Question about A500 Plus ram expansion

[dougal]

Looking through my Amiga misc items I found an A500 Plus trapdoor expansion.

Its like this: http://amiga.resource.cx/exp/marpet500

But it only has two simm slots with two 256k simms. Total 512K

It is def an A500 Plus expansion - No clock/bat and has a barcode sticker with A500+ written on it.

I have another one which i use with 4 slots and total 1MB for A500+

Question is, if I had to replace the SIMMS with 512K ones would it turn into a 1MB expansion ?

Or would I need to solder 2 extra angled simm holders onto the board and add another two 256mb simms ?

There are solder points for an extra two sockets/holders

 

[BLTCON0]

...

Question is, if I had to replace the SIMMS with 512K ones would it turn into a 1MB expansion ?

Absolutely not, even if such SIMMs actually exist (addressing is multiplexed on these DRAM chips (RAS/CAS), so the addition of another address line is equivalent to quadrupling the addressing capacity, hence from 256 KB the next logical size would be 1 MB and then 4 MB and then 16 MB).

Or would I need to solder 2 extra angled simm holders onto the board and add another two 256mb simms ?

Yes. The A500+ sees the 1 MB trapdoor expansion as two memory banks of 512 KB each. Each memory bank is 16 bits wide, so it requires two 30 pin SIMMs (each such SIMM is 8 bits wide).

There are solder points for an extra two sockets/holders

I would expect the cost reduced 512 KB version to simply skip the 2 SIMMs, but not the SIMM slots themselves! If this was a factory configuration it's rather shameful and resembles Commodore tactics