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Brother Calther 412 Electronic Calculator
Updated 4/6/2003
This is an interesting and unsual calculator. The Calther 412 appears to be the first desktop electronic calculator offered to the marketplace by Brother International. The machine appears to have been built sometime in 1969. The circuitry in the 412 is transitional, bridging the time between that of all-discrete transistor machines, and calculators based on Large Scale Integration IC's. It uses small-scale integrated circuit logic rather than all transistor logic such as that used in the Friden 130 and Wang 360 calculators. The machine uses a total of 111 Mitsubishi-made M53xx-series 14-pin IC's, packaged in plastic "DIP" (Dual Inline Plastic) packages. These devices are early bipolar IC's, with logic levels compatible with DTL/TTL IC's.
Inside the Calther 412
The Calther 412 is well built inside, with three stacked boards that plug into a hand-wired backplane. The whole electronics assembly is on a hinge, allowing it to swing up and out of the case for easier service. The circuit boards are made of phenolic, and have traces on both sides of the board, with plated through holes for interconnection. The edge connector fingers are gold-plated for reliability. The power supply is an uncomplicated linear transistor-regulated supply. The keyboard plugs into the bottom-most board in the stack via an edge connector, allowing the entire top half of the case to be removed for servicing.
The three circuit boards that make up the Calther 412
The display is made up of 12 Nixie tubes.
Each tube has 1/2" high digits, with a right-hand decimal point.
Decimal point placement is automatic.
Placed between each of the Nixie tubes there is a discrete Neon tube
that is used to separate the multiplicand from the multiplier when
multiplication functions are being carried out. The Calther 412 does not
perform zero suppression.
Magnetic Reed Switch Keyboard Construcion
The keyboard is based on magnetic reed switches. A circuit board has
the reed switches attached to it, and there are holes in the board
where the keystalk (with a small magnet attached) can move through the
board to trigger the switch corresponding to the key being pressed.
There is no interlock to prevent multiple keys from being pressed at once,
and doing so can result in strange results, e.g., Nixies with multiple digits
lit up at once. The keycaps are made of plastic, with moulded-in legends.
/FONT> Detailed view of keyboard The machine doesn't know about negative
numbers. Performing a calculation that results in what would be a negative
answer ends up displaying the 10's compliment of the result, IE: 5 - 10
= 999999999995. Pressing the '-' key when such a result occurs will complement
the result, IE: in the case above, the display would read 000000000005.
after pressing the '-' key. The machine also multiplies in a very strange
way. First, you enter the multiplicand, then press the 'X' key. When you
press the 'X' key, a neon tube situated between the Nixie tubes lights
up after the last digit of the multiplicand. Then, you type the digits
of the multiplier, and the multiplier 'pushes' the multiplicand and the
lit neon to the left as it is entered. For example, if 123 X 456 is performed:
000000000001. 1
000000000012. 2
000000000123. 3
000000000123|. X
0000000123|4. 4
000000123|45. 5
00000123|456. 6
0000123|4567. 7
000000561741. +=
Display after entry of 123 X 4567 (note 'tick' between digits 3 and 4) The '|' indicates the lit Neon tube
that denotes the place where the 'X' key was pressed. Doing multiplies
with fractional numbers is really interesting as the logic doesn't know
how to handle having more than one decimal point on at a time. The result
comes up correct, but the display looks very strange when entering such
calculations. In the picture above, you can see the display after entering
"23456789 X 569" before pressing the '+=' key to get the result.
You can see the lit neon 'bar' between the last digit of the multiplicand,
and the first digit of the multiplier. Oddly enough, division isn't a victim
of the strange method of display that multiplication is.
The Calther 412's "Brag Tag" The Calther 412 has the unusual
inclusion of a backspace key, making correction of erroneous entries
easier. The calculator has a primitive constant function, engaged by the
push-on/push-off "R" key (for Repeat). This function works for multiplication
only, and causes the multiplicand to be retained for successive calculations.
Accidentally leaving the "R" function engaged when doing division causes
the machine to return 0 for all division results. The "A" key (for Accumulate)
is a push-on/push-off key that, when activated, causes the calculator
to automatically accumulate the sum-of-products. The machine doesn't have any
notion of overflow, any overflow is simply discarded, and no indication
is given. The 412 also has some difficulty with division --- it gets
confused if the divisor is greater than 9999999999. The 412 takes about
400 milliseconds to perform the most-difficult 'all-nines' (which, in
this case is ten 9's) divided by 1 calculation.
