Saturday, December 24, 2022

Retro Review: HP 75C

Retro Review:  HP 75C

It's Christmas Eve!

Quick Facts

Model:  HP 75C 

Company:  Hewlett Packard

Years:  1982-1984

Type:  Scientific, BASIC Programmable

Batteries: Rechargeable Ni-CAD, with AC-Adapter**

Display:  1 line, 32 characters

Original Price:  $995

Memory:  16,000 bytes RAM with a slot for an 8K RAM Card

Other Ports:  3 ROM slots for applications 

Hard Case which stores the HP 75 and the cards, but not the AC adapter

The HP 75C (and later HP 75D) has an 8-bit CPU microprocessor named the Capricorn.   The Capricorn microprocessors where featured in the HP 85 computer.

For fans of tropical astrology, this article is posted on December 24, under the sign of Capricorn.   ♑  The Capricorn processor was designed by a Steve Wozniak, a Leo.  But I digress.


The HP 75C computer is a portable computer with an almost full sized QWERTY keyboard.  The 75C is a scrolling one-line display that can have up to 32 characters.  The keyboard works like a computer keyboard: the [SHIFT] key must be pressed and held to type capital letters.

The keyboard is nice to type and with the correct size of hands, touch typing can be accomplished.  It also has a numeric keypad that can be activated by holding down [CTL] while pressing [LOCK].   Pressing [LOCK] ends the numeric keypad mode.

[ 7 ]  returns 7

[ 8 ]  returns 8

[ 9 ]  returns 9

[ U ] returns 4

[ I ] returns 5

[ O ] returns 6

[ J ] returns 1

[ K ] returns 2

[ L ] returns 3

[ M ] returns 0

Other characters can be accessed by holding down the [CTL] key and pressing another key.   Example characters that can be typed this way are:

CTL + Space:  Δ

CTL + A:  °

CTL + D:  α

CTL + I:  σ

CTL + P:  θ

CTL + T:  π

CTL + plus key:  Σ

The keyboard also has an HP-IL port and a card reader.  The card reader is manual, requiring the user to pull the card at just the right speed to record programs.  Transferring programs to cards, which are long and skinny, takes a lot of patience.  Each card be pulled in two ways and each way can store a program up to only 650 bytes.   Every time a new program is stored in a track, that new program replaces the previous contents.     

The battery is a rechargeable Ni-CAD battery.  If you purchase a HP 75C, I advise you to get one with a working AC plug.  According to the HP 75 Owner's Manual, it takes about 8 hours for the batteries to fully recharge.  

See the Sources section for a download link to the Owner's Manual.  

How to turn the HP 75C on?   There is no ON key.   The [ATTN] (attention) key takes care of this.  To turn the 75C off, hold [SHIFT] and press [ATTN] or type bye then [RTN]. 


The HP 75C has three operating modes:

TIME: setting the computer's clock and time.  Thankfully the HP 75C is Y2K friendly and allows dates after 2000.

APPT:  set and check personal appointments.

EDIT:  the main mode of the HP 75C computer.  This is where all the basic programs are edited, stored, and ran.   The Edit mode can handle one line algebraic calculations, with the order of operations followed.  


The HP 75C has BASIC programming language, although not as extensive as the HP 71B.  It's probably the one slight problem I have with the 75C, but missing functions can be programmed.   In tomorrow's post, I have a simple linear regression which the 75C's list of commands lack commands for.

What is included?  A short list includes:

*  square roots, modulus, sign, maximum, minimum, random numbers, remainder, integer part, fractional part 

*  the constants PI, INF (9.99999999999E499), EPS (1E-499)

*  degree/radian angle conversions, trigonometric functions, angle (atan2), sec, sec, cot, logarithms (log10 and log (for ln, this is a computer language))

*  relational tests (evaluated to 0 for false or 1 for true), logic (and, or, exor (either or but not both), not)

*  strings, alarm settings, beep, read and data, goto and label, for-to-step structure, if-then-else structure, character, date and time strings, arrays

*  printing and user function commands

Remember, in programming the 75C, line numbers are required.   

Like the HP 71B, I find it a joy to program the 75C.  

The Updated HP 75D

The HP 75D was an updated HP 75C computer and produced in rare supply from 1984 to 1986.  The retail price of an HP 75D was $1,095 and included a port for a  bar code wand. Yes, the same type of bar codes that you see on grocery products and you scan at the self checkout lines.   

Final Thoughts 

The HP 75C is not an inexpensive purchase.  If you buy one, please be sure you see pictures from the seller with the unit turned on.  In addition, at the very least, make sure it has working batteries and the AC adapter.   The going price for the HP 75C these days from at least $250 US.   

It is a worthwhile machine especially for fans of BASIC and a good companion to the HP 71B.   I am happy to have the 75C in my collection, which is years I missed out on winning one as a door prize.  


"HP-75"  Wikipedia.  September 7, 2021.   Retrieved December 4, 2022.

"HP-75C/D"  Museum of HP Calculators.   Retrieved December 4, 2022.

Hewlett Packard.  HP-75 Owner's Manual.  Rev. B   November 1982.  

Download from the's literature page here:

Until next time, keep happy and sane,


All original content copyright, © 2011-2022.  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.