As many people know, I have extended my work with documenting the hardware and ROM versions of the TI-82 over to the TI-81, and my current findings are published here. Throughout the research, I purchased three TI-81s: 0719871 I-0790, ROM 1.1; 1210793 I-1292 'B', ROM 1.8K; and 0804828 I-0893 'D', ROM V2.00. I targeted the undumped ROM versions to add to my collection; ROM dumps can be expected in the next few months after this initial posting.

Hardware Versions
There are at least 7 distinct variations of the TI-81, each with a unique external physical appearance:

1. The original TI-81, which began production around the beginning of 1990. As well known, these do not have a backup battery; however, these do not have a bar below the [ON] button. Additionally, the place where the backup battery should be instead contains two plastic rings (see the detailed comparisons below for a picture).
   
2. Still in pre-'A' hardware revisions, the second variant of the TI-81 was introduced in May 1991 to prepare for the implementation of the TI-81 'A' and TI-85. A plastic bar was added below the [ON] button to prevent accidental presses. The backup battery's socket and labeling is present under a flat plastic cover, but there are no contacts under the cover.
   
3. The third variant of the TI-81, hardware revision 'A', added a backup battery. The battery information on the back of the calculator was updated to indicate this.
   
4. The timing coinciding closely with the release of the TI-85, the fourth variant of the TI-81 (hardware revision 'B') drastically improved the quality of the display and changed the texture of the screen cover to the even texture used in all later calculators until the TI-73. The screen was upgraded to one with viewing characteristics quite similar to the TI-85's screen, with blue-on-green pixels and a wider viewing angle. Due to a change in the contrast-setting code, these all have ROM 1.8K. This variant had a long production run, from April 1992 to sometime around May 1993.
   
5. Unfortunately, the fifth variant of the TI-81 undid these improvements. Externally, the most evident change is the housing plastic is no longer a bluish gray but rather a dark blue. The display bezel still uses the style of the TI-85 and variant 4 TI-81s. The mold used is very similar to that of the TI-82, differing only by the missing link port. Internally, this variant (and all future ones) use TI-82 PCBs and display drivers with TI-81 RAM and ROM chips. There are a few other changes, mostly relating to the slower CPU clock (either 4 or 5 MHz vs. 6 MHz) and different RAM/ROM chips. Different pinouts are compensated for by using zero-ohm resistors to connect the correct pins. All TI-81s of this variant that were checked use ROM V2.00.
   
6. Towards the beginning of 1994 or the end of 1993 (I am not certain due to the very poor readability of date codes on the dark plastic TI-81s) the display bezel was restyled to match the TI-82's styling, with the "TI-81" text bolder and to the right, while the TI logo was moved to the far left of the bezel labeling area. Internally, the RAM chip was changed to use a Suwa Seikosha SRM2264 in place of a Sharp LH5160N (due to a very limited sample size, this is not confirmed), and the ROM version was updated to V2.0V.
   
7. At the beginning of 1995, the TI-81, TI-82, and TI-85 all received a cosmetic change to their display bezels. On the TI-81, the display bezel was made much narrower, the background color is the same blue color as the calculator's plastic, and all text is white. "TI-81" is no longer italicized. Internally, the PCB was updated to version 31A (also seen on the TI-82). No other internal changes were made.
   

I have taken comparison pictures of my own TI-81s: the backup battery area between the first and fourth variants, the texture of the display bezel between the first and fourth variants, the printing on the display bezel between the first, fourth, fifth, and sixth variants, the serial number area between the first and fourth variants, and the [ON] button area between the first and fourth variants.


ROM Versions
Eight ROM versions are known, dumped versions are in bold:

• 1.0
  
• 1.1
  
• 1.1K
  
• 1.5K
  
• 1.6K
  
• 1.8K
  
• V2.00
  
• V2.0V

Very early TI-81s, along with at least one produced in December 1990, use ROM 1.0. Version 1.1 is uncommon, and is present in both known July 1990-produced TI-81s. Shortly after 1.1, 1.1K was introduced and is common towards the end of 1990. Around March 1991, ROM versions 1.5K and 1.6K were introduced. 1.8K is paired with hardware revision 'B'; V2.00 was introduced when the plastic color changed to dark blue, and V2.0V was introduced alongside the 1993 screen bezel redesign.

