Early Electronic Calculator Toshiba BC-1212 Calculator

Manufacturer:  Toshiba Model:  BC-1212 Year:  1970 Form:  Desktop Functions:  Basic four, accumulator, constant mode Number of Digits:  12 Display Type:  NIX Display: NEC LD-955A Logic Technology:  DIS,SSI,MSI Memory Technology:  MSI-MOS Diodes:  481 Transistors:  54 Principal ICs:  JMOS (TM4000,..) IC Complement: Toshiba TM4005 (7) Toshiba TM4006 (3) Toshiba TM4103 (15) Toshiba TM4105 (1) Toshiba TM4106 (2) NEC µPD13 (10) (38 total) Tech. Data Level:  3 Tech. Data Source:  RE & MF Tech. Data Pages:  17 Tech. Data: Schematic (pdf)

At the user level this is a standard-for-the-period 4-function-with-accumulator model.

Technically, it is notable for a nice, clean, canonical state-machine design, implementing straightforward multiply and divide algorithms. Compared to many of it's period, it's easier to understand at a detailed technical level. It's a good candidate for examination into how bit-serial machines of the era work. The reverse-engineered schematic includes a state diagram and state action descriptions.

Anomalies:

Division by 0 will result in an infinite loop. Multiplication of a negative number will result in a very long or infinite loop. Performing a division, followed by exchange and division by 1000 will expose the numerals of the first divisor which had been shifted into the hidden 4 digits of the registers. Changing the decimal point position switch between entry of the operands during an addition will result in an incorrect sum.

Notes:

Some units of this series were manufactured with some capacitors with an inadequate voltage rating. The consequence of these capacitors failing is the destruction of rare integrated circuits, although it possible to construct a substitute from modern CMOS. In more detail: The digit driver circuits for the Nixie anodes (see schematic below) contain DC-blocking capacitors on the input. These capacitors require a high voltage rating in order to isolate the high Nixie anode voltage (180V) from the low-voltage logic circuits. When one of these capacitors shorts, 180V is fed to a µPD10 IC in the digit-timing ring-counter. The diode in the circuit, although reverse-biased, is inadequate to block the 180V. The capacitor must be able to tolerate 180 + 24 = 204 volts. The rating on the installed capacitors is 200V. There are 12 of these capacitors, orange, "puffed-pillow" shape, located in a row on the smaller of the 2 logic boards. Physically smaller capacitors with a higher voltage rating are available today. I replaced them with 0.022µF at 400V and they fit better than the originals. Note MOS handling precautions are advisable during any servicing of these units as the ICs are MOS technology. 'See the BC-1212 simulation.'

Top of case removed.	Keyboard folded to the side.	The electronics package removed from the case. There are 3 stacked boards held in a frame.
The main logic board for the calculator.	The board on the left with the Nixie tubes contains the power supply. The board on the right holds the master clock, timing counters, the Y register and ALU, and display decoders and drivers. The column of orange capacitors are replacements for the originals which had an inadequate voltage rating. A breakdown/short failure of one of these capacitors blew one of the rare ICs in the unit.	Closeup of the main board showing the unusual construction to improve the discrete component density. The rows of diodes and resistors form the discrete logic gates. A row of diodes with a common bus-bar at the top is essentially one diode-logic gate: each diode is one input, the bus-bar is the output. The resistor rows are mostly gate load resistors with a common power supply connection on the bus-bar.