Calc Info
I own a TI-86, TI-83 Plus and Casio fx-115 ES Plus. The TI-86 is the one I mainly use, the TI-83 Plus, not so much, and the Casio fx-115 ES Plus.
Here is my TI-86:
There are many things the TI-86 can do that the TI-83 Plus cannot:
TI-83 Plus: All caps + theta. TI-86: Lower case and upper case, symbols and International characters + Greek.
TI-83 Plus: Equation solver has the add-on "0=", which needs to be gone, but cannot. TI-86: Easy-to-use equation solver.
TI-83 Plus: Has a "i" for Complex Rectangular number systems. (in the form of a+bi and reᶿⁱ) TI-86: Has a "∠" sign for Complex Polar number systems. (in the form of (a,b) and (r∠θ)
TI-83 Plus: Needs the Scientific Tools app from Texas Instruments to use Vectors. TI-86: Can do vectors without problems.
TI-83 Plus: Each letter is a variable, only having 28 variables, including theta and n. TI-86: With a lot of possibilities, you can have up to 257,575,523,205 8-letter variables.
TI-83 Plus: Have to go to the Catalog to get hyperbolic functions. TI-86: Has a menu with easy access to hyperbolic functions.
TI-83 Plus: Doesn't have Interpolates. TI-86: Has Interpolates.
TI-83 Plus: Calculus in the Math > Math menu. TI-86: Calculus in a separate Calc menu.
TI-83 Plus: Needs the Scientific Tools app from Texas Instruments to use constants. TI-86: The constants are built into the calculator, plus you can make your own.
TI-83 Plus: Needs the Scientific Tools app from Texas Instruments to use conversions. TI-86: Conversions come preloaded into the calculator, plus they don't always show the annoying scientific notation form.
TI-83 Plus: 5-letter statistical lists. TI-86: 8-letter statistical lists.
TI-83 Plus: Doesn't have Base numbers. TI-86: Has Base numbers.
TI-83 Plus: Needs an app for simultaneous equations and polynomials. TI-86: Already has simultaneous equations and polynomials..
TI-83 Plus: 10 functions. TI-86: 99 functions.
Et cetera.
This is similar to its cousin, the TI-85. Sadly, both the TI-85 and TI-86 are discontinued, but you can buy a used one, or use an emulator to use one. I use Virtual TI. It's a bit old, but still workable on Windows 10.
Here is a program called KeyCodes, where it helps you use the "getKy" command:
Lbl TOP
getKy→Key
While Key=0
getKy→Key
Disp Key
Goto TOP
When you press a key, you will get the key code to help you use the getKy command. For example, F1 yields "11" and Graph yields "41". To exit, press On.
This is another one of my faves. It has trig, hyperbolic functions, complex systems, parenthesis, statistics and more, along with 2ndF to select a second function.
Update: 3/5/2020
Unfortunately, when I turned on my TI-86 around this time, the screen corrupted and the keyboard basically becomes a brick. What happened is that I had a mesh backpack and when it rained, it probably got inside the calculator that was in my backpack. I still have my TI-83 Plus, but I currently can't use it.