I think that's only for 2.0 or higher ROMs.
Hello!
I investigated...
At the moment I didn't test what I'm going to say "for real" but the in theory this is what should be done to use K1.2 on 1.3 motherboards (rev.6a/7) or 2.04 motherboards (rev.8 - A500+) and vice versa (KS 1.3/2.04/3.1 on old rev.5 motherboards).
First of all one clarification (implicit above):
- A500 motherboard rev.5 (mounted KS 1.2 - OCS)
- A500 motherboard rev.6a/7 (mounted KS 1.3 - OCS/ECS 1MB Agnus)
- A500+ motherboard rev.8 (mounted KS 2.04 - ECS 2MB Agnus)
These are what I found here in Italy/Europe, in my personal experience (and not only).
By seeing the schematics (ROM related part can be seen below), in the rev.5 motherboards pin 31 of Kickstart ROM 1.2 is A17 (address 17), while on rev.6/7a ones pin 31 is connected to Vcc, while A17 is located on pin 1.
The same applies to rev.8 motherboards if you pay attention that these motherboards were designed with 1MB Kickstart in mind, so the ROM socket is not 40 pin (for "normal" 256kB/512K ROM) but 42 pin.
In fact numeration reported is referred to a 42 pin socket, but if you place the ROM from the 2nd pin (as printed on the motherboard), pin 2 becomes pin 1 of the ROM, so is the same numeration as rev.6/7a motherboards.
Hope I did not create confusion... re-read carefully with image...
So at the end:
- if you want to use KS 1.3/2.04/3.1 on old rev.5 motherboards you have to rewire pin 1 of the ROM to pin 31 of the socket, and put Vcc (5V) on pin 31 of the ROM (maybe some kohm resistor is not a bad idea, to limit current).
- if you want to use KS 1.2 on rev.6a/7/8 motherboards you have to rewire pin 31 of the ROM to pin 1.
Both modifications could be done with some wires and by bending pins (maybe better if you bend pins from a further socket in between) or creating a proper simple PCB (more work, of course).
In the past (I don't know now) Kickstart switchers provided jumpers for this purpose (but I don't know if they were done for working in all motherboard revisions with ALL Kickstarts, specifically 1.2).
Hope I clarified the thing completely.
If there are mistakes or imperfections let me know, but with precise references (eg. schematics).
I'm especially waiting comments from
alexh and
Cosmos, who are very competent on this arguments, from what I've read in their posts.
BTW, I'm NOT responsible for any damages to your motherboard, ok?!
Bear in mind that for doing this you need to have at least some basic skills of electronics.
Little tech note for novices:
A0-A17 address bus: addresses 2^18 (0-17) locations = 262144 = 256kB
A0-A18 address bus: addresses 2^19 (0-18) locations = 524288 = 512kB
A0-A19 address bus: addresses 2^20 (0-19) locations = 1048576 = 1MB
BY!
P.S. The schematics are available at the end of A500 user manuals (here pics are small, sorry - I don't know how to place bigger images; maybe a better account...)
