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NCR 18-2 Electronic Desktop Calculator

Updated 1/27/2017


Just about everyone knows that NCR (National Cash Register) made its fame by making cash registers, but few know that, for a time, they also sold electronic calculators. Note that I write "sold" rather than "made". The NCR 18-2 exhibited here is not designed nor manufactured by NCR. NCR partnered with Japanese calculating machine manufacturer Nippon Calculating Machine Corp. (NCM, also known as Busicom) beginning in the summer of 1968, selling calculators designed and manufactured in Japan by NCM. NCR, like many other American makers of mechanical and electro-mechanical calculating machines (a cash-register is a specialized calculating machine), was caught without electronic design expertise when electronic calculators began to erode market share from earlier mechanical calculators in the mid-1960's. The corporate management at NCR wanted to get in on this lucrative market, but didn't have the time to develop their own calculator design and manufacturing operation. NCR needed to find digital electronic engineering talent. The best place to look: Japan. At the time, Japan's Hitachi and NEC had some of the lowest-cost integrated circuit manufacturing capabilities, with lines of small to medium-scale bipolar and new Metal Oxide Semiconductor (MOS) devices that would serve well for electronic calculators (and eventually cash registers). Japan also had unmatched manufacturing capacity, with comparatively inexpensive labor rates. Busicom had managed to make the transition from mechanical to electronic calculators by buying up and analyzing the design of pioneering machines in the calculator business (the early calculators from Sumlock/Anita in the UK, and the advanced electronic calculators from IME in Italy), and then crafting their own designs based on what they had learned. Busicom's first electronic calculator, the Busicom 161, was essentially a re-engineered "copy" of the IME 84, IME's first electronic calculator, and one of an early group of electronic calculators that utilized coincident current magnetic core memory as the primary storage element in the calculator. Busicom successfully marketed their electronic calculators in Japan, but was looking for ways to grow their market. NCR had a great opportunity for Busicom to get into the very lucrative U.S. marketplace, with NCR providing a very well-established sales and service network. An alliance was formed, and Busicom's early electronic calculators started making their way across the ocean with NCR logos proudly adorning their cabinets.

Like so many other tidbits of history that are being lost every day, the historical details behind the story of NCR's foray into the electronic calculator market may be lost. My hope is that someone who was connected with NCR back in those days will read this and get in touch with me, and help add to the story of the relationship between Busicom and NCR. NCR's own web site has no information at all about the company even having been in the calcluator business. There is a large following of collectors that preserve and document NCR's wonderful cash registers, but there's virtually no information anywhere about NCR's calculator history. If you know anything more about the history of NCR's calculator business, or the company's relationship with NCM/Busicom, please write me.

Another View of the NCR 18-2

The 18-2 is the entry-level machine amidst the two Busicom-made machines that brought with them NCR's debut into the electronic calculator marketplace sometime in the early part of 1969. Along with the $1095 Model 18-2 exhibited here, the Model 18-3 stablemate was the flagship machine, with all of the features of the 18-2 along with a one-key automatic square root function. for a suggested retail price of $1,275. Both the NCR 18-2 and 18-3 are identical (other than minor cosmetic differences) to the Model 162-C and 162 calculators sold by NCM under the Busicom brand. The Busicom machines preceded the NCR versions of the machines to market (in Japan) by a period of about four to five months.

One fact that is not commonly known is that the only difference between the Busicom 162C/NCR 18-2 and 162/18-3 models is the keyboard assembly. Both machines have identical electronics, meaning that the Busicom 162C and NCR 18-2 have all of the necessary circuitry to perform square root, they just do not have a key on the keyboard to activate the square root function. The Busicom 162C/NCR 18-2 have a double-width backarrow key and a single keyswitch for the backarrow function, while the Busicom 162/NCR 18-3 keyboard assembly has two keyswitches, one with a backarrow key, and the other with the square root key. A Busicom 162C/NCR 18-2 can easily be converted to a 162/18-3 by installing the keyboard assembly from a 162/18-3. A number of NCR 18-2 calculators have been found that have benefitted from this modification, with all external and internal nomenclature indicating a model 18-2, but with the machine having the square root function. It is even possible (though not documented) that Busicom/NCR may have offered this as a field-installable upgrade to the 162C/18-2 calculators through their service organizations.

The 18-2 was quite a capable machine for the time. It performs the four basic math functions, offers a full sixteen digits of capacity and two independent memory accumulators. The calculator uses mixed floating/fixed decimal point logic, with a thumbwheel selecting the fixed decimal point location (at 0, 1, 2, 3, 6, or 9 digits behind the decimal point) for addition and subtraction (as well as recall of items from memory registers), and automatic decimal placement for multiplication and division operations. This mixed-mode decimal point logic is somewhat unusual, and takes a little getting used to, but is flexible and affords the greatest accuracy when multiplying and dividing. Other features include a convenient "back arrow" key that deletes the last digit entered for easy correction of mis-entered numbers, and a "clear indicator" [CI] key for clearing the display without disrupting calculations in progress, also useful for correcting input errors. For a retail price of just under $1,100, the NCR 18-2 was quite competitively priced against competitors like the Monroe 990, a machine with similar capabilities priced at $1,250.

