Can an A590 be upgraded to 8Mb (+ more)

[Zac67]

Hmm...

Never heard of this before but on checking the schematics there is a simple note "no connect on DRAMs are connected together to facilitate future conversion to 4M part (1Mx4)" - however, there's nothing more indicating that it's actually possible. It's not shown if the RAM controller U5 could provide muxed A9 for 1Mx4 operation, but it's fed the necessary inputs. You could check where pin 5 (NC) of the DRAMs leads to, but U5 is lacking the required output pin... Makes me seriously doubt whether this is possible.

As Azryl noted DIP20 packages of 1Mx4 DRAMs would be required, usually they come in DIP26.


Theoretically, the side expansion slot could be used for >8 MB RAM expansion, but only in conjunction with a 32 bit CPU upgrade to overcome the Zorro II space limitation. (Something in the line of a GVP A530, but with an extended RAM controller.)

[Zac67]

Makes me seriously doubt whether this is possible.

I'm not absolutely sure it's impossible, but unless you find someone who actually made it work I wouldn't believe in it.

PS: if you're really adventurous you could build an adapter board to fit DIP26 chips into the DIP20 sockets...

[Zac67]

Since DRAM uses a multiplexed address bus, the change from 256Kx4 to 1Mx4 requires an additional address bit (10 instead of 9) which the RAM controller (U205, 16L8 PAL) would have to generate (U8 & U9). From the schematics I have, there's little to indicate that this is actually the case. U205's output pins are completely used up and there's no spare anywhere to generate that bit. Unless a later rev (than the 7 I'm looking at) added this somehow there's no way this could work.

The A2091 controller is identical apart from the form factor and I haven't heard of anyone making it work with 8 MB RAM either, so finding DIP20 1Mx4 RAMs is probably the easier task...