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Tektronix Model 31 Electronic Calculator

Updated 11/9/2016

The Tektronix Model 31 calculator is a prime example of how companies can enter a marketplace by acquiring technology and talent. The technology involved was a pair of high-end calculators developed by a small company in California called Cintra, as well as the talent of those at Cintra that made the calculators a reality, combined with the expansive engineering resources that Tektronix had at it's disposal. The result was a calculator that entered a relatively small group of calculators in the market of the time: the (Wang 700- and Wang 600-series machines; Hewlett Pakard's 9820 and 9830 machines; along with Compter Design Corporation's second-generation advanced calculators). These calculators seriously blurred the line between calculator and computer. By adding enhancements to Cintra's calculator design, as well as leveraging advances in Large Scale Integrated Circuit technology, Tektronix built a machine that had very computer-like capabilities, but had the heart of the Cintra calculators.

While not commonly known, Tektronix, famous as a maker of high quality, high performance electronics test and measurement equipment, spent a short time in the high-end electronic calculator marketplace in the early to mid-70's. Tektronix acquired the assets of Cintra, Inc., located in Sunnyvale, California, in the spring of 1971, and by that acquisition, Tektronix entered the calculator business.

Cintra had introduced a duo of high-end calculators in the late 1969.... Before Tek purchased the company, Cintra was falling upon difificult financial times due to competition from the big boys in the high-end calculator marketplace -- Hewlett Packard, Wang and Compucorp. Along with the competition, Cintra also had some early production problems with their calculators that resulted in some bad press that hindered sales of their calculators, despite their impressive features, and prompt fixes for the problems. Tek's purchase of Cintra was a response to Hewlett Packard's very successful line of high-end electronic calculators. Tektronix and Hewlett Packard were fierce competitors in the test instrument marketplace. When HP entered the calculator market in 1968 with the HP 9100A, and soon after, followed it up with the expanded-capacity 9100B, Tektronix needed to come up with their own high-end calculator to counter Hewlett Packard.

A One-Off Tek Model 31 in "Computer" Color Scheme
Sincere thanks to Gary Laroff for donation of this one-of-a-kind machine

Initially, the newly-created Tektronix Calculator Products Division was headquartered in at Cintra in Sunnyvale, but in time, offers were made for some of Cintra's folks to move to Oregon to work for Tektronix, and eventually the remains of Cintra were shut down. The exact timing of the closure of the Cintra office in Sunnyvale are unclear at this time, but it likely occurred sometime in 1973 or early 1974. When Tektronix purchased Cintra, they rebadged Cintra's existing line of calculators and accessories (Early Tektronix Calculator Products Division Product Introduction Flyer) and sold them under the Tektronix brand for a period of time while Tektronix put some of Cintra's engineers, as well as some of it's own digital design engineers, to task to design new calculators capable of competing in the high-end calculator business.

On a side note, there is a very important historical twist in the story. One of Cintra's engineers was a young man named Michael J. Cochran. Cochran started out in electronics reparing TVs and Radios in a small shop near his home. He later ended up working for RCA dealing with missle testing, and then work for a time on monitoring systems that were involved in the Gemini space program test flights. Later, he ended up working at Fairchild Semiconductor, and while there further enhanced his digital logic design skills, and applied them to integrated circuits. Sometime in the latter part of 1967, Cochran was approached by a couple of former Fairchild employees who had left to go work for Cintra. At that time, Cintra was in the early stages of the design of a high-end electronic calculator. Cochran was intrigued, and was eventually convinced to defect from Fairchild and come to Cintra, where he became a critical force in the development of the calculator logic design and and microcode. The calculator, which became the Cintra 909 Scientist, came to market in the latter part of 1969. Saldy, due to market conditions, and the large amount of money involved in the development of the calculator, Cintra began to struggle financially. Coincidentally a few months before Cintra was acquired by Tektronix, Cochran was contacted by Texas Instruments, who offered him a position working on the development of integrated circuits for electronic calculators. Given his experience in the development of Cintra's calculator, it seemed a good fit, and, given the business difficulties at Cintra, Cochran decided it was time to leave. In March of 1971, went to work for Texas Instruments, just two months before it was announced that Cintra was to be acquired by Tektronix. Cochran joined a project in process at TI to develop a flexible microcoded single-chip architecture that could be used to power a calculator. Within 4 months, Cochran had made a major contribution to finalizing the design of the chip, as well as writing the microcode to to program the chip to operate as a calculator. Thus began Texas Instruments' meteroic rise in the calculator industry. Historians mark the development of what became the TMS 100 "calculator on a chip", which Cochran made a major contribution to, as the first commercial microprocessor, since it had all of the components necesary to be a general-purpose (albeit minimally complex) computer on a chip. Cochran and Gary Boone jointly are listed as inventors of the first commercial microprocessor on a chip on US Patent 4,074,351. This design became the foundation for Texas Instruments' calculators for years to come, and resulted in complex, high-end hand-held calculators, as well as basic business calculators.

