Detailed view of Toshiba BC-1211S keyboard. No divide key!

Division is the most complex of the basic four math functions to implement with electronics. Most electronic implementations of division rely on counting repeated subtractions of the dividend from the divisor until an overdraft occurs, then restoring the result from the last subtraction (generally by adding the dividend back in) performing a shift operation, and continuing repeated subtractions until all of the digits of the quotient have been calculated. Because of the complexity of the sequencing circuitry required to perform these operations, and given that division is the least-frequently used calculation in basic business accounting, a number of calculator manufacturers came to the conclusion that making a calculator that did not provide the division function, at a slightly lower cost, would entice business people to get rid of their mechanical calculators and replace them with new electronic technology. In some cases, the calculators were actually designed with division circuitry omitted, and in other cases, the divide key was simply not included on the keyboard. In either case, it was a psychological ploy by the manufacturers to get conservative business types to make the jump to electronic calculating machines. This market strategy was short-lived, as ever-increasing levels of sophistication in integrated circuit technology made it such that was actually more expensive to make a special design that didn't perform division than it was to make a general-purpose four function calculator.

Toshiba BC-1211S without upper cabinet

The Toshiba BC-1211S offers twelve digits of capacity, fixed decimal point locations at 0, 2, and 4 digits behind the decimal, constant mode for multiplication, and a sum-of-products accumulation mode. Addition, subtraction, and multiplication operate as usual for calculators of the day. Decimal point is floating on input, meaning that the decimal point can be located anywhere in the number that is input, but as soon as an operation is performed on the number, it is truncated to the number of digits behind the decimal selected on the decimal position switch. The machine allows a two-key squaring operation to be performed by entering the number, pressing the [X] key, then pressing the [+=] key. At the left end of the display is a neon indicator tube that lights when an overflow occurs. Overflows do not cause any physical or electronic keyboard lockout to occur, which can lead a distracted user to carry on calculating after an overflow has occurred.

The main circuit assembly of the Toshiba BC-1211S

It is interesting to note that the BC-1211S actually has a working register capacity of 16 digits. With the decimal point selector set at 4 digits behind the decimal, numbers larger than 12 digits can be entered and calculated upon. After the 12th digit is entered, the overflow indicator lights, but, as mentioned before, entry is not inhibited. As an example, entering 100000000000000 will display "000000000000" on the display, with the overflow indicator lit. Subsequently performing a multiplication by .0010 will result in "100000000000." in the display, clearly showing that the extra digits were properly kept track of throughout the calculation. The reason for this is likely due to a generalized calculator architecture devised by Toshiba that was scalable for different capacity machines. This approach allows for common circuitry between different models, saving cost. For example, a 10, 12, or 14 digit calculator could share the same logic circuitry, with jumpers or simple circuit changes.

The Bottom-most circuit board. Note the large array of diodes.

This machine was built at a time (late 1969) when integrated circuits were beginning to make major inroads into the calculator scene in Japan. At the time, IC's were mainly used for the more complex circuit elements such as shift registers and flip-flops -- the types of logic elements that required the highest discrete component counts to implement. Simple logic gates were implemented utilizing discrete components mainly because it was still considerably less expensive to create such logic elements from discrete components. As a result of the use of IC's for the more complex logic elements, the BC-1211S contains an unexpectedly small number of IC's, with only 32 IC devices used. Given that a lot of the logic is implemented with discrete components, the transistor count is relatively high, with 57 transistors used (not including power supply transistors). There are hundreds of diodes used, and countless resistors and capacitors.

