Amiga 500 vs 500 Plus Hardware Compatibility

[Tenacious]

What do you mean? The a500+ has 1mb of chip ram on the motherboard, if you want more ram then you can use the trapdoor.

Right.  There was a special version of the trapdoor for the plus that contained 1MB of memory (bringing the total to 2MB of chip).  Still available?

The plus has battery on the motherboard.

[Tenacious]

What version Agnus (probably won't make a difference, even OCS would see it as slow-fast ram, but still)...

I've always thought that Agnus provides the refresh for CHIP and SLOW/FAST memory.  The limitation is the amount of memory the various Agnus versions will refresh, the last version (as used in the A3000, A600, and A500+) refreshing up to 2 MB.  True FAST ram (like Supra's 500RX, etc) is accessed by the CPU alone and these expansions include their own refresh circuitry.

AS a side note, my A500 Rev 5 has empty sockets for another bank of 512k on the motherboard.  If the original chips are pulled and (1M x 1? chips) are installed in all positions, one can have 2MB of chip on the motherboard.  It also requires the 2MB Agnus be installed.  I did this about 20 years ago, but, now I can't find the docs I used for the mod.

[Tenacious]

Nice hack!  A500 obviously doesn't have an "upper row", different density of ram chips in most models... might still be do-able.  Interesting project, not for the faint of heart though!  

This is very similar to the modification I referred to above.  You are right about the trap door memory, mine had to be disabled except for the RTC.  This machine is still working great with 2 MB of CHIP on the motherboard.  I will look some more for the docs I used (Some one else wrote them, I simply benefitted ).

I haven't done it since because the 1 MB CHIP ram modification is adequate for my needs and that money is better spent on true FAST ram (even more memory and better performance gains).

[Tenacious]

I found some notes, and, it appears that the OCS Agnus 8371 (8370 for NTSC) was introduced with the early A500s.  It would address only 512kB of CHIP and 512kB of SLOW/FAST trapdoor memory.  It would produce PAL video modes.  

There was an earlier DIP design for the A1000, 8361 for NTSC and 8367 for PAL.

When Commodore introduced the ECS Agnus (8372A, 8375-PAL, 8375-NTSC), even though these would address 1 MB of CHIP, they still chose to map the trapdoor (512kB only) memory as SLOW/FAST on all A500s (not +).  This is where the pad-jumpering mod came in to remap the trapdoor to CHIP.  IIRC, Commodore actually recommended this mod for users of the A570.

This is where my notes get confusing.  The 2 MB Agnuses (8372B for the A3000, and the confusing part, 8375 for the A500+ and A600) were introduced and all the memory (motherboard and trapdoor, if present) was mapped as CHIP only.  Motherboard memory and trapdoor memory jumped from 512kB to 1 MB each at this time for the new A500+ and A600.  

The 8375 is listed as a 1 MB and as a 2 MB Agnus.  Would it do 2 MB all along?  Did it come along significantly later than the 8372A?  Perhaps it was the first 2 MB design?

Corrections?

EDIT:  Wikipedia has this to say, though it seems to not cover a number of exceptions.  http://en.wikipedia.org/wiki/MOS_Technology_Agnus

[Tenacious]

Long story short, there are more versions & revisions of Agnus than I have hairs on my head.  

Does that green helmet in your avatar hide a profusion of revision, err hair?