IIRC SCC is not just I/O based?, so, what slot/sub-slot don't matters.
I recall it contains memory mapper, and it is being accessed through memory addresses. Is there any good documentation / data sheet for this chip?
SCC is not I/O based but memory mapped (which means you can use multiple at the same time with different data). It can be used fine in a subslot, but not all (cracked) software searches for it in a subslot, which is wrong, of course. Many people use an SCC in a slot expander.
It's easy to try, just put an SCC in the slot but cut the SLTSL and solder it to the slot select of a subslot of slot 3 (The S3527 is you have a Philips).
Indeed there is software which does not work, eg. the original Solid Snake does not work when the SCC is in the same primary slot as the main ram for some reason. I also know two demo's, one does not support subslots and the other only knows slot 1 and 2.
Will SCC and other related software (e.g. games) work if connected to expanded slot - e.g. in SLT01, or SLT31 (with main memory in SLT32)?
Test: philips vg8235 and 8245 1024k internal ram expanded
if you use slot 3.1: sd cartridge with scc:
for example metal gear 2 or space manbow dsk version, game starts but you not get scc sound. However the same test works fine in slot 1 or 2
Usually they work if you select the subslot with the SCC in the expanded slot first. I've used that tool by Jasper Lautenbag for that with success.
with philips 8235/8245, metal gear 2 disk version, scc chip is detected in slot 3.1 without problem. However there is no scc sound (with 1024k internal ram scc sound must work).
