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Brother Calther 412 Electronic Calculator
Updated 8/26/2008
This is an interesting and unsual calculator. The Calther 412 appears to be the first desktop electronic calculator offered to the marketplace by Brother Industries. The machine exhibited here 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 ICs are relatively simple (a few gates or a flip-flop or two in one package) small-scale bipolar devices, with logic levels compatible with DTL/TTL integrated circuit logic. Brother Industries did not set any benchmarks with this design, as numerous other electronic calculator manufacturers had already begun using small-scale bipolar integrated circuits a couple of years before this machine was made.
Inside the Calther 412
It seems that the term "Calther" in the model name was the original name for the calculator. The earliest production machines seem to bear the Calther name. Based on advertising, it appears that the model name of "Calther 412" was later replaced by "Procal 412". The "Procal" moniker was carried through on later electronic calculators made by Brother Industries in their continuing (albeit rather short-lived) line of electronic calculators.
The Calther 412 has relatively high quality construction. The digital electronics are contained on 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 traces appear to be tin-plated copper. 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.
Situated between each of the Nixie tubes is a discrete Neon bulb
that is used to separate the multiplicand from the multiplier when
multiplication functions are being carried out. This allows the operator
to view both the multiplier and multiplicand on the display at the same
time, purportedly allowing the operator to make sure both numbers are
entered correctly. Why such indication is important in multiplication, but
not implemented in some fashion on the other three math functions is probably
not really for operator accuracy, but more likely due to the design architecture
of the multiplication metholodoly used by the machine. The Calther 412 does not
perform zero suppression, either leading or trailing, although the machine
does seem to try to minimize the number of trailing zeroes through its
automatic floating decimal point logic. The Nixie tubes are
driven by transistorized display driver circuitry on the top-most board
in the stack. The display is multiplexd, meaning that the machine is
constantly "running", even though no activity is being performed by the
operator. During idle time, the
display is continuously refreshed at a rapid-enough pace that the display
appears continuous to the human eye. In reality, each digit is lit for a short
period of time, as each digit circulates through the logic of the machine.
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 high-quality plastic, with
moulded-in legends.
Detailed view of keyboard The machine is not capable of
directly processing negative numbers. Performing a calculation that results
in a negative answer ends up displaying the 10's compliment of the result,
e.g, 5 - 10 = 999999999995. Pressing the [-] key after such a result occurs
will complement the result. For example, in the case above, the display
would read 000000000005 after the [-] key was pressed.
The machine also multiplies in an unusual fashion
way. First, the multiplicand is entered, then the [X] key is pressed. When
the [X] key is pressed, a neon tube situated between the Nixie tubes lights
up after the last digit of the multiplicand. Then the digits
of the multiplier are entered, 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 bulb
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 in the display 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. Division does not function this way, and
operates the same as most other electronic calculators. When the [÷]
key is pressed, the dividend disappears (it is stored in a hidden register),
and the display clears awaiting entry of the divisor.
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.
It appears that the later machines designated with the "Procal" model-name
changed this function key to [K] (a commonly-used nomenclature for
"Constant"), and modified the design of the calculator such that the
function worked both for multiplication and division.
On the Calther 412, 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. This limitation is
pointed out in sales literature, so it is likely caused by the fact that
the calculator uses the uppermost digit as a counter during division operations,
a common cost-savings methodology (by reducing the amount of circuitry needed
to keep track of the number of successive subtractions done while performing
division) used in many calculators prior to the introduction
of large-scale integrated circuitry.
The 412 takes about 400 milliseconds (0.4 seconds) to perform the
most-difficult 'all-nines' (which, in this case is 9999999999) divided by 1
calculation.
