In about a month, when I have a few good free days, I will try swapping T6A43 and T6A49A ASICs for myself and testing the results for fun as well. The T6A49A chip will come from my spare I-0990 TI-81 with corrosion and a bad display, so little of value should be lost.

Now, I can't seem to decide whether to try this procedure out on the TI-85 or TI-86, so I'm going to open this decision up to everyone. I may consider swapping to the unchosen model later, but I want to be real to myself about the time and risk involved.

Let me know if there's any gotchas to swapping the ASIC. I am not the best at telling pinouts, but the chips look compatible enough to me.

I'm not aware of anyone transplanting those early ASIC's ... I assume they would be pin compatible?

Maybe someone like @Zeroko might know more?

The T6A49A seems to be pin-compatible with the T6A43...there exist TI-81 revisions that have one or the other on the same motherboard & with the same other parts populated, as nearly as I can tell by looking at the pictures on DATAMATH. But I am not going to guarantee it will not explode in a TI-85 or TI-86, because you never know what exactly TI did change between them. On the other hand, it might not do anything at all different...it is possible that the differences only matter to the PS-6600 (which used the T6A43 as well & used some pins none of the calculators ever used), or something.

The TI-86 has extra hardware relative to the TI-85 (in particular, multiple RAM banks & a low battery detection circuit), so it would be more interesting to test it in one of those than in a TI-85. At least unless it fails to support the extra RAM, but that seems highly unlikely, given that switching between the two banks in a TI-81 with a T6A49A works fine.

Note that I have tested the T6A49 & T6A49A in TI-81s that come with them & not noticed any significant differences between them & a TI-81 with a T6A43. Especially not the T6A49A, which if I am not misremembering showed zero software-detectable differences. There was a difference with the T6A49 (that has zero effect on anything the OS does), & TI-81s with T6A49s seem to always contain some extra circuitry relative to those with T6A49As, so it is plausible that those differ more significantly.