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ICM (International Calculating Machines) Model 816 Desktop Calculator

Updated 6/17/2017

This machine is quite interesting, and appears to be fairly unusual. It was made by International Calculating Machines, of Woodland Hills, California, in the early 1971 timeframe. ICM was a consumer products subsidiary of the integrated circuit manufacturer Electronic Arrays(EA), located in Mountain View, CA. This calculator has quite a history behind it, with linkages back to the introduction of a very historical electronic calculator in the 1964 timeframe.

Electronic Arrays was founded in 1967 by a number of folks from General Micro-electronics (GMe), as well as a couple of gentlemen from Bunker Ramo. GMe had been a pioneer in development of MOS (Metal-Oxide Semiconductor) integrated circuits, which held the promise of providing much higher circuit complexity than earlier bipolar integrated circuits. In the first half of the 1960's, GMe had developed the first Large Scale Integration (LSI) electronic calculator "chipset" (consisting of 19 different chips) for Victor Comptometer, which were used in the Victor 3900, a revolutionary electronic calculator that, unfortunately, was a bit too far ahead of its time.

At its beginning EA, developed MOS shift registers which were very useful in computer and encryption circuitry. Later, the company moved into development of high-capacity (for the time) Read-Only Memory chips (ROM) that could store the operating code for computers. One of the founders of the company, Jim McMullen, one of the founders of EA, had been involved in the development of the Victor 3900 electronic calculator chips, and had a keen interest trying to make a calculator chipset again, this time using the more refined and advanced MOS IC fabrication techniques that EA had perfected. Beginning sometime in 1968, McMullen led a team of engineers who were put to the task, and in November, 1969, the company announced general availablity of the EAS100 six chip electronic calculator chipset. While Sharp had set the calcualtor market into a tizzy with the March, 1969 introduction (in Japan) of the Sharp QT-8D, which used an advanced 4-chip MOS chipset made by Rockwell, the Sharp chips were completely proprietary, while anyone could buy the EAS100 chipset from Electronic Arrays to build their own calculator, a fact that EA made abundantly clear in their announcement of the chipset. The EAS100 chipset was the first publically available calculator chipset ever offered.

In the early part of 1970, Electronic Arrays created the International Calculating Machines, Inc. subsidiary to produce low cost calculators using the EAS100 chipset. A facility in Woodland Hills, CA was established, and the process of putting a manufacturing operation in place was begun. By the fall of 1970, the factory had begun building the ICM-816 calculator. Along side the manufacturing operation, a sales and marketing organization was built up, and in the late part of 1970, the ICM 816 was introduced to the marketplace, as a price-leading, highly-featured desktop electronic calculator. The initial introduction price of $450 was rather steep, as Sharp was selling the QT-8D for $395.

The nameplate for the "Senator Mini Calc" version of the ICM-816


Serial Number Tag for Caltype Senator Mini-Calc

Sometime during the '71-'72 timeframe, fully assembled ICM 816 calculators, were sold under OEM contract to Caltype Corporation (a subsidiary of Transitron Electronic Corporation), of Los Angeles. Transitron Electronic Corporation was founded in 1952 to manufacture transistors, and did quite well for a time, but changes in transistor fabrication technology and the advent of integrated circuits eventualy led to their demise in 1986. Caltype Corp. was a spinoff of Transitron, incorporated in September of 1965 as a marketer of low-cost business machines manufactured by other companies. Caltype marketed the ICM-816 as the "Senator Mini-Calc". The only difference between the International Calculating Machines calculator and the Caltype Corp. version was the the name plate on the top cover of the machine, which reads "Senator Mini-Calc; Made in USA"; and the model/serial number tag, which replaced ICM's name with Caltype's. The model number of the machine listed on the model/serial number tag remains ICM-816, and the serial-numbering scheme follows ICM's sequence.

Along with the ICM-made calculators, the EA chipset was also sold to a small company in California called MITS (Micro Instrumentation and Telemetry Systems). MITS later became famous for developing the Altair 8800 microcomputer kit, that is generally considered the hobbyist computer that marked the beginning of the personal computer revolution. MITS designed a calculator based on the EA chipset that could be built as a kit, or ordered as a fully assembled calculator. The MITS 816 was introduced in the November, 1971 Popular Electronics magazine, and sold for $179 in kit form, or $275 fully assembled, setting a new low-price standard for a basic four function calculator. The MITS 816 used a vacuum fluorescent seven-segment display as opposed to the ICM-816's more expensive (though much more aesthetically pleasing) Nixie Tube display.

It is also very interesting to note that famous electronics manufacturer Sony, which entered the calculator marketplace in 1967, marketed a calculator called the ICC-88 beginning in fall of 1971 that utilized the same Electronic Arrays chipset as the ICM-816. The ICC-88 was Sony's first calculator to use non-Sony-made integrated circuits as the brains for a calculator. The display in the ICC-88 was a gas-discharge, seven segment planar display versus the Nixie-tube display of the ICM-816, and the Sony machine could operate on AC power, or through built-in rechargeable batteries. Otherwise, the ICM-816 and Sony's have the same features and operation.

In the early part of the 1970's, there was a dearth of upstart calculator manufacturers that were looking to try to grab a share of the market created by the dramatic decrease in price of calculating machine technology that larger-scale integrated circuits had made possible. At the time, IC technology was advancing very quickly, which made it difficult for the smaller players to keep up with big boys in the integrated circuit biz. Electronic Arrays fell upon hard times, and were later bought out by NEC Electronics(Japan) in 1978. Along the rocky road to Electronic Arrays' failure and buyout, and about the time that the electronic calculator market suffered it's first big shakeout, the ICM subsidiary was liquidated. The remaining inventory of ICM calculators were later sold off through a surplus liquidator in San Jose, CA. ICM 816's were sold for as low as $135 each. If anyone out there knows anything more about ICM, or Electronic Arrays, I would love to hear from you.

