Spotlight: TI-55 from 1977

Spotlight: TI-55 from 1977

Welcome to a special Monday Edition of Eddie’s Math and Calculator Blog.

Today, we have the original TI-55! Thankfully, I have the AC-adapter.

Quick Facts

Model: TI-55

Company: Texas Instruments

Timeline: 1977- 1979

Type: Scientific, Programmable

Memory: 32 programming steps, 10 memory registers

Power: AC adapter, 2 “AA” NiCd rechargeable batteries

Timeline of the TI-55

The original TI-55 was released in 1977 by Texas Instruments. It is the most basic programmable calculator in Texas Instrument’s line during the late 1970s, which the product line includes the TI-57 (Radio Shack EC-4000) and TI-58/58C/59.

The TI-55 was later updated in the 1980s twice. The TI-55 II was released in 1981. The good news of the update: memory increased to 56 steps, the sign, absolute value, fraction part, integer part functions, and numerical integration were added. Bad news, a crummy, unworkable keyboard! The keyboard was corrected with the TI-55 III in 1986. Even though it would be unofficial, I would consider the final version to be the TI-60 (no suffix) that would last from 1988 into the early 1990s.

Features and Keyboard

* Trigonometry

* Hyperbolic Trigonometry

* Logarithms, Anti-logarithms

* 8 sets of conversions: in/mm, gal/L, lb/kg, °F/°C, degrees/grads, grads/radians, degrees-minutes-seconds (DMS)/decimal degrees, polar/rectangular, all marked in blue font.

* Linear regression

* Storage Arithmetic

* Constant function for repeated calculation by pressing [ 2^nd ] (Const). On later TI calculators, such as the TI-30Xa, this would be labeled as K.

* Simple programming up to 32 steps

The [ 2^nd ] (CA) (marked in green): clears everything, including programs and registers.

The keyboard of the unit I bought is clean and all of the keys respond well. Knock on wood it stays that way.

Storage Arithmetic

The TI-55 has 10 memory registers, registers are numbered 0 to 9. Register arithmetic works as follows:

[ STO ] n: stores the number to register n

[ SUM ] n: adds the number to register n (STO+)

[ INV ] [ SUM ] n: subtracts the number from register n (STO-)

[ 2^nd ] (Prod) n: multiplies the number to register n (STO×)

[ INV ] [ 2^nd ] (Prod) n: divides the number by register n (STO÷)

[ 2^nd ] (Exc) n: exchanges the number with the contents in register n (x<>n)

Memory registers 0 and 1 are free to use, but the rest of them can be affected depending on what mode the calculator is in.

Conversions

There are eight sets of conversions that are marked in light blue on the calculator marked in this format: a ⋅ b

Pressing [ 2^nd ] before the key performs the conversion from a to b (left to right, →).

Pressing [ INV ] [ 2^nd ] before the key performs the conversion from b to a (right to left, ←).

Example:

[ 2^nd ] [ 4 ] (in ⋅ mm): convert from inches (in) to millimeters (mm)

[ INV ] [ 2^nd ] [ 4 ] (in ⋅ mm): convert from millimeters (mm) to inches (in)

Polar-Rectangular Conversions:

To Rectangular: r [ x<>y ] θ [ 2^nd ] (P ⋅ R): y [ x<>y ] x

To Polar: x [ x<>y ] y [ INV ] [ 2^nd ] (P ⋅ R): θ [ x<>y ] r

Statistics and Linear Regression

In single variable statistics, the y variable is used instead of the customary x. The one-variable results are:

[ 2^nd ] (S. Dev): sy (sample deviation)

[ 2^nd ] (Mean): y-bar (arithmetic mean)

[ 2^nd ] (Var): σy^2 (population variance)

[ RCL ] [ 5 ]: Σy

[ RCL ] [ 6 ]: Σy^2

[ RCL ] [ 7 ]: n

For linear regression (y = a * x + b), enter the x data point first, press [ x<>y ], enter the y point, finally press [ Σ+ ]. The [ 2^nd ] (Σ-) is used to erase points. Since statistics is accumulated in registers, we can enter as many points as we like.

[ 2^nd ] (Corr): correlation (r)

[ 2^nd ] (Slope): slope (a)

[ 2^nd ] (Intcp): y-intercept (b)

Predict with x’ and y’.

The TI-55 working primarily with the y variable carries over into linear regression mode too:

[ 2^nd ] (S. Dev): sy (sample deviation)

[ INV ] [ 2^nd ] (S. Dev): sx (sample deviation)

[ 2^nd ] (Mean): y-bar (arithmetic mean)

[ INV ] [ 2^nd ] (Mean): x-bar (arithmetic mean)

[ 2^nd ] (Var): σy^2 (population variance)

[ INV ] [ 2^nd ] (Var): σx^2 (population variance)

[ RCL ] [ 2 ]: Σx

[ RCL ] [ 3 ]: Σx^2

[ RCL ] [ 4 ]: Σxy

[ RCL ] [ 5 ]: Σy

[ RCL ] [ 6 ]: Σy^2

[ RCL ] [ 7 ]: n

The TI-55 has a feature called automatic line trend entry. That is we can enter data points as:

(n, y1)
(n + 1, y2)

(n + 2, y3)

and so on.

Start with entering n [ x<>y ] y1 [ Σ+ ]. Then just enter the rest of the y-data: y2 [ Σ+ ], y3 [ Σ+ ], and so on. The x values n + 1, n + 2, etc. are entered automatically. It sounds like a nice, convenient, possibly underrated feature that is worth exploring. I think this also works on the TI-57, TI-58/58C/59, and TI-66, but please don’t quote me on it.

Keystroke Programming

The TI-55 has basic algebraic programming, which can hold up to 32 steps (step numbers 00 to 31). The programming module is basic: no loops as far as goto/labels, no prompts, and no comparisons (although some tests can be used using backdoor methods such as 1/0).

There are only four commands:

[ 2^nd ] (R/S): Run-Stop

[ 2^nd ] (Rst): Reset, go to step 00 and continue execution.

[ 2^nd ] (Lrn): This sequence is the toggle between learn (program) mode and calculator mode.

[ 2^nd ] (Sst): Single Step through the program.

The TI-55 is in program mode when the display is in the format: SS RC.

SS: step number

RC: key code. R is the row number coming from the top down, C is the column code is the column going left to right. The left most column: C = 1 (unshifted), C = 6 (shifted). Right most column: C = 5 (unshifted), C = 0 (shifted).

There are couple of exceptions:

Pressing [ 2^nd ] adds 5 to the column number. For example, [ % ] has the code 22, while the [ 2^nd ] (Δ%) sequence has the code 27. The steps with the [ 2^nd ] key are merged, while the [ INV ] key (key code 21) is not.

Pressing a number key gives RC in the format 0#. The [ 9 ] gives the key code 09.

SS represents the current step you are on. 05 00 means that you are on step 5 and nothing has been entered. To review codes, we must use Rst (reset) and Sst (single step).

A sample program: f(x) = x^2 + 1.

32	x^2
75	+
01	1
85	=
86	R/S
87	Rst

Hidden Secrets

The TI-55 has hidden key codes, which I have yet to explore, but here is an article from rskey.org:

https://www.rskey.org/CMS/the-library/?view=article&id=98

Source

Woerner, Joerg. “Texas Instruments TI-55” Datamath. December 5, 2001. Retrieved November 11, 2024. http://www.datamath.org/Sci/MAJESTIC/TI-55.htm

Manual: http://www.datamath.net/Manuals/TI-55_US.pdf

To those in the United States, Happy Veteran’s Day! Gratitude, take care, and be safe,

Eddie

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