Monroe 620 Calculator
The 620 is a very close relative of the Canon Canola L121, using the same Texas Instruments-made four-chip LSI set for its brains. The only major differences between the Monroe 620 and the Canon L121 is the obvious difference of case styling, and also the fact that the 620 gets an additional digit of capacity, with 13 digits instead of the 12-digit capacity of the L121.
The 620 is member of the 600-series of Monroe calculators, which include the 610, 620, 630, 640, and 650.
Interior View of Monroe 620
The 620 is a basic four function electronic calculator with a single memory accumulator register. The calculator has a 13-digit Nixie tube display, and uses fixed decimal point logic, with the decimal point location selected via a rotary switch on the keyboard panel, at 0, 1, 2, 3, 4, or 6 digits behind the decimal point.
Closer Internal View Showing Texas Instruments LSI Devices, Nixie Display Array, and Power Supply
The LSI chips take up most of the real estate of the main circuit board, with only a few discrete components which generate the master clock signals that orchestrate the timing of operations within the chips. The calculating logic of the Monroe 620 is divided up amongst the four LSI's in the chip set. The "Entry Chip" (TMC1754) provides the logic for handling the input from the keyboard; It takes care of key de-bouncing, logically locking the keyboard on overflow, keyboard encoding, and other input-related functions. The "Data Chip" (TMC1733) contains the majority of the working registers of the calculator, logic for routing and some processing of data as it is channeled 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 based on the externally-generated master clock, as well as providing some miscellaneous functions such as keeping track of decimal point location. Last, the "Arithmetic Chip" (TMC1807) contains the logic that 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 Nixie tubes, JPC B-5755's (Japanese-made clones of Burroughs parts of the same part number), use discrete-transistor driver circuitry, and are multiplexed. The keyboard is of the standard magnet-activated reed-switch design. A special circuit in the Entry chip monitors the keyboard causing an overflow error should more than one digit key be activated at a time, making it more difficult for data entry errors to occur. When the overflow condition is raised, the keyboard is ignored until the [C] key is pressed to clear the calculator. The keyboard chassis is made of a high density and very sturdy plastic frame in which the key-switch modules are contained.
Detail of the Monroe 620's Overflow Indication
The calculator performs the standard four functions. A push-on/push-off key 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 [CD] (Clear Display) key to clear the display and re-enter the number. The [CD] key also clears the machine in the event of an overflow condition. The [C] key clears everything, excluding the memory register. The [C] key also removes any error condition except if the memory register has overflowed. To clear a memory overflow condition, the memory must be recalled with the [*] key, which recalls the content of the memory to the display (with any overflow digits discarded), and clears the memory register. The memory of the machine operates somewhat differently from the Canon L121, though they both share the same LSI chip set. The differences are likely programmed by jumpers on the circuit board that define the various different configurable functions of the logic chip set. The [=+] key acts as an "=" key, and adds the result of the calculation to the memory register. Likewise, the [=-] key subtracts the result of a calculation from the memory register. In either case, the display is not affected. To recall a memory register content without clearing it, the [⋄] key performs the task. The [*] key, as mentioned before, recalls the memory register to the display and clears the memory register. 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 in the shape of a "-" sign indicates negative results, and another neon indicator is positioned behind a jeweled indicator near the lower left end of the display that lights up the word "OVERFLOW" (see photo) when an overflow/error condition occurs.
The model ID tag on the Monroe 620 (note the "Litton" logo at the left end of the tag)
Dividing by zero results in no indicated error condition, and the calculator enters an undefined state, where the machine counts in an internal (not displayed) register at a rate of approximately 10,000 counts per second. Pressing a single digit key, followed by the [+=] key will display the count as of the time that the digit was pressed. Other key presses result in unpredictable results. Pressing the [C] key resets the logic of the calculator and returns it to normal operation.