The cabinet of the 18-2 is all-metal, with the only plastic part of the cabinetry being the smoked plastic display window that the Nixie tubes shine through. The base of the machine is a thick metal casting, with nicely machined bosses for the electronics of the calculator to mount to. The upper half of the case, again all metal, is a thinner-gauge casting, but still quite hefty. The upper half of the case fastens to the base via four screws that are easy to access, which made it nice for service personnel.

Overall Internal View of the 18-2

The chassis of the machine is a combination of stamped metal parts and plastic. The backplane, situated along the bottom of the chassis, is built upon a rather heavy metal casting that makes up the base of the chassis, mounting on the bottom half of the cabinet. The circuit boards plug into a plastic frame that provides card edge guides that align the cards and keep them from moving around during transport. A stamped metal panel covers the top of the card cage to keep the circuit boards from working themselves out of the connectors they plug into.

The Keyboard Connector

The keyboard assembly, a rather massive item by itself, is built on a stamped metal frame that sets into plastic section of the chassis. A large multi-pin connector provides the electrical connection between the keyboard and the backplane. The design is such that getting the keyboard disconnected from the connector is difficult, because the cables are short enough that the keyboard assembly can't be lifted too far, meaning that it takes small hands to get under the keyboard while holding it up. Plugging the keyboard connector back in after the keyboard has been removed is even more challenging.

The "Business" side of the keyboard

The keyboard design is quite unusual, using leaf-type contacts actuated by the plunger of the keystalk. Most calculators of the time used magnet-actuated reed switches, which provide a much cleaner switching action (less contact bounce) and offer longer service life. This design was likely used to help reduce the cost of the calculator, however, this seems somewhat contradictory, though, given that the rest of the machine seems to be so overbuilt.

A close-up view of the keyswitch modules in the NCR 18-2

Each switch has a fixed set of contacts and another contact that moves. When the key is depressed, the plunger pushes the movable contact aside as it moves downward, causing it to come into contact with the fixed set of contacts. A capacitor mounted directly to the leads of the switch helps absorb some of the switching transients caused by the mechanical action of the switch. Other circuitry helps clean up and shape the switch closure waveform so that reliable keypresses are detected. Each switch is encased in a snap-on clear plastic housing that keeps out dust, but allows removal for adjustment and cleaning of the switch contacts by service personnel.

The Backplane of the NCR 18-2

The backplane that provides interconnection between the circuit boards, keyboard, power supply, and display subsystem is hand-wired, with point-to-point connections painstakingly put in place by workers that had to have immense patience to be able to do this type of wiring day-in and day-out. Each of the ten circuit boards that make up the logic of the machine plugs into four edge-connectors in the backplane. Each connector has 20 gold-plated contacts, for a total of 80 possible connections for each circuit board. One extra set of two edge connectors provide an eleventh slot for the display subsystem to plug into.

The Power Supply of the NCR 18-2

The power supply for the machine takes up the back portion of the chassis, and, like most of the other aspects of this calculator, seems quite overbuilt. It is a complex power supply, with it appearing that all voltages (with the exception of the Nixie tube drive voltages) being transistor-regulated, and heavily filtered. Power supply voltages are +4.5 Volts DC for the logic supply for the Diode-Transistor Logic (DTL) integrated circuits; +6.5 Volts (variable within a range (+5.5V to +8V) based on temperature) for the core memory driver circuits; -10 Volts DC also used as a bias voltage in the core memory system; +100 Volts DC used as the cathode supply for the Nixie tube display; and +200 Volts DC for the Nixie tube anode supply.

Detail of the 18-2's Nixie Display (note lit "Overflow" Indicator)

The display subsystem of the 18-2 is a modular assembly that plugs into the backplane. The subsystem consists of a circuit board to which the Nixies are soldered, with a fairly elaborate metal frame to align and secure the display tubes. A metal bezel with stamped digit designator nomenclature presents the faces of the Nixie tubes to the user. Along with the sixteen Nixie tubes, which have 5/8-inch tall digits and a right-hand decimal point, are two incandescent lamps situated behind jewels at the left end of the display which light to indicate a negative number and/or overflow of the calculator.

A typical logic board from the NCR 18-2

The 18-2 uses small-scale integrated circuits for its logic. A total of 134 IC devices combine forces with a small core memory array, and an assortment of discrete components to make up the logic that runs the machine. The IC's used in the 18-2 are a bit of an enigma. I had expected, given that the machine was clearly manufactured in Japan, that it would use Japanese-made integrated circuits. This machine is an exception, as it uses small-scale bipolar DTL IC's from the American IC manufacturer Signetics.

