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 Four-Input Expandable NAND gates; ST629A Single RS/T Flip Flop; ST670A Triple Three-Input NAND gates; and ST680A Quad Two-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. Feed-throughs 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 rigidity for the boards.

The circuit boards are numbered one through ten, with board #1 closest to the front of the machine. Board 1 contains the add/subtract and memory total control logic, binary full adder, clear function Logic, negative indicator driver, 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 designated Y, Z, and MQ that are used in all operations of the calculator, along with logic for right-shift control and the round-off. Circuit Board #4 has the master clock generator and timing chain logic, register selection control, shifting control, and memory 1 round-off logic. Board #5 contains a sequencing counter, operation start control logic, right-shift gating logic, numeric entry control, function control logic, overflow control, and left shifting of the decimal point when digits 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 current sources and sinks, and four core memory sense amplifiers. Board 8 has the core memory row current sources and sinks, and sense amplifier strobe timing. Board 9 provides the Nixie tube display anode drivers, more numeric entry logic and 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 calculator 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 with the capabilities of the NCR 18-2. The NCR 18-2 uses a small magnetic core memory array, manufactured by Japanese electronics component and equipment manufacturer Mitsubishi, as its register storage system. 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 ferrite magnetic material, 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, potentially as a result of some key caps being replaced over the life of the machine. Key caps are colored white, ivory, beige, or gray, in a mixture that doesn't seem to correlate with their function. The [4] and [6] keys are white, while the rest of the digit keys are ivory in color. It seems that perhaps the [4] and [6] key caps were replaced, and at the time they were replaced, there were differences in the color of the replacement keys, or the difference could be that the [4] and [6] are newer and haven't discolored from as much as the other digit keys 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, key cap 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 in the last group.

At the left end of the keyboard assembly is a thumb-wheel switch that is used to select the decimal point position for addition, subtraction, and memory recall operations. The decimal point selection thumb-wheel provides selections for 0, 1, 2, 3, 6, and 9 digits behind the decimal point. Above the decimal point selection thumb-wheel 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 the usual-for-the-time 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 thumb-wheel (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 thumb-wheel, 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 after a the calculator has been powered off, as can be done on some other calculators using magnetic core memory 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 electronic calculators of th era, mainly because two digits in the working register are used as temporary counters during the division calculation. 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 useless. Division by zero results in the machine becoming quite confused, causing it to become unresponsive to any keyboard input except the [C], or [CI] key, which will stop the futility of the effort, though 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). Generally the manufacturer-quoted calculation times are averages rather than worst-case.