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Cybernetic Systems Mathiputer (CPD-35)

Founded in New Mexico in 1974, Cybernetic Systems, Inc. assumed responsibility for manufacturing and distributing the Mathiputer educational computer after the partnership between Omni Corporation - the original manufacturer of the Multiputer - and Audio-Visual Marketing Corporation, its exclusive distributor, came to an end. In 1975, Audio-Visual Marketing Corporation released a machine called the Digitor Learning Arithmetic Module, which Omni believed bore a striking resemblance to the Multiputer. Manufactured by Centurion Industries, Inc. in Redwood, California, the Digitor remained in production for over a decade. During that time, it underwent cost-reduction and parts-obsolescence redesigns and ultimately sold more than 50,000 units to schools.

However, credit must be given to Texas Instruments for introducing a similar device in June 1976 - the famous Little Professor - bringing it into children's homes and ultimately selling millions of units. Remarkably, fifty years after its introduction, the Little Professor remains in production.

Similar to arithmetic flashcards, the Mathiputer CPD-35 helps students practice addition, subtraction, multiplication, and division facts. The device is programmed to generate two random numbers and display them along with instructions indicating whether the numbers should be added, subtracted, multiplied, or divided. The student mentally computes the answer and enters it using the device’s keyboard. The Mathiputer then compares the student's response with the correct answer calculated by the machine and indicates whether the answer is correct or incorrect by displaying either a happy face or a sad face.

Three rotary switches allow the user to configure the difficulty level, math topic, and session length before the results are graded:

The Mathiputer grades the performance of the student by showing the number of correct answers after a session. Learn more about the CPD-35 Operation and Logic Implementation.

The first-generation Mathiputer, much like the original Multiputer, utilized Fairchild Semiconductor's PPS 25 (Programmed Processor System – 25 Digits) Building Blocks architecture. Around 1974, as Fairchild shifted its focus away from the PPS-25 and toward the newly introduced F8 processor family, Cybernetic Systems responded by developing a second-generation Mathiputer with a smaller and more compact enclosure. In its third generation, the Mathiputer transitioned from the F8 processor to an 8-bit single-chip microcomputer produced by American Micro-systems, Inc. (AMI). The fourth and final generation focused primarily on cost reduction, substituting features such as the rotary selector switches with simpler push-button controls while still relying on an 8-bit AMI chip.

Our research has so far identified six different versions of the Multiputer and its predecessor Mathiputer:

The Mathiputer features a distinctive design consisting of two large hemispherical metal shells stacked on top of each other. The lower section houses the power supply, on/off switch, and keyboard, while the upper section contains the main electronics, including the LED display, happy and sad face indicators, rotary switches, reset button, and carrying handle.

Dismantling the Mathiputer CPD-35 manufactured in November 1975 in New Mexico, USA by Cybernetic Systems, reveals a relatively simple design with extensive hand-soldering, necessitated by the lack of through-hole plating in the double-sided printed circuit board (PCB). The power supply located in the lower housing section employs extensive flying-wire connections and interfaces with the Main-PCB in the upper section through a three-wire connection, while the keyboard unit is attached using a hand-soldered flat ribbon cable.

We acquired this Mathiputer CPD-35 educational computer in Spring 2026 not only to better understand the innovative PPS-25 Building Blocks, but also to explore the evolutionary path that led to Texas Instruments' Little Professor. To gain some knowledge about the PPS 25 design used with the Multiputer, we decided here at the Datamath Calculator Museum to give it a "Teardown Treatment" and share our findings accordingly.

Microprocessor System: A minimal PPS 25 design uses six Integrated Circuits interconnected by 4-bit data buses and various control signals, corresponding directly to the block diagram of a computer based on the Harvard architecture. At the core of the system is the CPU, composed of the 3805 Arithmetic/Logic Unit and the 3806 Control Unit. The 3808 and 3809 Data Memories each provide three dynamic 25-digit shift registers, while the 3810 Program Memory provides 256 × 12 bits of storage. The 3803 and 3807 Input chips support up to 32 keys and 16 switches each, and the 3811 Output chip drives displays of up to 16 digits. The clock frequency of the PPS 25 Building Blocks is 400 kHz, resulting in a Digit Time of 2.5 us and a Word Time of 62.5 us. The Mathiputer CPD-35 manufactured in November 1975 is using as expected six PPS 25 Building Blocks on its large double-sided PCB:

The PPS 25 System is supported by some additional SSI (Small Scale Integration) and MSI (Medium Scale Integration) chips in TTL (Transistor-Transistor Technology), for segment decoding of the BCD data for the 7-segment display and some housekeeping logic.

Display: The featured Mathiputer CPD-35 manufactured in November 1975 utilizes five Litronix DL707 LED seven-segment displays along with five discrete LEDs used to indicate the selected math function and equal sign. Small masks with cutouts for the addition, subtraction, multiplication, division, and equal sign symbols are mounted on the red protective screen directly in front of these LEDs. The happy-face and sad-face "emojis" are represented by slightly larger masks on the same protective screen and are illuminated by two low-voltage incandescent lamps.

Keyboard and Switches: Located in the lower half of the Mathiputer CPD-35, the 11-key keyboard module connects to the Main-PCB through a 16-pin cable. The 3807 Input Device generates a unique 5-bit code for every key press and release, while also monitoring the three rotary switches responsible for selecting difficulty level, math topic, and session length.

