A Detailed look at the HP Classic Series: 35, 45, 55, 65, and 67

No serious discussion of scientific calculators can exclude the HP classics. These machines set the standards of their time for quality, performance, and function. The series also demonstrates a stately growth of functionality in a few iterations. Notice that each machine has the same number of keys, yet the 67 has almost 4 times the number of functions of the original 35 . In this section, I examine the life and functionality of these machines. This is done in order to determine some benchmarks by which to gauge other machines. An attempt is made to catalog the functions of each of the machines. This catalog was done to produce a visual rating scale for lesser known machines in the Collection Area. This is done using a five box rating system as shown below:


Functionality Color Codes for Classics

The color coding scheme will be used in the collection descriptions to allow the reader to rapidly see which functions of a particular calculator are common to the HP-classic series.

Production Cycles

The calculators in this series were relatively long lived. The table below shows the years of production for the members of the family:

   '72  '73  '74  '75  '76  '77  '78  '79  '80  '81
Table 1. Production Years of the Classic Series

By the standards of today (1998), these product lifetimes are astoundingly long. As we will see a bit later, another calculator series, the HP41 series also had long life (but for very different reasons). The reasons for the long lives of these machine can be found in the basic engineering realities of their time:

  • Most Mechanical Engineering design was manually done.
  • Electronic Design and Schematic capture was either manual or very expensive.
  • Photo Plotting was in it's infancy.
  • Semiconductor Design tools were basically non-exsistent.
  • Embedded Software tools and development systems didn't exsist for the processors in use.
  • Plastic tooling costs were very high and lead times to production could reach almost to a year
  • General purpose computing was a centralized corporate function.
  • Logistical and Materials ordering systems were not yet mature.
  • The planning was manually done, the engineering was difficult and the documentation required a large infrasturcture. There were no standard software tools so changes in processor technologies had to be made carefully. Most processors were custom designs and they were exceeding difficult to change. Semiconductor parts were not easy to design and the planning systems for manufacturing made it difficult to forcast availability. And finally, no one (except for Dave Packard ) knew that there would be a market for scientific calculators.

    In the decade prior to HP-classic series, men were dying inVietnam while other men were walking on the moon. The United States was going through an educational metamorphosis that was being driven by the fears of the Cold War and the pessimism of the '60s. It was a period of great anger and intellectual turmoil, but the war was a catalyst for technology and rapid advances were being made in computing and automation. The 1970's were a bit more optimistic, but as a nation we began to slow down . The country took an intellectual vacation in the mid to late 70's: quality began to slip in all quarters, the economy was a shambles, inflation was high and inerest rates were higher. The calculators of those days, such as the Woodstock series were a reflection of the malise of the times. That's why you don't see them on this site. HP was trying to make 'em cheaper, not better. The number of display digits went down. They rebounded with the later 30 series and the voyager series.

    HP Classic Function Summary

    The HP classics encompassed a wide range of functions. The chart found below is a summary of the functions found in each of the HP classic series. The column colors represent the each model . The calculators are arranged in order of year of introduction. Note that the HP-65 preceeded the HP-55.


    Monroe, the Calculator Company


    Monroe was a major force in the mechanical calculator marketplace. It developed a reputation for high quality, high levels of service and dependability. Monroe understood the customer and the market. They developed a direct sales force and saw the product grow in markets that were not traditional markets of the day: ie. accounting and business. The mechanical calculator couldn't perform mathematical functions such as sines and cosines. They could perform relatively high precision multiplies, so they could be used in series calculations for these functions.

    As electronics began to evolve to a point that an electronic calculator could be made, it became evident that the entire fate of the company was at stake. During this period, a small high technology company, Computer Design Corporation, was developing electronic calculators. Monroe had the name and the marketing clout, and Computer Design Corp had a headstart in this infant technology. The calculaor design group of Computer Design Corp was formed as the Compucorp corporation

    Computer Design and Monroe collaborated on a number of designs that found favor in the scientific community. Some of the earliest electornic machines had tremendous feature sets for there time. During the late 1960's and early 1970's there was a growth of the use of statistics in the manufacturing process.

    Desktop Technology

    The Monroe/Computer design collaboration produced machines of execptional durablitiy and accuracy coupled with unique styling. The 1900 series (shown on either side of the photo above) demonstrated the state of the art in all areas of calculator design.

    The Panaplex display formed an easy to read image under nearly all lighting conditions. Two slide switches allowed for reconfiguration of the the two switches labled "I & II". This unique keyboard configuration allowed for a limited size keyboard that concentrated the most often used functions on the key board, and the enhanced functions assigned to the slide switches and multifunction keys.

    The 300 family

    The 300 family represented the state of the art of portable calculators. The family shared a common hardware set with particular features determined by rom programming. Calculators in this family where used for scientific, statistics, surveying and financial purposes.