The HP-10 was a complete departure from previous HP calculators - neither a financial model nor a technical one, but instead an "adding machine". It was the smallest HP calculator so far to have a built-in printer - easily held in one hand, it did not use RPN, it had only + - x - and % keys; not even a square root, though it had an automatic constant feature and a single memory register. No
wonder the calculator's codename was KISS - "Keep It Simple, Stupid".

A fairly small label on the back describes all the HP-10 operations - there was no need for a pocket guide, and the manual was not that big. Next to the ON/OFF switch it had a mode switch to select how results were to be displayed - in the LED display, on the printer, or on both.

Using only the printer or only the LED was a useful option as it saved power - even so, the battery pack is larger than that used on earlier handheld models. The option to use both is called ALL, and is equivalent to the TRACE mode on other printing HP calculators.

Oddly enough, the HP-10 looks very much like the original HP-35 - white keys for digits, blue keys for simple maths functions, and black keys for the more advanced functions - though on the HP-10 even x and - are treated as advanced functions!

There is also a yellow shift key, used for the automatic constant, for selecting display modes, and for a few printing operations, including printing a row of ###### symbols to separate one calculation from another.

There are so few functions that the calculator has only 22 keys (zero occupies the double-width key) which means that the keys are larger than those on other HP handheld models - making it easier to use this as a simple printing model.

It seems clear that this was an attempt by HP to enter the small business printing calculator market, but judging by its rarity, not many can have been sold. Apart from HP's usual reluctance to advertise, further reasons for the low sales may be that the target market was not aware of HP calculators, and that the tiny printer was reputed to be unreliable. Despite its unusual format, I must say that I very much like the HP-10!

This article is part of the WMJARTS file. This file contains a series of articles written by Wlodek Mier-Jedrzejowicz and published in DATAFILE, the journal of the HPCC. The article was reproduced with permission of the author.