SCM Cogito 412 Desktop Calculator

The SCM (Smith Corona Marchant) Cogito 412 is a fairly late Nixie tube, yet relatively early Large Scale Integration IC-based, desk calculator. It is a rather basic four-function calculator, but unusual in that it performs just about all its functions in ways different than most electronic calculators. This particular Cogito 412 was likely built in the mid-1970 time-frame, with date codes on IC's ranging from late-1969 through early 1970. The IC's in the machine are made by American Micro-systems, Inc. (AMI). AMI was one of the earliest chip makers to develop large-scale MOS multi-chip calculator chip-sets, generally fabricating the devices from proprietary logic designs developed by calculator companies, or in partnership with them. In the case of the Cogito 412 and Cogito 414 calculators, the logic design was created by jointly by SCM engineers and AMI, and when the logic design was complete, AMI developed the chip design, implementing the logic on the eight MOS chips, each of which contains from hundreds to up to 1,200 transistors, making the chips very advanced for the time. AMI also fabricated complex multi-chip calculator chip sets for Computer Design Corporation (also known as Compucorp), as well as for Tektronix in its Model 21 and 31 calculators).

SCM Cogito 412 with Upper Cabinet Removed

There is a sprinkling of discrete components making up the unusual switch-mode power supply, Nixie tube drive, incandescent lamp drivers, and the master clock generator circuit. The calculator certainly has much fewer components than calculators with less sophisticated LSI integrated circuits.

The SCM 412/414 Switch-Mode Power Supply

The power supply is quite unusual for a calculator of this vintage. A traditional (for the time) linear power supply uses a heavy and bulky multi-tap mains-transformer with rectifiers and capacitive filters on the output tapes, followed by zener-diode or resistor-bridge refernece voltage regulators, with pass-transistors providing the final regulated output. A switch-mode power supply dispenses with the large mains transformer. Instead of the AC line power driving large transformer primary winding(s), the mains power in the Cogito 412 (and 414, which uses the identical power supply assembly), goes through an RFI filter, power switch, and fuse, all of which are normal for any calculator for minimizing radio frequency interferance, as well as safety, is directly rectified into a DC voltage. This DC voltage is reduced through a resistor divider network and a few transistors into a lower regluated DC voltage that provides power for a transistorized oscillator circuit that typically oscillates at somewhere between 15 and 40 Kilohertz (15,000 to 40,000 cycles per second). The output of the oscillator, which is typically a square wave, is integrated into a low-voltage sinewave running at the oscillator frequency. This is fed into the primary windings of a small transformer (many times smaller than the transforer used in a comparable linear power supply). The secondary windings of the transformer which createa AC outputs in the vicinity of the voltages needed by the circuitry of the calculator. The transformer can be much smaller than that of a linear power supply small because of its much higher operating frequency. Transformers operate more efficiently at higher frequencies, allowing for much smaller and more efficient transformers to be used in switch-mode power supplies. The output windings of the transformer produce AC voltages that a bit higher then what is needed. The AC voltage is rectified by diodes or a rectifier bridge into a choppy, but high-frequency wave, and passed to small filter capacitors that smooth out the choppiness of the rectified voltage. Much smaller filter capacitors can be used here because they work more efficiently at higher frequencies. Typical linear power supplies required much larger filter capacitors because of the lower frequencies involved, contributing to the size reduction of switching power supplies. After being filtered, the voltage is passed to a voltage regulator circuit that maintains the required DC voltage at the necessary current level to operate the circuitry of the calculator. In the case of the Cogito 412 (and 414) the power supply voltages generated are quite numerous. The main logic voltages used are -15V and -8V. These voltages are used to provide power to the integrated circuits. The number of components in a switch-mode power supply can sometimes exceed the parts count for a more conventional linear power supply, but the reduction in size, as well as the increased efficiency of the power supply means that less heat is generated by the power supply, reducing the requirements for cooling grates in the cabinet of the calculator in order to keep it cool enough inside. The reduction of the power required to operate the calculator is also a plus for the owner of the calculator, because even in the short run, it will cost less to operate than similar calculators that utilize Switch-mode power supplies also are significantly more efficient than an equivalent linear power supply, generally Advertising for the Cogito 414 (which was introduced first, follwed by the 412) touted the machine as being the smallest and lightest electronic calcualtor on the market. The primary reason SCM was able to make this claim was two-fold. Both the the use of the switch-mode power supply, along with the high level of integration used for the logic of the calculator combined to make the calculator significantly smaller than anything on the market at the time of its introduction in August of 1969. making for ligher power output technology. Four of the LSI's are plugged into the main board with Molex-style strip IC sockets, with the other four soldered onto two removable daughter-cards that plug into the main circuit board. The main circuit board and its ICs are shared between the 412 and 414, but the plug in circuit boards contain different ICs, which are specific to each model of calculator.