Interesting stuff! I've updated the ROM list on ticalc.org.

I didn't know that the casing color changed during production. I did notice differences in screen/key printing in some photos when I was young, and I used to wonder about that. For instance, my TI-85 manual has a photo with slightly smaller digits on the number keys than my (relatively late) revision calc. Do any of yours have this difference?

The display cover difference is interesting, too. The smooth covers are held on around the edges with adhesive and can be pulled off (to remove dust that collects underneath, for instance). I wonder how the two-texture cover works.

Neat! Any new versions I find will be reported here, but given Datamath's searching it is unlikely I will find anything they haven't.



Yes, and I can tell you when it happened . Sometime between March and May 1993, TI updated their product line (then consisting of the TI-81 and TI-85) to feature these larger key labels; when this changed, so did the housing color of the TI-81. Also note that the earliest TI-82s found so far are from May 1993, which is within the date range that the key label size changed. Are these all related? Probably.

I use the key label size change to quickly identify older TI-85s from image thumbnails, as it was changed very close to a year after it entered production. For TI-81s, the different housing color is an obviously faster method, and due to the large number of 'B' TI-81s, doesn't really help me find older ones. Instead I use the paler screen color and presence of specular reflections on the screen to quickly spot the pre-March 1993 ones, and further narrow this down by checking for a bar below the [ON] button. These differences are incredibly helpful in spotting older ViewScreen TI-81s, as these usually (for pre-1993 models, always?) lack date codes or even serial numbers.


The two-texture cover is still made from one piece of plastic (if you zoom in on the picture, you can see there is no discontinuity in the plastic itself), but was probably harder/more expensive to manufacture since the textured area had to line up with the background ink. The actual texture of the clouded area is much finer than the texture of the later display covers, so it was more prone to cosmetic damage. The glare introduced by the glossiness of the display cover seriously hampers viewability, and the display itself is crappy enough. Other than these differences, the cover is exactly the same.

Bit of a necropost but I just found a very interesting TI-81.

The back of the case is light blue but in the mold for the 5th version (Made in taiwan ROC. with the markings for a backup battery) but there is no backup battery. The backup battery area is like the 1.6k's but its some weird shade of blue.
It has an engraved R with the sticker residue from a refurb sticker (which might explain some of the weirdness), but it also has an engraved N??

The front half of the case is the rev B variant with the thick on-button bar so it seems the back and front case plastics aren't from matching molds...
I don't know if the number in place of the serial number is original since its very different from the normal stickers and I've never seen such a sticker...
If the serial number is in fact a TI serial, that would place it in 0493, a very interesting time since TI was making big changes to the TI-81 and presumably working hard on the TI-82 in that month. This would also place it around the right time for 1.6v
Anyways here's a few pics
I'll take a close look at it when it arrives.

Very interesting - I'd imagine there would have been quite a few refurbs over the years. I think I saw one with the same battery backplate style somewhere too (ebay?).

It arrived today and I'm a little intrigued by what's inside.
There is a tantalum bead capacitor that was added to the main battery terminals which essentially serves as a backup battery, keeping the RAM powered for a short amount of time after a battery pull. I'm not sure why they decided that they needed to do this since that board doesn't normally have a backup battery...




Another interesting thing is that since the mold used for the back portion of the case was the dark blue 5th version, it had a spot for the backup battery which was covered up with some random piece of plastic cut out from something.



The ROM version is 1.6K which is a fairly common version from 1991.
The board itself is also a common known revision that was produced from 0990 to 0192. Judging by the date codes on the chips, this particular unit would have been initially manufactured towards the tail end of that range (after august 1991). I've got no way of knowing when it was refurbished but its pretty safe to assume it would have been at the very least a few months after it was manufactured since getting things to stores and stuff takes time. By then, the revision A was being manufactured which had a backup battery.
I still don't know what the stamped "N" stands for. Maybe "No crashy crash" lol