Inside the ICM 816

The ICM 816 has much more than a passing similarity to another interesting and unusual machine in the museum, the Master H-1. The machines operate in a very similar manner, and in fact, share similar logic, both based on LSI chipsets made by Electronic Arrays. The ICM 816 seems to be a little earlier design than the Master H-1, as it uses a less-highly integrated six-chip LSI chipset rather than the four-chip set of the H-1. The chipset in the ICM 816 is dated in early 1971, whereas the chipset in the Master H-1 is from late 1971 to early 1972. During this particular time in the development history of large scale integrated circuits, the time difference between the chips in the ICM 816 and the Master H-1 represents a long time, as LSI integrated circuit technology at the time was advancing at a break-neck pace. The ICM 816 also uses older Nixie tube display technology rather than the early planar gas-discharge display panel used in the H-1.

Close-up View of ICM 816 Keyboard

As with the Master H-1, the ICM 816 uses an 8-digit display to provide 16 digits of integer capacity. A special key (a double-ended arrow, designated as [<->] in this text) on the keyboard toggles the display between the most and least significant eight digits of results which exceed eight digits. For example, multiplying 12345679 by 18 would result in "22222222" showing up on the display. The clue that there is more of the result to be displayed is that there is no decimal point in the display. (If the result were actually 22222222, it would be displayed as "22222222.".) Since there is no decimal point shown, pressing the [<->] key changes the display to "2 ", showing the least significant digit of the result. Subsequent presses of the [<->] key toggles the display back and forth between the most significant and least significant digits of the result. If there aren't any additional digits to be displayed, pressing the [<->] key results in a blanked display.

Though the ICM 816 and the Master H-1 share similar logic, there are some subtle differences in their operation. The ICM-816 will only allow entry of up to eight digits of input. If more than eight digits of entry are attempted, the machine goes into input overflow state, requiring a press of the [CE] or [C] key to clear the invalid entry. The Master H-1 allows input of up to 16 digits (with the least significant eight digits not visible to the user while they are being entered). Another subtle difference between the machines is that the Master H-1 ignores attempts to set the decimal point position to 8 or 9, wheras the ICM 816 seems to get somewhat confused by doing this.

The ICM 816 uses fixed decimal point logic, with a setting from zero through seven digits behind the decimal point. Setting the decimal point location is not intuitive, as there is no obvious switch or dial provided to make the setting. At power-up, the calculator defaults to zero digits behind the decimal point, making the machine effectively an integer-only calculator. Based on experience with the Master H-1, pressing and holding the [CE] key, while at the same time, pressing a digit from 0 to 7 on the keyboard, properly sets the fixed decimal point location. Pressing 8 or 9 as the decimal point position selection is accepted, but results in the machine blanking the display, and immediately causing an input overflow as soon as any non-zero digit is entered. This method of setting the fixed decimal point position is very much like that used on the Marchant Cogito 412 and Cogito 414 calculators.

Closer view of ICM 816 LSI's

The ICM 816 is based on a EA's EAS100 six-chip LSI chipset, with each ceramic-packaged chip having 24 pins. All of the chips are dated within the first few weeks of 1971. The US Patent (Number 3,800,129) filed by Electronic Arrays indicates that the chipset is a microcoded processor, utilizing ROM to provide the microcoded instructions that orchestrate the operation of other chips in the system to provide the brains for the calculator. The devices include an input chip (scan and encode keyboard), an output chip (multiplex and generate display signals), an arithmetic chip (perform logical and arithmetic operations), a register chip (provides working storage registers), and at least one (in this machine, it appears that two are used) program control chip (essentially, ROM with microcode to control the the system). The chipset IC's are numbered "150-5001", "150-5004", "150-5005", "150-5013", "150-5014", and "150-5017". The machine's construction is of high quality, with individually replaceable keyswitch modules (using magnetic reed switches for reliability), high-quality silkscreened fiberglass circuit boards, and overall very durable construction.

The Display Panel (note negative indicator lit at left end of display)

The ICM 816 uses eight genuine Burroughs B5853ST Nixie tubes for its display. Each tube contains the digits zero through nine, and a right-hand decimal point. The display is driven by an unknown IC (no part number or other identification on it at all) in a 14-pin DIP package, as well as discrete transistor drivers. The display provides leading zero suppression, which is a feature that the Master H-1 does not provide. The Nixies sit behind a red filter that tints the orange Neon-glow display of the Nixies more toward the red end of the spectrum. Sign and overflow conditions are indicated by small discrete neon tubes located at the left (NEGative) and right (OVerFlow) ends of the bank of Nixie tubes.

The calculator is good about catching error conditions. Division by zero results in an immediate "OVF" condition, requiring a press of the Clear key to unlock the keyboard. Any result that would exceed the 16-digit integer capacity of the machine causes the "OVF" indicator to light and the keyboard to be locked out. Entry overflow (typing in more digits than the machine can handle) results in an overflow condition that can be cleared with the "CE" key.

The ICM 816 is fast calculator, with 99999999 divided by one requiring about a blink of an eye to complete. 99999999 squared (99999999 X 99999999) takes just a shade longer, but certainly less than 1/10th of a second.

The calculator in the museum is missing its model number plate, as evidenced by the rectangular area on the cabinet that has a significantly lighter color. This is where the nametag was placed. Apparently the adhesive failed over time, and the tag was lost.


Sincere thanks to the Estate of James T. Lapsley, Jr. for donation of the late Mr. Lapsley's ICM-816 calculator

Thanks to LaFarr Stuart for information on chip maker Electronic Arrays and their association with ICM.

Text and images Copyright ©1997-2014, Rick Bensene.