Clock: The PPS 25 System is using a 2-phase, non-overlapping clock for its dynamic and static logic with a large voltage swing between V_SS and V_GG. The featured Multiputer CPD-35 manufactured in November 1975 is using a clock oscillator with discrete transistors and additional level shifters to accommodate the PPS 25 requirements. We measured a clock frequency of 115 kHz with the evaluated device.

Power Supply: Located in the lower portion of the Mathiputer CPD-35 is a center-tapped transformer, paired with a two-diode rectifier and filter capacitors to create a full-wave DC supply of roughly ±17 volts. On the Main-PCB, a three-terminal linear regulator provides a stable +5V output for the PPS 25 Building Blocks V_SS rail, the TTL logic devices, and the LED displays. The negative PPS 25 Building Blocks V_GG supply of -9.7V is generated through a Zener diode and transistor regulator circuit.

A Main-PCB of a second Mathiputer CPD-35 manufactured in October 1975 was donated to the Datamath Calculator Museum (Thanks to Jeri) for the sole purpose of decapping its six Building Blocks and we added the die photos to their respective Datasheets.

Die photos courtesy of Sean Riddle.

CPD-35 Operation and Logic Implementation

The Mathiputer Learning System CPD-35 employs a range of controls, indicators, and digital displays for user interaction.

The base of the CPD-35 houses the power switch and an 11-key keyboard used for numerical entry, along with an Execute key. The upper control panel contains three 4-position rotary switches for selecting the difficulty level, math topic, and session length, as well as a push button used to start a teaching session. The control panel display consists of seven indicators and five seven-segment LED displays arranged in three groups.

Controls

[POWER] On/Off switch, right side of CPD-35 base.

11-key Keyboard, front side of CPD-35 base:

[RANGE] 4-position rotary switch, CPD-35 control panel, left position. Each quiz generated by the CPD-35 uses two random numbers based on the selected math topic. The [RANGE] switch determines the set of possible random numbers used for the second addend, subtrahend, second factor, or divisor, which is shown in the second seven-segment display field:

[MODE] 4-position rotary switch, CPD-35 control panel, center position. The CPD-35 provided math quizzes to train addition, subtraction, multiplication and division. Depending on the selected math topic and difficulty level, the possible number range of the first addend, minuend, first factor, or dividend is chosen. The table below is based on the [RANGE] switch set to '0-9':

[ASSIGNMENT] 4-position rotary switch, CPD-35 control panel, right position. The number of quizzes per teaching session can be set to 10, 25, 50, or 100:

[START] The red push button below the [MODE] rotary switch is used to initiate a teaching session. Any changes made to the positions of the 4-position rotary switches are registered only after the [START] button is pressed. Otherwise, the CPD-35 may exhibit erratic behavior..

Indicators and Digital Readouts

The visual feedback of the CPD-35 is provided by various indicators and digital readouts arranged in seven groups and covered by a red acrylic display bezel. Their functionality, from left to the right, is provided with the following table:

• WRONG – Illuminated after a wrong answer to the question is entered • 88 2-digit 7-segment display – Displays the first addend, minuend, first factor, or dividend during a quiz, or the number of correct answers at the end of a teaching session • + − × ÷ – One of four indicators showing the selected math topic • 8 1-digit 7-segment display – Displays the second addend, subtrahend, second factor, or divisor during a quiz • = – Indicator for the equal sign, always on during quizzes, off at the end of a teaching session • 88 2-digit 7-segment display – Displays the entered sum, difference, product, or quotient if a correct answer is entered. Showing the correct answer if a wrong answer is entered • RIGHT – Illuminated after a right answer to the question is entered or all answers were correct

Operation

Operation of the Mathiputer Learning System CPD-35 consists of three steps:

The configuration of a teaching session is carried out with the three 4-position rotary switches to select the difficulty level [RANGE], math topic [MODE], and session length [ASSIGNMENT] and confirming the selections with the red push button [START].

Example: [RANGE] = '0-9', [MODE] = '− SUB', and [ASSIGNMENT] = '10'.

After pressing [START], the CPD-35 is configured and the display would show in its seven groups:

The student begins the teaching session by pressing the [EXC] key, after which the CPD-35 presents the first of ten quizzes:

If the student enters the correct answer [2] with the numeric keypad, the result appears in the answer field and the RIGHT indicator illuminates:

5

−

3

=

2

RIGHT

Pressing the [EXC] key again presents the second quiz:

If the student enters a wrong answer [4], the correct answer is shown but the WRONG indicator illuminates:

WRONG

12

−

7

=

5

Entering an incorrect answer locks the numeric keypad, and the [EXC] key must be pressed again. The CPD-35 then presents the same quiz again:

These steps are repeated until all quizzes in the teaching session have been answered correctly.

The CPD-35 is then grading the finished teaching session with showing the number of correct answers in the display:

If the student answers all questions correctly, the RIGHT indicator also illuminates:

10

RIGHT

Known Bugs

If the [RANGE] switch is set to 0–3 or 0–9 and the [MODE] switch is set to DIV, division-by-zero problems may appear in the quizzes. In such cases, all entries, including '0', are treated as correct answers.

Changing the [MODE] switch during a teaching session may result in unexpected quiz behavior.

If you have additions to the above article please email: joerg@datamath.org.

© Joerg Woerner, May 16, 2026. No reprints without written permission.