Will these RAM chips work for DIY upgrade?

[Zetr0]

Off the top of my head,

To add an additional 1MB of DRAM (bringing to a total of 2MB) to the A500+ (A500 plus) motherboard you will also need to take the RAS and CAS from the piggy-backed DRAM chips (lift the relevant legs) and feed this to the RAS1 and CAS1 on the trapdoor slot.

You could easily achieve this by daisy-chaining 30awg wire from each RAS of the Piggy Backed RAM to the next - leading up to the trap-door, and then do the same for each CAS as well... I would suggest differentiating colours (Blue and Red work well )

That is all there is to it.... I would suggest checking the height of the piggybacked RAM chips in regard to hitting the underside of the keyboard.

[Zetr0]

From my research here is the following.

Your IC's should be compatible - I have checked these against the TMS44C256-80 that are on my A500+

Here is the pin out -




The pin-out to your chips can be found by clicking here

The RAS0 and RAS1 are not touched in the following augment only the addition of BCASL1 and BCASU 1 are used.

(big picture - sorry about that)




As you can see the banks are dealt with in blocks of 256k by 16 bit wide (512KB each bank)  The A500+ uses BCASL 0 and BCASU 0 for the 1MB of RAM on the motherboard.  And should there be RAM in the "trap door" expansion slot then it uses BCASL 1 and BCASU respectively for those two further banks.

512KB Expansions only make use of the BCASL 1 signal as far as I can work out.

Alternatively you could piggyback 512KB of ram on an A501 half-meg RAM upgrade (or clone) and just pull-up the CAS Pin (17) on the chips and then daisy chain this signal across the 4 piggyback chips and tie this to Pin 36 of the expansion bus for BCASU 1

[Zetr0]

@Astral

No worries, glad it helps.  I would suggest to buzz-out BCASL1 (pin 35) on the CHIP expansion bus - just to ensure that the A501 is using the Lower CAS (1) and not the Upper CAS (1).

Also - remember to take pictures for our fellow forum chummies =)