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Monroe 650 Calculator
Updated 5/10/2005
The 650 is Monroe's version of
Canon's Canola L163 desktop calculator, sharing
the same basic internals, but packaged with Monroe's inimitable 'style'
of machines from the era. The 650 is the top-end machine of Monroe's 600-Series, which include the 610, 620,
630, 640 and 650.
The 650 works virtually identically
to the Canon Canola L163, except for a couple of minor deviations.
The 650 does not provide the [%] key (useful for percentage
calculations) that is provided on the Canon L163. To make up for this,
the Monroe 650 offers more capable memory functionality than the L163.
All of this done in the interest of market differentiation, allowing
Canon and Monroe to co-exist in the marketplace, yet sell machines that
are essentially clones of each other internally.
Monroe also marketed the Monroe 640,
identical to the 650 with the only change being the omission of the
square root function. Internally, the 640 and 650 are identical.
The electronics of the 640 are fully capable of calculating square roots, but
Monroe simply removed the [√] key to access the function on the 640.
The additional cost for the square root key on the 650? $70! Just for
a keyboard switch and key cap.
Interior of Monroe 650
Both the 650 and its Canon counterpart
share the same basic internal calculating platform, made by Canon in Japan.
The exhibited Monroe 650 was manufactured in late 1972, as opposed to the
mid-1971 production of the Canon L163 in the museum. It is apparent
from comparison that a few minor changes were made, but the basic platform
is identical. The five-chip LSI chip set that makes up the brains of the
Monroe 650 is identical between it and the Canon 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 direct the operation of the calculator. 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 the
exhibited Monroe 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 Monroe 650 or a Canon L163.
The state of these jumpers are read by the microcode at power-up to
customize certain aspects of the calculator (such as the [%] key on
the Canon L163 versus memory function key(s) on the Monroe 650)
Main Board of the Monroe 650 (note daughter board)
The 650 also contains an add-on daughter board that contains five Toshiba
small-scale integrated circuit devices in dual-inline packages. This board
is not included in the Canon version of the machine. The function this board
performs is not known, 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
silk screen on the main board.
Detail of Display Subsystem The Monroe 650, like the Canon
L163, has 16 digits of capacity, using Nixie tubes for the display.
The Nixie tubes are driven by discrete transistor driver circuitry, and
are driven in a multiplexed fashion. Along with the sixteen Nixie tubes, a bank of 15 individual neon indicators are situated above
and between each of the Nixies, providing indicators that help
with display readability by lighting up to group the digits in the display
into groups of three. The display subassembly connects
to the main circuit board through 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 while
preserving as many digits behind the decimal point as possible..
A neon indicator with an orange jewel labeled "OVERFLOW" is situated
at the left end of the display, below the Nixie tubes. This indicator
lights when an illegal operation has been performed (for example, division
by zero), or when the capacity of the machine has been exceeded.
When the OVERFLOW indicator is on, the calculator keyboard is logically
locked, ignoring any key presses other than the [C] key to clear
the calculator and the overflow condition.
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 in the display panel.
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, [K], enables a constant function for multiplication and division.
A three-position slide switch selects the rounding mode of the calculator, 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 is provided that swaps the two operands in
multiplication and division operations, and a [←] 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, as well as resetting
any error condition. The 650 has two completely independent accumulating
memory registers. Each register has a neon indicator in the keyboard panel
that 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 "Σx) allows for automatic summation of the
first operand in multiplication and division operations. 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 [+=] or [-=] key. Each time either of
these keys is pressed, the counter increments by one count. This counter
is useful as an item counter for functions such as calculating
averages of groups of numbers. 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, 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 simple diode arrays. The numeric keypad is encoded into
four bits of BCD (Binary-Coded Decimal), and the function keys are encoded
into unique eight-bit function codes. The keyboard assembly connects
to the main circuit board through 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
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 The exhibited Monroe 650 spent its life as a dedicated servant of the
U.S. Department of Agriculture, Forest Service Division,
Stanislaus National Forest, California, as evidenced by two asset tags
affixed to the machine. Date codes on IC's in the machine, along with two
Quality Assurance tags inside the cabinet place the manufacture of the machine
in the late part of 1972. The calculator was obviously was well-cared for, as
it was quite clean inside, and the cabinet shows only minimal signs of
wear, fading, and grime accumulation.