Retro Review: HP 65

Retro Review: HP 65

Quick Facts

Company: Hewlett Packard

Years: 1974 - 1977

Type: Scientific, RPN (Reverse Polish Notation)

Memory: 9 memory registers, 100 steps

Batteries: originally Ni-Cad, there are battery packs that can use AAA batteries (Ebay seller: waterhosko, https://www.ebay.com/usr/waterhosko)

DISCLAIMER:  I am not page for referring the website on eBay, nor I do not guarantee that inventory is available.

The HP 65 is first programmable calculator. I recently purchased a HP 65 from Persnickity Antiquity in Pomona, California. I saw that HP 65 a year earlier. What got me to purchase was it was for two reasons, (1) I have a HP 67 and fell in love with the classical HP calculators and (2), it had the AAA battery pack (see waterhosko above).Originally the HP 65 is ran with rechargeable Ni-Cad batteries.

Format Settings

The HP 65 has two format settings:

Scientific Format: [ DSP ] # (0-9)

Fixed Format: [ DSP ] [ . ] # (0 – 9)

Modifier Keys

There are three modifier keys: two orange shifts [ f ] and [ f^-1 ] and one blue shift [ g ]. The label for the orange shift is above the key and the label for the blue shift is below the key.

Inverse Key Table

[ f ]	[ f^-1 ]
LN	e^x
LOG	10^x
√x	x²
SIN	SIN^-1
COS	COS^-1
TAN	TAN^-1
R→P (to polar)	P→R (to rectangular)
D.MS+	D.MS-
→D.MS (decimal, minute, seconds)	D.MS→ (decimal)
→OCT (to octal base)	→DEC (to decimal base)
INT (integer part)	FRAC (fractional part)
SF 1/SF 2 (set flag 1 or 2)	CF 1/CF 2 (clear flag 1 or 2)
TF 1/TF 2 (is flag 1 or 2 set?)	TF^-1 1/TF^-1 2 (is flag 1 or 2 clear?)

Programming

Program Steps

In program mode, the HP 65 displays only the key code. The key code is usually two digits, the first is the row (top-down), second is column (left-right). The exception is the digit keys where they would be labeled in the format 0#.

Editing is limited to SST (single step forward) and delete key ([ g ] [ Clx ] (DEL)).

Partially Merged Steps

The first programmable calculator holds up to 100 steps. Steps are partially merged. The program commands that are merged are:

STO # (1 – 8) * does not include storage arithmetic	Swap X and Y: x<>y	Comparisons: x≠y, x=y, x≤y, x>y
RCL # (1 - 8)	Roll Down: R↓	All but STO/RCL are followed by the [ g ] shift key.
NOP (No operation)	Roll Up: R↑

Memory Registers

The HP 65 has nine memory registers R1 through R9. There is no R0 (register zero), which would be added in later calculators.

Two registers are used for specific purposes:

R8: Register 8 is used the counter in the DSZ command (Decrement and Skip if Zero command).

R9: Register 9 is used as a temporary register from trigonometric function calculations, rectangular/polar conversions, and for comparison tests, R9 is used as a Last X register.

R8 and R9 can be used for general use, but would be subject to change.

Comparisons and Labels

The HP 65 has four comparisons (x = y, x ≠ y, x ≤ y, x > y) and the DSZ command operate somewhat like most RPN calculators: if the test is true, the next step is executed. However, if the result is false, the next two steps are skipped. That’s right, the next two steps.

Example:

x > y

[ if true, x > y, execute this step; if false, x ≤ y, skip this step ]

[ also skip this step if x ≤ y ]

[ third step ]

Why two steps? The goto (GTO) command takes two steps on the HP 65. The HP 65 has 15 labels, 0 – 9 and A – E. If both steps are not needed, one can be filled by the NOP (No Op (Operation)) command.

Subroutines and Instant Labels

The labels A through E (A, B, C, D, E) are user programs that can be accessed by keys. They are the only labels act as subroutines. There is no XEQ/GSB command, subroutines are automatically called by pressing the corresponding key. Only one subroutine can be called at a time.

One quirk for the HP 65, if there is a program without a label A, then pressing [ A ] in run mode, execution starts from the first step.

The HP 65 loads five short default programs every time the calculator is turned on, which function is printed above the key in white:

[ A ] 1/x

[ B ] √x

[ C ] y^x

[ D ] R↓

[ E ] x<>y

Non-Continuous Memory

The memory on the HP 65 is not continuous. When the calculator is turned off, all memory is lost. The only way to save the memory registers and the steps require the use of thin memory cards and the built-in card reader. Each card can hold 100 steps.

I love the classic RPN HP calculators from the early 1970s. I have three of them now, the HP 45 (1973), HP 65 (1974), and HP 67 (1976).

Sources

“HP 65” The Museum of HP Calculators (MoHPC). https://www.hpmuseum.org/hp65.htm Retrieved April 1, 2026.

