Toshiba BC-1411 Circuit Board #8

Board #8 is the circuit board plugged into the rear-most slot in the card cage. This board contains capacitive dynamic memory circuitry that provides the storage for the two working registers of the calculator and the memory register along with some addressing logic.

The memory system used in Toshiba's first-generation electronic calculators is unique in the industry. To the curator's knowledge, no other calculator manufacturer utilized anything even close to this memory technology in any other electronic calculator. Most electronic calculators of the discrete component era, and even into the early integrated small-scale integrated circuit phase used other forms of memory technology to store the bits needed to represent the numbers in the working and memory registers of the calculator. The usually used technologies were magnetic core memory, magnetostrictive delay lines, flip-flop based registers, or rotating magnetic storage similar in principle to today's hard-disk drives. Toshiba came up with another method, which was both simple and highly reliable. The concept was to use capacitors as the memory storage elements. Capacitors are similar to rechargeable batteries that can be charged comparatively quickly, hold electrical charge for a period of time with the charge either used by some kind of load applied to the capacitor, or if no load is present, will naturally bleed away over time. The amount of charge a capacitor will hold is a function of the construction of the capacitor, and the length of time that the charge will remain at a detectable level is also dependent on the design of the capacitor. There are huge capacitors used in equipment such as the Large Hadron Collider. These huge capacitors are charged up with high DC voltages over a many hours such that they store up gigantic amounts of electrical charge. Once fully charged, capacitors like this are capable of discharging themselves completely almost instantaneously releasing the massive amount of energy stored within them in a burst lasting just a few nanoseconds. This burst of very high energy is used to generate a beam of subatomic particles moving at nearly the speed of light. These beams of particles smash into each other such that the equivalent of temperatures exceeding that of the center of the Sun are generated for extremely short periods of time, with subatomic particles smashed together such that they decompose into new types of particles that are expanding our understanding of the underlying construction of our universe. Clearly, capacitors of this nature aren't needed for memory in an electronic calculator, but the principle of the capacitor storing an electrical charge by charging it, then attempting to "use" that charge to indicate whether the capacitor was charged or not can be applied to make a memory system. The capacitors used are quite small, about the size of a

The circuitry on the left part of the circuit board countains the fourteen driver transistors (silver colored cans) and associated discrete components to select one of the fourteen Nixie tube anodes which will display a digit during a particular digit time. The circuitry on the right side of the board contains buffers/inverters that create inverted and buffered versions of bits 8, 4, and 2 in the Binary-Coded decimal(BCD) code presented to the circuit, and buffers for bits 8, 4, 2 and 1 of the BCD number that is to be displayed at a given digit position. These signals are passed to an array of diode gates located below the transistors that decode the BCD code for the digit to be displayed into a 1 of 10 selection representing the digit cathode (0 through 9) to be lit in the Nixie tube. The single signal for the decoded digit is passed to its driver transistor (the gold-colored cans) which provides the current path to the cathode of the coded digit, causing it to light up in the Nixie tube selected by the circuitry on the left side of the board.

The board also has driver and decoding circuitry for the decimal point in its appropriate Nixie tube. The decimal point position is encoded by a special non-BCD digit stored in the digit position within the register where the decimal point resides. This is why there are fifteen capacitors in each row of the memory array, to allow for one extra digit position that has a special non-BCD code in it to represent that the decimal point is to be lit in the digit position currently being displayed, with the digit displayed first, and the decimal point then lit after the digit has been displayed. Thus, the decimal point is also multiplexed along with the digits. You will note that there are eleven transistors in the digit decoding circuity at the top right of the board. The special non-BCD four-bit code for the decimal point is decoded in the same way as the digits zero through nine, but it is decoded by the circuity that turns on the eleventh driver transistor to light the decimal point at the currently selected digit.