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Canon Canola L121 Calculator

Updated 10/02/1998

A calculator identical to this one played a part in my younger life in developing an interest in numbers and math which helped me in later years in my career in computer science. My father had one of these machines for use at his office, and during one particularly busy time, he brought the machine home with him over a weekend to get some overflow work done. Once I got my hands on that machine, my Dad literally had to pry it away from me so he could get his work done. From that time on, he would bring the machine home from time to time just for me to play with. Alas, after years of service, the machine failed. He brought it home for me to take apart. I wanted to fix it, but my knowledge and skills in electronics were pretty limited at that time, and so the machine ended up being taken apart and eventually 'parted out' for the resistors, capacitors, and other parts which were interesting. The IC's were a total mystery, and beyond comprehension at the time. I still have the bank of Nixies from that original machine.

Interior of Canon L121

The Canon L121 is a basic four function electronic calculator with memory. The machine had a 'little brother', the L120, which is essentially identical, except it does not have the memory functions of the L121. The calculator has a 12-digit Nixie tube display, and operates with floating decimal input and fixed decimal output. The fixed decimal point position is selected via a rotary switch on the keyboard panel, at 0, 1, 2, 3, 4, or 6 digits behind the decimal point. The calculator has a very 'solid' feel about it, seemingly built with the same care and precision that Canon's cameras are famous for. It has a small plastic carrying handle which extends out of the back of the machine for easy transport.

Main Circuit Board and Display Detail

The Canon L121 is an LSI IC-based calculator, with this particular machine being made in the mid-to-late 1972 time-frame, based on date codes on parts in the machine. The calculator's smarts come from a four-chip set, made by Texas Instruments. Each of the four chips provide a specific functional part of the logic of the machine. The Entry Chip (TMC1754) provides the logic for handling the input from the keyboard; It takes care of key de-bouncing, locking the keyboard when errors exist, keyboard encoding, and other input-related functions. The Data Chip (TMC1733) contains the majority of the working registers of the machine, logic for routing and some processing of data as it is routed between the various registers, and some Nixie display decoding/multiplexing logic. The Timing Chip (TMC1753) provides for controlling and coordinating the operations of the calculator, as well as providing for some miscellaneous functions such as keeping track of decimal point location. Last, but not least, the Arithmetic Chip (TMC1807, apparently an updated version of the TMC1755 mentioned in the service manual for the machine) contains the logic that actually performs arithmetic operations. All of the main logic of the machine is packed onto one circuit board, with hand-wired interconnects between the Nixie display module, keyboard module, and power-supply module. The Nixies are driven with common transistor driver circuitry, and are multiplexed rather than directly driven. The keyboard is made up of key caps with molded in nomenclature, with a return spring under each key cap. The key stalk has a small magnet attached to it, which moves within the proximity of the magnetic reed switch when the key is pressed. The magnet activates the switch, closing the circuit for the pressed key. A special circuit monitors the keyboard, causing an overflow error should more than one key be activated at a time.

Closeup of the Data and Arithmetic LSI's in the L121

The calculator performs the standard four functions. A slide-switch labeled "K" enables or disables the constant function, which works for multiplication or division only. The [RV] key swaps the content of the hidden operator register with the display, useful in cases where the divisor and dividend in a math operation need to be swapped. The [←] key deletes the last digit on the display, shifting the display to the right, to allow for correction of input errors. The backspace key doesn't handle undoing the decimal point if it is backspaced over, so if a decimal point is entered in error, the backspace key can't be used to fix the error, and the user must resort to using the [CI] (Clear Indicator) key to clear the display and re-enter the number. The [C] key clears everything, as expected. It also removes any error condition EXCEPT if the memory register has overflowed. If the memory register has an overflow condition, the [CM] (Clear Memory) key must also be pressed to clear the overflow indicator. The memory functions of the calculator operate traditionally, with a larger key with a white [M] for adding the content of the display to the memory register, and smaller key with a red [M] for subtraction. The [CM] key clears the memory register, and the [RM] key recalls the content of the memory register into the display. A slide switch selects whether or not the calculator performs roundoff. A neon tube indicator at the left end of the display, with a cut-out window showing an arrow shape pointing to the left indicates any overflow condition, and a similar indicator, at the right end of the display, with a window in the shape of a minus sign, indicates if the number on the display is negative. Dividing by zero results in no indicated error condition, and the calculator enters a somewhat strange state, which can be cleared by pressing the [C] key.

Canon L121 Block Diagram

From the service manual for the calculator, some technical details. The machine runs on a two-phase clock that cycles at 100KHz. The clock signals are generated by a transistor-feedback oscillator, conditioned, then fed to the Timing Chip. The keyboard is divided into two sections, the digit (0-9) keys, and function keys, which are encoded separately. The digits encode into standard four-bit binary-coded decimal(BCD), and the function keys encode into a five bit pattern. The Entry and Arithmetic chips are both of similar design, using the notion of a PLA (Programmable Logic Array) to implement the logic of each chip. A Programmable Logic Array consists of a general assortment of large number of logic elements (AND, OR, and INVERT logic gates, as well as flip flops) that are arranged on the chip in such a way that the inputs and outputs of the elements can be connected by means of a special layer of metal that is custom designed to implement specific logic functionality. This way, a generalized chip can be 'wired' to implement a given functionality. This concept was invented by Texas Instruments, and is a concept that is still in use in chip designs today. The Nixie display is multiplexed, with a 4-bit register that the display register continuously circulates through, digit at a time. The BCD digit in this register is decoded into a 1-of-10 signal which drives the individual digits of the Nixie tubes. The digit register and the BCD to decimal decoder reside on the Data chip, while the digit timing and selection logic resides on the Timing chip.

Text and images Copyright ©1997-2023, Rick Bensene.

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