The machine uses a 12-digit Nixie display, with each tube containing zero through nine, and a decimal point. At the left end of the display, a legend marked "OVERFLOW" is back-lit by an incandescent lamp when results of calculations exceed the capacity of the machine. At the right end of the display panel, a "-" legend illuminates to indicate a negative number in the display.

While the Cogito 412 doesn't have a memory function in the conventional sense, it operates in such a way that a memory is part of the intrinsic operation of the machine. The memory is actually an accumulator which is normally not displayed. The accumulator register is where the results of addition and subtraction operations are stored.

Looking at the keyboard, one sees the usual [+], [-], [X], and [÷] and [=] keys, as well as the [TOTAL], [SUBTOTAL] keys (which would seem more in place on an adding machine). There is also an unusual blue-colored [=+] key. The rest of the keys on the keyboard consist of the common [CLEAR] key (although it performs an unusual secondary function, more on that later), along with the [0] through [9] and [.] keys.

The accumulator register, which is not normally displayed, may be called up to the display through the use of the [TOTAL] or [SUB TOTAL] keys. For example, to add 123 to 456, one would press 123 then press the [+] key, press 456, and press the [+] key again. When the [+] key is pressed after the second number, the display will still show "456", making one wonder where the answer is. The sum is stored in the hidden accumulator. To see the answer in the accumulator, you could press the [SUB TOTAL] key, which would bring 579 up on the display, or you could press the [TOTAL] key, which recalls the accumulator to the display, then clears the accumulator. To explain this in the context of a more conventional calculator with separate memory function keys, the [+] key actually behaves more like a "M+"(add to memory) key; and the [-] key like a "M-"(subtract from memory) key; the [SUB TOTAL] key behaves like a "MR"(memory recall) key, and the [TOTAL] key acts like pressing an "MR" key followed by a "MC"(memory clear) key. When first sitting down to use this machine, this scheme takes some getting used to, but actually is rather powerful, with this functionality useful for financial and statistical applications.

Close up of one of AMI LSI IC's

Multiplication and division work in algebraic fashion, with the [=] key displaying the result. The unusual but very useful [=+] key generates the result for multiplication and division like the [=] key does, but also adds the product or quotient to the accumulator. The [=+] key comes in very handy for performing sum of products calculations and averaging.

Detail of SCM Cogito 412 Display Construction

The Cogito 412 is a fixed-decimal point machine, with an unconventional method used for selecting the decimal point position. The [CLEAR] key provides a dual purpose. It's first function is to clear the calculator. However, below the [CLEAR] legend on the key, is a (hard to see in the pictures) embossed area of the key that says "DECIMAL POSITION". Pressing down the [CLEAR] key and holding it down, then pressing a digit on the keyboard from 1 through 0, selects the decimal point position from one digit behind the decimal point to ten digits behind the decimal point (when 0 is selected). Also, if a digit key is held down when turning on the calculator, that digit is used to determine the decimal point position at power-on. If no key is pressed at power-on, the machine places the decimal point at two digits behind the decimal, most useful for financial calculations. This method of decimal point selection was patented by SCM Corporation, under US Patent number 3622768.

A closeup of the display, showing dimmed insignificant zero digits

The 412 also is rather unique in the presentation of the Nixie display. The machine performs leading and trailing zero suppression in an interesting way. Rather than blanking superfluous zeroes completely, it instead reduces the brightness of the insignificant zeroes to about one-quarter of the brightness of significant digits. Why the designers opted to do this rather than simply blank the digits is perhaps to assure the user that the tubes are working at all times. The calculator performs operations fairly quickly, providing a result to 99999999999.9 divided by 1 in less than 1/2 second. Calculations proceed with little fanfare on the display, though the display isn't blanked during calculations, calculations occur fast enough that the display doesn't really jump around all that much.

For more information about a companion machine to the Cogito 412, have a look at the Cogito 414 page.