A close-up view of one of the IC's (Signetics ST670A Triple Three-Input NAND Gates)

A total of four different IC part numbers are all that are used in the machine. The devices are from Signetics' SP/ST 600A low-cost, high-reliability DTL integrated circuit family. The four devices used are the ST616A Dual 4-Input Expandable NAND gates; ST629A Single RS/T Flip Flop; ST670A Triple 3-Input NAND gates; and ST680A Quad 2-Input NAND gates.

Each circuit board is approximately 11 inches wide by 4 1/2 inches tall, and is made of a phenolic circuit board material. Edge connector fingers are gold-plated, which makes for a very reliable and corrosion-resistant connection. Components are mounted only on the front of the boards, with interconnection traces etched on both sides of the board. Feedthroughs connecting the two sides of the board are implemented by placing a pad on each side of the board, with a hole drilled through. A piece of wire is inserted in the hole and soldered on each side of the board. Jumper wires are occasionally used on either side of the board to provide interconnections when etched traces couldn't be routed. A metal stiffening bar is riveted across the top edge of the circuit board to provide some structural ridgidity for the boards.

The circuit boards are numbered 1 through 10, with board #1 closest to the front of the machine. Board 1 contains the add/subtract and memory total control logic, binary full ddder, clear function Logic, negative indicator driver and sign determination logic, and decimal point placement logic for divide and square root. Board 2 provides sequence control logic for multiply, divide, and square root, as well as overflow detection for multiplication. Board #3 contains three counters (Y, Z, and MQ) used in all operations of the calculator, as well as right-shift control and the round-off logic. Circuit Board #4 has the basic clock generation and timing chain logic, register selection control, shifting control, and memory 1 round-off logic. Board #5 contains a sequencing counter as well as operation start control logic, right-shift gating logic, numeric entry control, function control logic, overflow control, and left shifting of the decimal point when digts are entered behind the decimal point. Board 6 contains overall decimal point placement logic, as well as additional divide and square root sequencing logic. Board 7 contains the magnetic core memory module, core memory column drivers and sinks, and four core memory sense amplifiers. Board 8 has the core memory row drivers and sinks, and sense amplifer strobe timing. Board 9 provides the Nixie tube display anode drivers, more numeric entry logic, as well as additional logic for divide and square root functions. Lastly, Board 10 has the Nixie Display cathode drivers, numeric entry decoding, and function key decoding and storage.

The Core Memory Board (Core Array at Center)

At the time this machine was designed, during the late part of the 1960's, the levels of integration possible in IC technology were such that it wasn't practical in most cases to implement the operating registers of an electronic caluclator using individual flip flops. Some of the Japanese MOS (Metal-Oxide Semiconductor) IC's could implement a 10 or 12-bit shift register in a single chip, but even at that level of integration, it would require quite a number of IC's to implement all of storage required for the working registers of a calculator of this physical size. A number of schemes were used for register storage during this period, including magnetostrictive delay lines (see the Monroe 990 exhibit for an example of this technology) and magnetic core memory. The NCR 18-2 uses a small magnetic core memory array, manufactured by Japanese electronics component and equipment manufacturer Mitsubishi, as its register storage means. The core array consists of four 8x8 core planes for a total of 256 bits of storage. Magnetic core memory uses small doughnut-shaped rings of a special magnetic material, all woven into a grid of wiring that allows each individual core to be magnetized in one direction or another, as well as allowing the magnetic state of each core to be individually detected. With such an arrangement, it is possible to store the working registers of the calculator (the display register, a working register, and the two memory registers) as a series of magnetic ones and zeroes within the core array. Magnetic core memory technology evolved out of work by Dr. An Wang, a pioneer in the field of magnetic core memory technology, and later, the founder of Wang Laboratories, a company that made a fortune in the high-end electronic calculator business from the mid-1960's through the late 1970's.

Keyboard Detail on the NCR 18-2

The color scheme of the NCR 18-2's keyboard is somewhat odd...leading me to believe that perhaps some keycaps were replaced over the life of the machine. Keycaps are colored white, ivory, beige, or gray, in a mixture that doesn't make a whole lot of sense. The [4] and [6] keys are white, while the rest of the digit keys are ivory in color. I believe that perhaps the [4] and [6] keycaps were replaced, and perhaps there were differences in the color of the plastic, or simply the fact that the [4] and [6] are newer and haven't aged as much is why they keys have the lighter color. Another example is the [X] key, which is gray, while the other math function keys are beige. Perhaps the [X] key was also replaced and somewhere in the process, the colors were changed. In any case, keycap coloring aside, the layout of the NCR 18-2's keyboard is logically thought-out and attractive from an aesthetic point of view. There are four groups of keys, with the left-most group providing miscellaneous functions, including the [→] key, memory register recall and clear keys, along with the clear all and clear indicator [CI] keys. The next group of keys is the traditional numeric keypad, with oversized [0] key, and a raised area on the [5] key to allow the user to orient their fingers on the keyboard by touch. The math functions make up the next group of keys, with the memory accumulation function keys rounding out the groupings.

