8520 CIA's: are odd/even socket-specific, to replace?

[kippy]

Maybe this is dumb, but why are socketed DIP 8520's always sold in pairs, as odd/even, but the manufacturers markings in photos on eBay etc. are identical between the 2 chips. If I blow one by a klutzy move, do I have to replace both? I vaguely remember some post about, 'if you have problems, just swap them and see if you now have different problems'. Are they identical internally? If not, how do you tell from outside? Is my eyesight just bad?



Also, can they be replaced by home-programmed EEPROMs of some sort (if you have the technology)? I've read that nobody has the original specs anymore, does that mean that when all the MOS/CSI chips are gone, everybody is SOL?


Inquiring minds want to know ...


{dont have a sig yet}

[Galahad/FLT]

They are identical, most times one cia chip goes down, and swapping them around usually confirms it as what was previously not working, now works.

Sometimes both fail at the same time

[torsti76]

They are, unfortunately, much more than simple memory devices like EPROMs.

For these kind of chips you need something like a CPLD (complex programmable logic device), and for the more advanced chips probably even a FPGA (field programmable gate array).

However, for these devices you need to recreate the chips in a kind of programming language first, which is a very difficult task considering that the inner workings of the chips aren't as meticulously documented as they'd need to be.

Nonetheless, there are several ongoing attempts to create replacements (or even upgrades) for the custom chips, so not all hope is lost...

[Bruce Abbott]

Originally Posted by torsti76

which is a very difficult task considering that the inner workings of the chips aren't as meticulously documented as they'd need to be.

Not that difficult. Apart from simplified Time-of-Day circuitry the 8520 is almost identical to the 6526 used in the C64.

CIA operation is not as complex or critical as other Amiga custom chips like Agnus and Denise. You could probably reproduce enough of the circuit to keep the OS and most games happy with a dozen or so standard TTL chips or a medium sized CPLD.

MOS Technology CIA