Back to the story of the Tektronix calculator. The result of almost two years of engineering effort, with the architecture of the Cintra calculators embedded within the LSI chipset fabricated by AMI, were the Tektronix Model 21 and Model 31 calculators, both of which were introduced to the marketplace in July of 1973.

Tek 31 with top cover removed

Unfortunately, the 21 and 31, while capable calculators, had a hard time competing with other well-established programmable calculators from Hewlett Packard and Wang. As it turned out, a combination of forces combined to make Tek's venture into the high-end calculator market a disppointment to Tektronix' management First off, Tektronix salespeople did not really know how to sell calculators. The Tektronix sales force was extremely skilled in selling Tek's best-in-class test and measurement equipment, however, calculators just didn't fit their sales expertise. HP, Wang, and Computer Design Corporation (Compucorp) had well-seasoned calculator sales experts out in the trenches that could generally out-savvy the relatively green Tektronix sales folks. Along with that, Tektronix got out of the gate rather late in the game. At the same time, the general calculator market was beginning to experience its first major shakeout, with many companies falling by the wayside between 1973 and 1975 due to saturation of the market and general economic conditions. This combination of factors resulted in disappointing sales figures for the calculators. In time, it became difficult to justify the existence of the calculator division. By late 1975, Tek realized that that the return on investment simply wasn't occuring, and disbanded the calculator division. Tektonix' venture into the calculator market wasn't a complete waste, though, as many of the engineers involved in the calculator development ended up moving into development of an early "personal computer", introduced in 1975, the Tektronix 4051, which put Tektronix into the true computer marketplace, providing a stand-alone desktop computer that provided advanced programming and, most importantly, high-resolution graphics capabilities, at a price that put it within reach of small-to-medium sized businesses, as well as educational institutions and even well-to-do individuals.

The Tektronix Model E31

Before Tektonix' calculator business was disbanded, some attempts were made to recoup as much investment as possible. The sales price of the calculators was cut dramatically. A special low-cost version of the Model 31 was introduced in May of 1975, designated the E31, that omitted the I/O interfacing capability to reduce cost for calculator buyers that didn't need I/O capabilities. These actions met with little response from the marketplace. Finally, the remaining stock of Tek calculators was disposed of through the Tektronix company store (with machines selling for pennies on the dollar to Tektronix employees), and later en-masse to scrap dealers, who unceremoniously parted out the machines for metals reclaiming.

This particular Tek 31 was apparently a lease return (note the Intel property tag at the right front of the machine), and showed up at the Tektronix company store in 1979, where I purchased for $7.00, which was a price determined by the weight of the device, a common practice at the Tektronix company store.