At the left end of the keyboard assembly is a thumbwheel switch that is used to select the decimal point position for addition, subtraction, and memory recall operations. The decimal point selection thumbwheel provides selections for 0, 1, 2, 3, 6, and 9 digits behind the decimal point. Above the decimal point selection thumbwheel is the push-on/push-off power switch. At the right end of the keyboard, a three-position slide switch selects the rounding mode of the calculator. The switch has no detectable nomenclature on the keyboard panel, either it was never there, or the nomenclature was printed on the keyboard panel in such a way that it wore off over time and usage. The rounding mode of the machine operates only with addition and subtraction operations, where the fixed decimal point setting of the machine comes into play. The uppermost position of this switch forces the calculator to round the least-significant digit up in all cases. The center position of the rounding switch causes the calculator to round up if the next less-significant digit is five or greater, and to truncate if the digit is four or less. The lower-most position of the rounding switch causes the calculator to ignore the next less-significant digit, leaving the least significant digit alone in all cases. As an example, performing 1 divided by 3, followed by pressing the [+] key (thus forcing the machine to fix the decimal at the selected position), with the decimal position set to '2' would result in: 00000000000000.34, 00000000000000.33, and 00000000000000.33 with the rounding switch in the upper, middle, and lower positions. Performing 2 divided by 3 with the same settings would result in 00000000000000.67, 00000000000000.67, and 00000000000000.66.

The Model/Serial Number Tag on the NCR 18-2

The 18-2 uses a curious mix of algebraic and arithmetic logic. Addition and subtraction operate arithmetically, with the function entered after the operand, like an adding machine. For example, to subtract 15 from 30, the problem would be entered as '30', [+], '15', [-], with the result of 15 showing in the display after the [-] key is pressed. Multiplication and division are entered algebraically, with the [=] key completing the operation. As mentioned before, the decimal point location in multiplication and division is fully floating, with the calculator placing the decimal point wherever needed to wring the maximum accuracy out of the result. However, as soon as a result of a multiplication or division is submitted for addition or subtraction, or stored in a memory register, it is forced to the number of digits behind the decimal point as selected by the decimal point selection thumbwheel (and rounded according to the setting of the rounding mode switch).

Internal QA and Modification Record Tag (located under the keyboard, glued to the cabinet base)

The memory capabilities of the 18-2 are quite handy, with two completely independent accumulating memory registers. Each memory register has keys to clear, recall to display, add to memory, subtract from memory, and add product/quotient (acting as "=", followed by add to memory function) to memory. The memory registers adhere to the decimal point location selected by the thumbwheel, with rounding occurring as defined by the position of the rounding mode switch. The memory registers, although residing in non-volatile core memory, are automatically cleared when the calculator is powered up, so it isn't possible to recall a number in a memory register through a power off period, as it is on some other calculators that don't clear the core memory at power-on time.

The NCR 18-2 does have a few quirks in its operation. Division with dividend larger than 14 significant digits causes incorrect results. This is a common limitation on many early electronic calculators, mainly due to the fact that some digits of the working register of the calculator are used as temporary storage during the accumulation of the quotient. The overflow detection on the machine is a bit dicey, sometimes failing to indicate an overflow (mainly in large multiplication operations) when one should occur. When an overflow does occur, the keyboard is not locked out, meaning operations can continue, although the results obtained when the calculator overflows are generally uesless. Division by zero results in the machine getting quite confused, causing the machine to become unresponsive to the keyboard. Pressing the [C], or [CI] key will stop the futility of the effort, but pressing [CI] leaves the machine in a strange state, requiring a full clear (using the [C] key) to restore it to normal operation.

The 18-2 is not a speed demon, with some division operations taking nearly a second to complete. The "all nines" (which in this case, is 14 nines due to the limitation mentioned above) divided by 1 division problem takes just a shade under 1 second to perform. During the calculation, the Nixie tubes put on quite a light show, as the displays are left active as the calculator subtracts, shifts, and accumulates the quotient. Manufacturer-stated calculation times are 430 milliseconds (0.43 second) maximum for multiplication, division and square root. Addition and subtraction are quoted as taking 20 milliseconds (0.02 seconds).


Sincere thanks to Mr. Peter Broughton for providing a wealth of information on Busicom's early calculators.
Thanks to Laura and Michael Greenfield for the opportunity to acquire this machine for the museum

Text and images Copyright ©1997-2017, Rick Bensene.