Today, Runer112 contacted me with a rumor that the TI-83+ (the original black edition, as opposed to the Silver Edition of the TI-84+/SE) did not perform the type of port mirroring that the community has long assumed and is documented in WikiTI. The evidence came from another community member, Juju, who failed to have the adverse affects on his calculator that writing to port $FF should have caused. This evening, I dug through my dozens of calculators and found one with a physical z80 chip, serial 131424476N-1099A. With a small test program I made using port $41 to read from and write to the keypad (normally port $01), I verified that reads and writes are properly mirrored. I then tried on a newer calculator with the combined bus arbiter/processor ASIC, serial 2265064268S-0402C, and verified that it performed the same way. For reference, its mainboard is labelled 9TA83ASMB-31G.

Therefore, the question remains whether there are some calculators that do indeed display this odd asymmetry in port mirroring that was reported, but not my calculators, or if there's something else at play here. I'll continue testing a few of my other calcs.

Actually, I had a lot to do with this too. What I wrote at the top of the ports page on WikiTI is exactly what happened on Juju's calculator, or at least what we believe happened.

If your calculators do something different, then we'll have to figure out what the pattern is. I wouldn't doubt that different hardware revision handle the extra ports differently.

Sadly though, I have only seen 2 83+s in my life, so...

As I said, I tried two of my TI-83+s, one with the discrete Zilog z80, one with the integrated ASIC. Both displayed identical behavior, mirroring both port writes and reads. I'm trying to get Juju to post details of his calculator in this thread; among the 15 or so TI-83+ calculators that I have on my shelf, I'm sure at least one of them exactly matches his hardware revision. Based on taking all of mine apart over the years, I believe there are at most four different possible versions, so if you're sure that the behavior that Runer112 described to me is accurate, I can try a few of my other calculators and see what they do.