jumper is used when jp2 is changed from the default configuration to having the middle and top pin soldered then, jp7a controls if the slow ram is enabled slow ram is enabled by grounding the middle pin you can do this by soldering the jp7a to the left ground pad, so it is always enabled, or by soldering it to the right pin going to the trapdoor expansion port, allowing the external ram to control if it enables it (by grounding it) jp2 pin 1 2 (default) = jp7a ignored jp2 pin 2 3 = jp7a when grounded adds upper ram or expansion ram to slow ram expansion ram setup the recommended setup is jp2 pins 1 2 (default) and jp7a floating, to assign all ram to chip ram to make the above simple to understand here is what will happen, given your console has all 1mb of ram (512kb lower, 512kb upper) installed, and an expansion ram board with 1mb optionally installed or removed jp2 jp7a expansion ram total ram pin 1 2 (a23) pin 1 2 (gnd) 1mb won't boot pin 1 2 (a23) floating 1mb 2mb chip ram pin 1 2 (a23) pin 2 3 (ext port) 1mb 2mb chip ram pin 1 2 (a23) pin 1 2 (gnd) empty won't boot pin 1 2 (a23) floating empty 1mb chip ram pin 1 2 (a23) pin 2 3 (ext port) empty 1mb chip ram pin 2 3 (a19) pin 1 2 (gnd) 1mb 512kb chip / 512kb slow ram pin 2 3 (a19) floating 1mb 512kb chip ram pin 2 3 (a19) pin 2 3 (ext port) 1mb 512kb chip ram pin 2 3 (a19) pin 1 2 (gnd) empty 512kb chip / 512kb slow ram pin 2 3 (a19) floating empty 512kb chip ram pin 2 3 (a19) pin 2 3 (ext port) empty 512kb chip ram if the expansion ram grounds this pin, then the expansion will be used as 512kb slow ram if expansion ram grounds jp7a, and 1mb if internal ram is installed, it won't boot note when jp2 is default, and jp7a gets grounded, it won't boot this is likely caused by the 512kb upper ram being installed and conflicting i will confirm this in future that with a 512kb system that jp7a grounded allows the expansion ram to map to the chip / slow ram if jp2 is left in the default configuration a23 (pin 1 2, bottom and middle), then jp7a must be left floating, otherwise it will not boot if grounded by the board or expansion ram (this is possibly only when 1mb internal ram is installed), and the expansion ram is then assigned to chip ram if jp2 is set to the a19 pin (pin 2 3, top and middle), then the upper ram (or expansion ram if not upper ram is installed) is mapped to slow ram when jp7a is grounded (by the board or expansion board)