Thursday, September 26, 2019

Retro Review: Texas Instruments TI-66

Retro Review:  Texas Instruments TI-66

Quick Facts

Company:  Texas Instruments
Years in Production: 1982 to 1985
Original Price: I estimate the original price to be from $70 to $100
Battery:  2 x LR44 or 2 x AR76
Type:  Scientific, Keystroke Programmable
Operating System: AOS
Number of Digits: 10
Memory:  512 steps, which can be allocated to a maximum of 64 memory registers
Accessories: Carrying Case, PC-200 Printer

I paid $7.99 at a Goodwill in Cerritos, CA.  The calculator came with the carrying case, User Guide, and Quick Start Guide, all boxed up.  What a lucky find!

The Next Generation of TI-58C

The TI-66 was slated to be the next generation of the TI-58C of 1978.  Virtually the keyboard design and commands of the TI-58C are present in the TI-66, but there are some differences:

*  The TI-66 is an landscape form with a gray keyboard, where as the TI-58C (and the related TI-58 and TI-59 has a black keyboard).

*  The TI-66 lacks a card reader.

*  The TI-66 also lacked a slot for modules. 

*  However, the TI-66 runs on coin batteries (AR76 or LR44), which can be found easily.  The TI-58C family had rechargeable battery packs, which cannot be easily purchased (few vendors sell them online).

*  In learn mode, when a step is registered, the TI-66 displays the key you just pressed with its mnemonic. 

Programming With The TI-66

Keystroke programming is fairly simple with the TI-66.  As I mentioned before, the TI-66 displays a mnemonic when a key stroke is registered (think HP 42S, HP 41C, or HP 32SII).  Examples are:

[ x^2 ] displays x↑2
[ π ] displays PI
[ Σ+ ] displays Σ+
[ √x ] displays √x

However, when it comes to programming the TI-66, patience is key!  The TI-66 takes on average, 1.5 seconds to register a new keystroke in learn (LRN) mode.  This is not for the speedy programmers among us.  This is my down point about the calculator, but it is not a deal breaker.

Other Programming Features

*  User labels A, B, C, D, and E; along A', B', C', D', and E'.  There are five keys dedicated to user labels.

*  Local labels have an unusual scheme.  They are defined by (almost) any key that is not a numeric key.  Therefore, we have labels such as 1/x, sin, and ÷.   This is carries over from the 1970s TI programmables (TI-57, TI-58, TI-58C, and TI-59). 

* 10 user flags: 0-9.

*  Indirect addressing

*  Storage arithmetic:  SUM (STO+), INV SUM (STO-), Prd (STO*), INV Prd (STO÷)

*  Comparisons:  compare the value in the display with register t.  Store amounts in register t with [ x<> t ].  Example:  a [ x<>t ] b [ 2nd ] (x≥t) tests whether b ≥ a).

*  An [ OP ] key gives access to additional functions and printing commands. 

A Partial List of OP Commands

OP 01 - 08 deal with printing, while 09 is not a valid code.

OP 10:  sign(x)
OP 11:  variance(x), variance(y)
OP 12:  slope, intercept  (linear regression)
OP 13:  correlation (linear regression)
OP 14:  x'  (predict x, linear regression)
OP 15:  y' (predict y, linear regression)

OP 2#:  increase register R0# (0-9) by 1
OP 3#:  decrease register R0# (0-9) by 1


Other than registering new programming commands, the keys are immediately responsive.  The keys are nice to the touch. 


I would recommend the TI-66, especially if you want a TI-58, TI-58C, or TI-59, but don't want to deal with 1970s rechargeable batteries, or you are on a budget.  It is a very nice, feature rich calculator to have. 


All original content copyright, © 2011-2019.  Edward Shore.   Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited.  This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.


  1. Good review, Eddie! The ** major ** drawback with the TI-66 is its terribly slow execution speed. So much slower than the TI-58 nearly makes this unusable for any actual programming where you care about getting a solution.

    The LCD display and alpha keycodes and landscape are all wonderful, but if only they had made it faster. :-)

  2. Thanks for the review, Eddie!

    My cover of this model oriented to the hardware, here:


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