Toshiba BC-1411 Circuit Board #1

Board #1, which is the circuit board plugged into the card cage closest to the front of the calculator. This board contains the Nixie tube driver and decoding circuitry.

The Nixie tube display of the Toshiba BC-1411 is multiplexed, which is a method of driving a display utilizing a minimum amount of circuitry by lighting each digit to be displayed for a short time, then turning that tube off, and moving to the next tube in succession, scanning across the entire display. This is done at a rapid enough rate that the human eye perceives all of the digits in the display as all being lit at the same time. This method means that there only needs to be a group of drivers for the anodes of the individual tubes, and another group of decoders/drivers that light up the correct digit cathode in the selected tube based on the Binary-Coded Decimal digit code presented for that digit.

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.