Monroe also marketed the Monroe 640, identical to the 650 with the only change being the omission of the square root function. Internally, the machines are identical...the electronics of the 640 are fully capable of calculating square roots...they just removed the keyboard key to access the function on the 640. The additional cost for the square root key on the 650? $70!

Interior of Monroe 650

Both the 650 and its Canon counterpart share the same basic internal calculating platform, made by Canon in Japan. This 650 was manufactured in late 1972, as opposed to the mid-1971 production of the L163 in the museum, and it is apparent that a few minor changes were made in the guts, but the basic platform is identical. The five-chip LSI chipset that makes up the brains of the 650 is identical between it and the L163, with the TMC1761 (Shift Register), TMC1763, TMC1764 (Display), and TMC1765 chips combining forces with two ROMs (TMC1768 and TMC1793) that contain the microcode that runs the machine. The TMC1793 ROM is a later part number than the TMC1769 occupying the same spot on the circuit board in the Canon machine, apparently containing 'bug fixes' or other enhancements that came about due to the later production of this 650.

The main circuit board, shared between both the Monroe 650 and the Canon L163, differentiates the function between the two machines by virtue of a pair of wire jumpers that select whether the board is being used in a 650 or a L163.

Main Board of the Monroe 650 (note daughter board)

The 650 also contains an 'add-on' daughter board that contains five Toshiba small-scale devices in DIP packages. This board is not included in the Canon version of the machine. I don't know what function this board provides, but it is clear that its inclusion was anticipated in the design of the main circuit board, as the locations where the wires that connect the daughter board to the main board are clearly marked in silkscreen on the main board.

Detail of Display Subsystem

The 650, like the L163, has 16 digits of capacity, using Nixie tubes for the display. The Nixie tubes are driven by discrete transistor driver circuitry. Along with the sixteen Nixie tubes, a bank of 15 individual neon indicators are situated above and between each of the Nixies, providing 'comma' indicators that help with display readability by lighting up to group the digits in the display into groups of three. The display assembly is a subassembly that connects into the main circuit board via an edge connector and three individual wires that provide power supply connections. Each Nixie tube contains the digits zero through nine, and a right-hand decimal point. The display system provides leading zero suppression by blanking insignificant zero digits. Trailing zero suppression is provided by the machine always positioning results so that they are right-justified on the display. A neon indicator with an orange jewel labeled "OVERFLOW" is situated at the left end of the display, below the digits. This indicator will light when an illegal operation has been performed (for example, division by zero), or when the capacity of the machine has been exceeded. The sign of the number on the display is indicated by another neon lamp at the right-end of the display, that lights through a minus-sign-shaped cutout.

Keyboard Detail of Monroe 650

The Monroe 650 provides addition, subtraction, multiplication, division, and square root functions. The machine uses arithmetic logic, functioning like an adding machine does, with the addition or subtraction operations coming after each numeric entry. The calculator can operate in fixed or full-floating decimal mode, as defined by a knob with positions for fixed decimal at zero through ten digits behind the decimal point, or "F" for floating decimal. A push-on/push-off keyboard key labeled "K" enables a constant function for multiplication and division. A three-position slide switch selects the rounding mode of the machine, with positions for truncation, rounding, or force up. A feature that exists on the 650 that doesn't exist on the Canon machine is the addition of two neon indicators situated on the keyboard panel above the multiply and divide keys that light to indicate when a multiplication or division operation is pending.

As with just about all Canon-designed machines, an "RV" key swaps the two operands in multiplication and division operations, and a 'back arrow' key provides for correcting numeric entry by backing out digits one at a time. The "CD" key clears the entire display without affecting already entered operands, and the "C" key clears everything except for the memory registers. The 650 has two completely independent, fully functional memory registers. Each register has a neon indicator in the keyboard panel which lights to indicate that the register has non-zero content. Each memory register has independent keys to add to, subtract from, recall, and recall/clear the memory register. Each memory register has a two-position slide switch associated with it that enables a special function of the memory register. Memory register 1's special function switch (labeled "Sigma"x) allows for automatic summation of the first operand in multiplication and division operations, and memory register 2 will automatically accumulate the results of multiplication and division if its special function (labeled "ACC") switch is enabled.

Like the Canon L163, the Monroe 650 provides a two-digit electro-mechanical, solenoid-activated counter that is triggered by depressions of the "+=" and "-=" key. Each time either of these keys is pressed, the counter increments by one count. This counter is useful as an item counter. A small button next to the item counter resets the counter to zero when depressed.

Keyboard Assembly

The keyboard assembly of the Monroe 650 is quite complicated, with a good-sized circuit board that serves both as the 'backplane' for the keyboard switches themselves, as well as containing the keyboard encoding logic. The keyboard uses tried-and-true magnetic reed switches for reliability and simplicity. Keyboard encoding is done by diode arrays. The numeric keypad is encoded into four bits of BCD (binary-coded decimal), and the function keys are encoded into an eight-bit function code. The keyboard assembly connects to the main circuit board via an edge connector that plugs into the front edge of the main board. A separate individual wire connection feeds back to the power supply to provide the high-voltage necessary to drive the neon indicator bulbs for memory status and pending multiply/divide pending indicators. Two separate wires, with insulated plug-in style spade connections provide the connection to the "OVERFLOW" indicator.

A Profile View of the Monroe 650