Closer View of Printer(Left), Power Supply(Center), Tape Drive(Right)

The Model 31 exhibited here was built sometime in late 1973, based on date codes on parts within the machine. The Model 31 was the big brother of the somewhat "stripped down" Model 21. Both machines were programmable, had a seven-segment gas-discharge display and optional 16-column thermal dot-matrix printer, provided for off-line storage of programs (a magnetic card in the Model 21, and a mag-tape cassette in the Model 31), and the Model 31 also has an expansion bus that allowed external devices such as test equipment, printers, plotters, paper tape readers and punchers, graphic display, and other peripherals.

Example of a Tek 31/53 Instrumentation System

Tektronix published a detailed guide for interfacing external devices to these machines. The interface was such that just about anything could be interfaced to these calculators with fairly simple glue logic including address decoding, timing, and bus-driver/receivers. Up to 99 different devices could be connected up to the calculator via a daisy-chain cable arrangement (similar, but not identical to GPIB/IEEE-488). This I/O interfacing design was based off of the Cintra 909's "REMOTE" capability, which was Cintra's means to interface their calculator to external devices. Tektronix ended up targeting the Tek 31 into two markets. The main market was for test system control. With the Tektronix 31/53 instrumentation system (introduced in March, 1974), it was possible to connect modified versions of some of Tek's popular TM-500 line of modular test instruments (such as digital volt/ohm meters and frequency counters) to the calculator. The calculator could then read the results of measurements by the instruments, allowing automated test systems to be built using the calculator as a much-less expensive controller than the typical mini-computer system used at the time. The other market for the Tek 31 was that of a small computer system. The Tek 31 could be purchased in a package called the 31/10, which included an interface that allowed the 31 to drive a Tektronix Model 4010 "DVST" (Direct View Storage Tube) graphic terminal. With this arrangement, it was possible to use the calculator as an interactive number cruncher, displaying the results of calculations in graphical form on the 4010 terminal. This product set ended up being the precursor to the prior-mentioned Tektronix 4051.

Tektronix Model 21 and 31 Calculators

The technology in the machines is a combination of MOS/LSI circuitry (the calculator board) and TTL (7400-series logic) integrated circuits. Both the Model 31 and Model 21 share a common calculator board which contains fifteen Large-Scale Integration (LSI) ICs that implement the logic of the Cintra-designed 909 Scientist calculator that Tektronix acquired when it purchased Cintra in 1971. The LSI chips consist of (Tek Part Number in parenthesis): Adder-Subtracter (156-0243); Working Register Storage(W, X, Y and Z register, 165-0238x2); Function Decoding (156-0239, 156-0240, 156-0242); Microcode Routine Address ROM (156-0235); Microcode ROM/Next Address Latch (Same basic chip, Microcode Mask-Programmed: 156-0231 through 156-0234); Timing and Constant Storage ROM (156-0237); K Register Storage (156-0236x2); and A & C Operation Control ROM (156-0241). These chips were manufactured by AMI as custom parts, exclusively for Tektronix, and made to Tektronix logic design specifications. Along with the LSI's on the calculator board, a group of mostly 7400-series small and medium-scale TTL devices carry out functions such as generating the master clock (5 MHz) for the machine, and providing the interfacing between the LSI IC's on the calculator board and other parts of the machine.

The programmability of the calculators is where the Model 21 and Model 31 differ dramatically. The Model 21 is somewhat limited in the number of steps and complexity of programs that it can handle. The programming capabilities of the Model 21 are very similar to the earlier Cintra 909 and 911 calculators, since the calculator board in the Model 21 is essentially a re-implementation of the logic of the Cintra 909. The main difference between the Model 21 and the Cintra 909 is that the Tek 21 has the ability to load and store keystroke sequences on a magnetic card, whereas the Cintra 909 had no way to enter program steps except through the keyboard, and when the calculator was turned off, the program was lost. Programs on the Model 21 are essentially linear sequences of keystrokes that can be played back at high speed, with no real way of performing logical operations or branching.