“HP-65 Programming” The Museum of HP Calculators (MoHPC). https://www.hpmuseum.org/prog/hp65prog.htm Retrieved April 1, 2026.

Hewlett-Packard. HP-65 Owner’s Handbook. Cupertino, CA. July 1974.

Eddie

All original content copyright, © 2011-2026. 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.

Review: Casio fx-55 Plus

Review:  Casio fx-55 Plus

General Information

Company:  Casio

Type:  Fractions, Elementary Math

Memory:  1

Battery:  Solar with battery backup (1 LR44)

Years:  2012 - current

Cost:  $10.00 - $20.00, I paid $12.99

Operating Logic:  Algebraic

Target Audience:

* Elementary and middle school students

* Anyone who wants a few more functions than the regular four-function calculator, but not a scientific calculator

* You want a calculator with an emphasis on fractions

Features

The major emphasis on the Casio fx-55 Plus is on fractions.  We’ll discuss this in the next section in detail.

The screen has a textbook display, meaning fractions and all other calculations are displayed as you would write them.  Fractions are simplified.  If MathO (Math Output) is selected, square roots and terms of pi (π) shown in exact form.  If LineO is selected, all answers (except fractions) are shown in decimal form.  No matter what the display mode is, you can always decimal approximation by pressing [SHIFT] [ = ].

Example (MathO):

√12 returns 2 √3

5^2 * π returns 25π

You enter expressions the way you would write them with the Casio fx-55 Plus.  The display is a mutli-line display, the expression on the top of the screen and the answer is on the bottom of the screen. 

The keyboard is simple where only four keys have shifted functions.  The OFF function is shifted as well. 

Other functions include reciprocal, random numbers, including random integers, and calculations in Degrees-Minutes-Seconds [ ° ‘ “ ]. 

The memory has four dedicated keys: store, recall, add to memory (M+), and subtract from memory (M-).

The percent keys work a bit differently from the normal four-function calculators (and is not very intuitive). 

[%] divides the argument by 100 and that’s it.  Its fine when you are working with multiplying or dividing percentages, but adding and subtracting percent will take an algorithm:

X + Y%:  enter X + X * Y %

X – Y%:  enter X – X * Y %

[>%] multiplies the result by 100 and displays the answer in percentage notation. 

Example:  3 [ = ] [ SHIFT ] (>%) returns 300%

Let’s Talk Fractions

The fraction keys are marked lime green on the Casio fx-55 Plus.  Let’s go over the keys in detail:

[ []/[] ] (1st key from the left):  fraction template

[ [] []/[] ] (2nd key from the left):  mixed number template

[ F←→D] (3rd key from the left): switches the result between exact (fractions, exact square roots, terms of π) and decimal answer

[ a b/c ←→ d/c ]:  (4th key from the left)

Manual Simplified mode:  This key does nothing.

Auto Simplified mode: This key converts the result between improper and proper fraction notation. 

[ Simp ]:  calls the simplify conversion to the entry.  In Manual Simplified mode, any fraction that is not reduced to its irreducible form will have a down arrow indicator ( ↓ ).  Also executing Simp in Manual Simplified mode not only returns the reduced fraction, but the factor used into reducing such fraction.

[x^-1]:  The reciprocal function

I also want to point a very handy key, the remainder division function ( [÷R]), which is located on the bottom right hand corner of the keyboard.  This function returns both the quotient and remainder. 

Example:  77 ÷R 13 returns 5, R = 12 

Only the quotient is stored in last answer (Ans).

Let’s compare this Casio fx-55 Plus with an older fraction oriented calculator, the Texas Instruments Math Explorer (aka TI-12). 

Comparison:  Casio fx-55 Plus (2012-present) vs. Texas Instruments Math Explorer (1978-1998)

TI Math Explorer	Casio fx-55 Plus
1 line display	Multi-line, textbook display
No shifted functions	4 shifted functions
N/A	Degrees-Minutes Seconds calculations, random numbers and integers
Constant arithmetic storage function through the [ Cons ] key.	N/A
10^n is limited to integers 0-7	10^x allows for all real numbers between up to, but not including, 100.  Used to work with numbers in scientific notation
Int÷ works with only positive integers	÷R works with real, positive numbers (any non-positive numbers makes ÷R work like ÷)

Verdict

I like this calculator for middle school and elementary students. The Casio fx-55 Plus presents a few extra functions but without the many scientific functions that may intimidate some students.  (emphasis on some)  I wish the percent would work like we would write expressions when it comes to addition or subtraction (i.e. X + Y% instead of X + X * Y%), but this is understandable.  I also wish the proper/improper toggle worked in Manual Simplified mode.

The display is great because the user can see the expression and answer at the same time.  I really like that this calculator also returns exact answers in calculations involving square roots and π. 

I can see this calculator being used beyond the classroom, especially in everyday basic math calculations. 

In any case, the Casio fx-55 Plus serves a great introductory calculator.

Eddie

All original content copyright, © 2011-2018.  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.  Please contact the author if you have questions.