The Middle Circuit Board

The IC's used in the machine are small- and large-scale (for the time) MOS (Metal-Oxide Semiconductor) IC devices manufactured by both NEC and Toshiba. Japanese electronics manufacturers including Toshiba, Hitachi, and NEC created a number of mass-produced lines MOS small-scale devices in the late '60's. All three also created early "large scale" MOS ICs acting as shift registers and arithmetic logic. The ICs used in the BC-1211S are from the NEC's µPD1x and Toshiba's TM4000 small-scale MOS IC families. There was a lot of cross- cooperation between the big-three Japanese IC manufacturers (Hitachi, NEC and Toshiba) at the time, so it wasn't uncommon to see calculators made by Hitachi with NEC IC's in them, as well as this example of a Toshiba-made calculator with NEC IC's. The ICs are packaged in metal cans with either ten or twelve radial leads. The small-scale devices include a few logic gates, or a few flip-flops. The large-scale MOS devices consist of shift registers that contain either 60 or 64 bits per IC, with the 60 bit shift registers utilized for digit shifting operations. These shift register IC's are used to implement the working registers of the calculator. The machine also benefits from larger-scale IC technology by virtue of a single-chip adder IC that performs bit serial addition of BCD (Binary-Coded Decimal) numbers. Along with the integrated circuits, there is a substantial amount of discrete component-based logic also, made up of diode-resistor gates and transistorized level converters, buffers, and inverters. It appears that the machine also uses a diode-based ROM for some of its sequence control. It is doubtful that the ROM contains true microcode, as the general design of the machine indicates that it uses mostly hard-wired logic. It is more likely that the diode ROM contains sequence tables for orchestrating the various elements of the architecture for processing the math operations the machine.

The Nixie Display & Power Supply Board

The logic of the calculator is contained on three circuit boards, two of which contain the main logic, and the third carrying the Nixie tube displays, their drive circuitry, and power supply components. The boards are interconnected by unusual connectors and sockets that connect the boards together in a vertical stack. The circuit boards are made of phenolic, and have traces on both sides of the board, with plated-through feedthroughs. The boards are of average quality, with a green solder-mask on the component side only, and silk-screened nomenclature. The boards are held in position by a sheet metal chassis that screws into the baseplate of the machine.

A closer view of some of the Integrated Circuits in the BC-1211S

The power supply is of conventional linear design, using a transformer, diodes for rectification and electrolytic capacitors for filtering, followed by zener-diode controlled pass transistor voltage regulation. Supply voltages include -24V, -14V, and +180V for the Nixie tubes. The main power supply components except the transformer are located on the Nixie tube display board. The transformer is mounted to the chassis.

The keyboard of the BC-1211S is of conventional magnetic reed switch design. The keyboard circuit board does have an un-populated location which might be the location where the divide key may go on the BC-1212, so that the same keyboard circuit board can be used for both the BC-1212 and the BC-1211S. The key-caps are of high quality plastic, with molded in color and nomenclature. A slide switch provides for decimal point setting at 0, 2, or 4 digits behind the decimal point. Two rocker switches enable the automatic accumulation of products and constant mode. A larger rocker switch located on the keyboard panel serves as the power switch. The machine uses a hard-wired power cord, which exits through the rear panel of the machine.

The Model/Serial Number Tag

The BC-1211S has a few quirks. First, the machine does not provide for negative numbers. This seems odd on a machine that was likely targeted at business users. Any calculation that would result in a negative answer displays the tens-complement of the answer and causes an overflow condition. For example, performing 1 - 2 displays 999999999999, with the overflow indicator lit. Pressing the [-] key will complement the number in the display, and clear the overflow indicator. Another quirk is that large multiplies can confuse the machine. For example, performing 123456789012 X 987654321098 causes the machine to go into a state with numbers rapidly whizzing through the display. If left for a while, sometimes the churn will stop on its own, with a nonsense answer in the display (which has no relation to the original problem that I can detect), and other times the churn runs without ending (at least for as long as I was willing to wait for it). Pressing the [C] key stops the chaos, and restores the machine to normal operation. Overflow detection on some multiplications is intermittent. Some multiplications which should cause an overflow condition don't end up lighting the overflow indicator. This probably has something to do with the fact that the machine actually has a working capacity of 16 digits, though only 12 digits are displayed.

The BC-1211S is about average in calculating speed for machines of the day. Addition and subtraction complete almost instantly, with only a slight flicker of the display while the calculation occurs. Easy multiplications also complete with just a little more of a flicker of the display. The benchmark full-capacity multiplication of 999999 X 999999 takes just under 1/2 second, and results in an amusing amount of display activity.