The programmer in the Tektronix 31 provides significantly more-powerful programming capabilities over that of the Mode 21, including decision making and branching by address or label, as well as subroutine capability. Interestingly, like the Model 21 calculator is essentially a reimplementation of the Cintra 909 calculator, the Model 31 calculator takes this one step further, by essentially adding the features of the Cintra 926 Programmer to the Model 21 calculator, although substantially improving upon the capabilties of the 926, and building it all inside the Model 31 cabinet. Along with the additional programming features, the programmer in the Model 31 provides access to much more memory (made using 1024 bit Static RAM chips made by Intel or Signetics) which can be used to store program steps and numeric memory storage registers, as well as providing for handling of alphanumeric characters. The memory of the Model 31 is expandable in increments, and can be divided up between program storage steps and numeric memory registers based on the setting of jumpers on the memory board.

Memory Allocation Selection Switch on Later Model 31 Calculators

On later model Tek 31 calculators, a three position rocker switch was added to the front panel of the calculator, beneath the slot for the magnetic cartridge drive, which allowed the user to select how the memory for the calculator was allocated, with the left-most (STEPS) position providing a selection that provided more program step memory, the center position selecting a 50/50 split between program step and register memory, and the right-most position (REGISTERS) selecting a preference for more memory registers. The actual splitting of the memory in the "STEPS" and "REGISTERS" positions of the switch was selected by internal jumpers within the machine.

The calculator exhibited here is configured with 5120 steps available for program storage, and 640 memory registers. Base memory in the Tek 31 provides 512 program steps, and 64 memory registers. The maximum memory configuration allows for 8192 program steps and 256 memory registers, providing capacity that would rival some small computer systems of the time. The Model 31 also provides a very computer-like capability for external devices to utilize a Direct Memory Access (DMA) facility to transfer data or program steps directly to/from the calculator's memory.

Block Diagram of Tek 31 Architecture

The programmer board on the Model 31 is made up almost entirely of 7400-series small and medium-scale TTL IC's, with only 4 MOS IC ROMs containing microcode that sequences the operaton of the programmer. The Model 21 has a significantly less complex board in place of the programmer board of the Model 31, called the f(x) board. The f(x) board essentially implements the [LEARN] and [f(x)] keys and program memory of the Cintra 909/911 calculators. The f(x) board of the Model 21 provides 128 or 256-step program storage and sequencing logic to allow learning and playback of keypresses, as well as interfacing for the magnetic card reader and optional printer.

The Calculator Board with the 15-Chip AMI-Manufactured "Cintra 909" Chipset

The Programmer and Memory Boards

The calculator board performs computations to a total of 12 significant digits, with ten digits displayed, and two digits which are not displayed acting as guard digits for extra accuracy. The calculator board operates on floating point numbers, which allows the calculators to operate in scientific notation, with ten digits (plus a digit for negative sign) displayed for mantissa, and and exponent from -99 to +99.

Tek 31 Display in Program Mode showing Error Code E3

The display uses Sperry-made seven-segment plasma display modules. Each module contains two or three digits. The mantissa display is made up of one 3-digit module and four 2-digit modules, with the exponent display made up of one 3-digit module. These display modules have a bad tendency for leaking, meaning that the special gas mixture inside the display element that makes the segments glow when energized leaks out of the display module, rendering it non-functional. The vast majority of Tek 21/31 calculators that have survived to this day may still function, but will likely have digits that won't light up, or worse, no display at all as a result of the display modules failing. Replacement display modules are made by Babcock's Display Division, but are quite expensive. Another common failure on the Tek 21/31 display system is the high-voltage display decoder/driver chips, which also seem to also have a high failure rate. Unfortunately, these chips are very difficult to find today, and no one is making replacement chips. The display segments glow orange when energized, but a red filter in front of the display makes the digits appear red to the user. If an error condition exists, the display blinks at 1Hz and can also display a numeric error code in the exponent digits that the user can use to determine what kind of error occurred.

