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Brother Calther 412 Electronic Calculator

Updated 1/22/2012

This is an interesting and unsual calculator. The Calther 412 is the second electronic calculator offered to the marketplace by Brother Industries. The first machine, the Brother Calther 130, debuted in October of 1966, and was an all-transistor calculator sold only in the domestic Japanese market. The 130 was a rather large desktop machine with 13 digit capacity, and was a basic four-funtion machine. Brother Industries waited quite some time before introducing the Calther 412, which appears to have been introduced sometime in the late 1968 timeframe. The delay between the introduction of the Calther 130 and the Calther 412 was likely due to Brother focusing on building an international distribution network to allow its calculators to be sold on the world-wide market rather than just in Japan. Along with the distribution reasons, another possible reason for the delay was the development of small-scale integrated circuit technology by Japanese semiconductor manufacturers, which did not begin to mature until the 1967 timeframe.

The Calther 412 exhibited here appears to have been built sometime in the early part of 1969, fairly early in production run of the machines. 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 small-scale bipolar devices, similar in design to US-developed TTL (Transistor-Transistor Logic) integrated circuit devices. The most complex IC in the machine is a serial-in/serial-out 8-bit shift register IC, used to implement the working registers of the calculator. The most notable aspect of the design of the Calther 412 is that the vast majority of the logic of the machine is implemented with integrated circuits, with very little in the way of discrete diode-based gating. This makes this machine one of the earlier "all IC"-based machines. Earlier MOS IC-based calculators from other Japanese manufacturers utilized a large number of discrete component gating along with the ICs.

Inside the Calther 412

It seems that the term "Calther" in the model name was the original name for the calculator, carried over from the original Calther 130. It appears that the earliest production 412s bear the Calther name, while later, the Calther moniker was abandoned and replaced by "Procal". Based on advertising, later production 412's were sold as the "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 digital electronics of the Calther 412 are contained on three stacked circuit 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 ease of 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 algorithm 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.


Text and images Copyright ©1997-2011, Rick Bensene.