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Canon 141 Desktop Calculator

The Canon 141 is yet another example of the fact that Canon produced calculators for the US maker Monroe. The 141 is a scaled down version of the Monroe 950 machine also found in the museum. There are some minor differences, but overall the machines share more than they differ. The Canon 141 has a capacity of 14 digits versus the 16-digit capacity of the Monroe 950. The Canon 141 does not know how to deal with negative numbers directly (opting for 10's compliment representation), while the 950 has a '-' indicator and displays negative numbers in the correct fashion. The 141 does not have the 'comma' indicators that help make answers easier to read on the Monroe 950. The Canon machine also skips the "CM" (Clear Memory) key that the Monroe offers. Other than these basic differences, the two machines operate almost identically. Both are four-function, fixed decimal point calculators. Both machines provide a single memory register, and a constant function for multiply and divide. A large rotary knob on the keyboard panel selects the fixed decimal point location, and the machines also have a 'truncate, round, force up' switch on the keyboard panel to determine the handling of the least significant digit in any result. The keyboard layout between the Monroe 950 and the Canon 141 are virtually the same, with only subtle differences in keycap color (the Canon opting for more 'colorful' keycaps, with the Monroe opting for more monochromatic presentation). The Canon 141 in the museum appears to be a slightly earlier machine than the Monroe 950. The 141, based on date codes on IC's, and a date of October 29, 1969 on a Quality Assurance stamp on the NEC-made acoustic delay line, was made during the late part of 1969, versus the early 1970 manufacture of the Monroe 950.

View of Canon 141 sans-case

The similarities between the Canon 141 and the Monroe 950 are more than just cosmetic. Electronically and mechanically they share a very common design. The common elements include a 7-slot motherboard, a virtually identical card cage, very similar power supply circuitry, a common Nixie tube display mechanical design, similar driver circuits for the Nixie tubes, and the use of an acoustic delay line (made by NEC) for working register storage. In fact, the machines share the exact same part number delay line and delay-line driver circuit board. The remainder of the differences between the machines are very subtle, with similar use of small scale integrated circuits making up the logic of the machine. The Canon 141 'backplane' is populated with four boards (leaving three slots filled with 'dummy' circuit cards (the Monroe 950 fills an extra slot, with only two 'dummy' cards).

Keyboard Layout of the Canon 141

The Canon 141, like it's kin, uses TI-made small scale IC's for its brains. A few discrete transistors are sprinkled here and there on the circuit boards, but the majority of the logic is based on the IC's. The 141 contains a total of 73 IC's, all of which are 14-pin DIP devices. The Monroe 950, in contrast, uses 94 IC's. The differences in chip counts between the machines is likely due to the extra two digits of capacity on the Monroe 950, the fact that the 950 properly handles negative numbers, and the 950 has the added logic required for the 'comma' indicators.

A Look at the NEC Acoustic Delay Line

The Canon 141 is a fixed-decimal point machine. The decimal point location is set via a large thumbwheel-style rotary switch located at the upper-right of the keyboard panel. The decimal point location can be set to 0, 1, 2, 3, 4, or 6 digits behind the decimal point. A slide switch located near the decimal location wheel selects whether the machine truncates, rounds up, or forces up the least significant digit on the display.

The Nixie tube display on the Canon 141 is the same as that of the Monroe 950, though the 141 does not have the comma indicators and associated circuitry, and the '-' indicator. The same Nixie tubes are used, and the display control board is of very similar design. The 141 also does not have any indicator to show when the memory register has zero content, a feature that the Monroe 950 does have.

The Nixie Tube Display of the Canon 141

From an operator's point of view, the Canon 141 has all of the expected Canon quality and operating convenience. The left-most group of keys on the calculator are general control keys. The "AM" key, a 'push-on/push-off' key, puts the memory into 'accumulate' mode, where the results of multiply and divide operations are automatically added into the memory register. The "RV" key swaps the operands of math functions. The "back arrow" key works just like the backspace key on your PC keyboard, allowing the user to 'back out' incorrectly entered digits without having the clear the display and start over. The "RM" key brings the contents of the memory register up on the display. The remaining keys clear various sections of the calculator; with "CI" clearing the display register, and "C" clearing the entire machine including the memory register. To the right of the standard digit-entry keypad are the math functions. Addition and subtraction operator 'adding machine' style, with sums and differences accumulating as each "=" key is pressed. The black "=" key adds, the red "=" key subtracts. The multiply and divide functions work as expected, with either "=" key providing the result. The "K" key, a push-on/push-off key, selects a constant operator for multiply and divide operations when actuated. Two "M" keys, one white, one red, control accumulation of numbers in the memory register. The white "M" key adds the content of the display to the memory register, and the red "M" key subtracts the display number from the memory register. The memory register on the 141 has no immediate overflow detection. Any carries out the most-significant digit of the memory register are simply discarded. However, if the memory register has overflowed anytime since the machine was cleared, recalling the content of the memory register to the display (with the "RM" key) causes an immediate overflow condition.

The Canon 141 runs off 110V AC power only, and uses a conventional transformer-based, transistor-regulated linear power supply to generate all of the working voltages needed to operate the machine's logic and display. A circuit board mounted at the back of the machine contains the various components used to filter and regulate the various supply voltages. The 141 uses fuses for the logic supply protection, while the Monroe 950 uses resettable circuit breakers.

The 141 is not very fast, with even simple addition operations taking a noticeable period of time to complete. I'd estimate that the 950 takes perhaps 1/20 second to perform an add or subtract operation, and a good part of 1 second to do an all nines times all nines multiply (correctly providing the least significant 14 digits of the answer, along with overflow), and a little under one second to do 99999999999999 divided by 1. The 141 is just slightly faster than the Monroe 950, likely because it has two less digits do deal with. The Canon 141 does not exhibit the 'bug' that exists in the Monroe 950, such that the 'all nines' divided by 1 expression results in an overflow. Division by zero results in the calculator going into a confused state. The display shows all zeroes, with no decimal point on the display, and no overflow indication. Pressing digit keys results in varying results, sometimes all the decimal points come on, other times the display 'flickers' for a fraction of a second. Pressing function keys causes no noticeable effect. Pressing the "C" key resets the machine when it is in this confused state and returns it to normal operation. The machine leaves the Nixie tubes active during calculations, which results in interesting 'spinning' of the digits while the longer calculations are occurring.

Text and images Copyright ©1997-2011, Rick Bensene.