The Display Board

The Tek 31 calculator provides a huge array of pre-defined math operations, including: standard and hyperbolic trig functions (with arguments in degrees or radians), factorial, natural and base-10 logarithms, ex, xy, squaring, square root, and integerize, along with various summing functions for statistical calculations. The calculator uses algebraic logic, with parenthesis keys allowing entry of calculations as they would be written -- within limits. Parenthesis cannot be nested, which seems to defeat the purpose of having them in the first place. The reason for this is that the so-called "Heirarchy Control" feature of the Cintra 909's architecture was left out of the chipset, likely for cost-containment reasons. This omission left the Model 21 and 31 calculators at a bit of a disadvantage from the user's perspective, as the change made the machines less adept at solving problems "as they would be written on paper", which the vision of Irwin Wunderman (founder of Cintra). Memory functions include a ten register constant (K) storage, and access to up to 1000 numeric storage registers (depending on the amount of memory available in the calculator, and how it is divided between program and data storage). The Model seems to run at a fairly decent clip, but can take a little while to perform some operations such as trig or factorial functions. It is definitely not as fast as the mostly bipolar implementation in the Cintra 909 calculator, which is among the fastest calculators that the museum has come across. A 'BUSY' indicator lights while the machine is performing calculation functions. Some specification documents from the Tek 31 Sales Engineer Training Manual lists addition and subtraction as taking 8 milliseconds (as opposed to the Cintra 909's published time of 0.6 milliseconds), with trigonometric operations taking from 70 to 160 millisconds (Cintra 909: 100mS)

Programming functions on the Model 31 are extensive. Programming mode is entered by pressing the [LEARN] key on the keyboard. Successive keypresses are learned (stored) as 8-bit codes in memory, with some instructions (such as GOTO, or register access) requiring multiple keypresses to complete. When such multi-keystroke instructions are entered, an "ADDR INCOMPLETE" annunciator on the display lights until the instruction entry is completed. Program editing functions include positioning the current instruction display, and insert and delete functions. Branching operations include absolute and indirect (via address on display) GOTO, GOTO display (branch to address in four least-significant digits on the display), conditional branches based on display content >=0, =0, <0, or if error condition exists. There is also a simple flag bit which can be set or cleared, and a branch if the flag is set. The programmer has a simple subroutine facility, where where a section of code can begin with a label, and a subroutine branch to the label will cause the code after the label to be executed. When the branch to the label is executed, the return address is stored in a single register (not a stack, making subroutine nesting challenging). To return from the subroutine, the return address register is recalled to the display, then a 'GOTO Display' command is executed. This makes subroutine programming quite tedious as compared to machines from competitors such as Hewlett Packard and Wang Laboratories, which provided a return address stack to allow easy nesting of subroutines.

Sample of Program Listing Printout from Tek 31

The printer shared by the Model 21 and Model 31 calculators is a very quiet thermal dot-matrix printer which can print up to 16 characters on a line. When in calculator mode, the printer prints the content of the display when the [Print Display] key is pressed, or can dump out the content of program memory when the [List] key is pressed. The printer on the Model 21 can print numeric data only. The printer on the Tek 31, while sharing the same basic mechanical design, adds additional electronics to allow it to print alphanumerics. In learn mode, the printer lists program steps as they are entered, and can also list out programs. When a program is running, the printer acts as a programmed output device, allowing output via program instructions.

Tek 31 Mag-Tape Cartridge

The Model 31 has a magnetic tape cartridge drive which can be used to store programs and content of data registers. It is a sequential file type device, with file 0 being the first file on the tape, followed by file 1, etc. Up to 6 files can exist on a tape, with the [TO TAPE] and [FROM TAPE] keys on the keyboard allowing the tape to be positioned to a given file and have content read/written to/from the tape. Reads and writes to the tape device can be performed as programmed instructions, making program chaining and overlays possible.


Sincere thanks to Gary Laroff for access to old Tektronix Calculator Division Archive Material as well as a large donation of Tektronix 31 calculators, accessories, and Tek 21/31 documentation.

Text and images Copyright ©1997-2016, Rick Bensene.