Tektronix Model 31 Electronic Calculator

The Tektronix Model 31 calculator is a prime example of how a company can enter an established marketplace by acquiring technology and talent. It is also a prime example of how, especially in the high technology market, timing can be everything.

The technology involved was a powerful calculator known as the Cintra 909 Scientist, developed by a small Sunnyvale, California-based company called Cintra, Inc. Cintra had developed the 909 Scientific and 911 Statistician electronic calculators during the late 1960's using mostly small- and medium-scale TTL (7400-Series) bipolar integrated circuits, with high-speed bipolar ROMs for sequence storage, long-length MOS shift registers for memory, and a number of early LSI MOS ROMs for microcode storage. The timing in the situation was related to competitive and financial pressures, as well as the opportunity that these forces created for Cintra to be acquired by the large Pacific Northwest-based test instrumentation giant, Tektronix, Inc. As a result of the acquisition, Tektronix embarked on a project to take the Cintra calculators to the next level, resulting in two calculators that entered the relatively elite market of high-end electronic calculators in the Summer of 1973, the Tektronix Model 21 and Model 31.

While this exhibit primarily concerns the Tektronix Model 31 calculator, the high-end machine of the duo, information is also presented on the Model 21, which, thus far has remained elusive to find for exhibit in the museum. If the reader knows of the whereabouts of, or has a Tektronix Model 21 calculator in a dusty corner somewhere, in any condition, please get in contact with the museum by pressing the "EMail" button in the menu-bar at the top of the page. The museum would very much like to add a Tektronix Model 21 to complete its exhibition of the Cintra/Tektronix calculators.

The Tek(a common shortening of Tektronix, hereafter used to refer to the company) Model 31 calculator joined the likes of Wang Laboratories' 700 and 600 series calculators; Hewlett Packard's 9820 and 9830 calculators; as well as Computer Design Corporation's second-generation of advanced calculators, as well as a number of other high-end calculators from other manufacturers. These high-end calculators began to blur the line between calculator and low-end minicomputer in terms of capability and capacity, but at prices significantly lower than even the lowest-cost minicomputer systems of the time. By adding enhancements to Cintra's existing calculator design, as well as leveraging advances in Large Scale Integrated Circuit technology, Tektronix built a calculator that had powerful computer-like capabilities, but at a considerably lower cost than the competition.

Within the new Tek 21 and Tek 31 calculators beat the heart of the earlier Cintra calculators. The re-use of much of the core architecture of the Cintra calculators helped save engineering time in the development of the new calculators, as well as being a sure thing in terms of knowing that all the bugs in the logic had already been worked out.

In late 1969, Cintra had introduced the 909 Scientist, a feature-packed calculator that very favorably compared to the higher-end calculators of the time marketed by Wang Labs and Hewlett Packard. However, being a new and comparatively small and unknown player compared to giants in the calculator industry, Cintra had a tough row to hoe to make a place for itself in the high-end electronic calculator marketplace.

One feature that Cintra's calculators had that was only matched by Hewlett Packard's 9820A, was fully algebraic expression entry, meaning that expressions were entered into the calculator just as they would be written on paper, with the calculator following the mathematical rules of precedence, as well as providing for parenthesis of expressions to change the default order of evaluation. This made using the Cintra calculators for solving complex expressions very easy, without the user having to think about how to break down the expression into the "language" of the calculator. This was a very powerful concept that made the Cintra 909 and 911 calculators unique in their price bracket. The Cintra 909 and 911 were also substantially less-expensive than HP's 9820A.

Before Tektronix purchased Cintra in the spring of 1971, Cintra was falling upon difficult financial times due to competition from the big players in the high-end calculator marketplace. Cintra had spent a substantial amount of money on the development of their calculators, and sales just were not coming in at a sufficient rate to help recoup the investment made to create them. Along with the problems selling the calculators amidst the competition, Cintra also had some early production problems with their calculators that resulted in delays to fulfill initial orders, as well as a batch of calculators that simply would not work due to some out-of-spec transistors when they hit the end of the production line, requiring extensive re-work of one of the circuit boards to get them to work. Word got out about this problem, which resulted in some initial bad press that hindered sales, all despite the calculators' impressive features and value for its price.

Tektronix' purchase of Cintra was the company's response to Hewlett Packard's very successful line of high-end electronic calculators. Tektronix and Hewlett Packard (HP) were fierce competitors in the test and measurement instrument marketplace. When HP entered the calculator market in March, 1968 with the HP 9100A, followed in October by the HP 9100B, Tektronix management felt that Tektronix needed to come up with their own high-end calculator to counter Hewlett Packard's already imposing high-end calculator market presence.

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

Initially the newly-created Tektronix Calculator Products Division was headquartered at Cintra's facility in Sunnyvale, but in time, offers were made for many of Cintra's employees 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 facility in Sunnyvale is unclear at this time, but it likely occurred sometime in late 1972 to early 1973. When Tektronix purchased Cintra, they re-badged Cintra's existing line of calculators and accessories (Early Tektronix Calculator Products Division Product Introduction Flyer) and sold them under the Tektronix brand while Tektronix put its engineers (some of whom came from Cintra) to task to design a new calculator capable of winning sales in the high-end calculator market.

On a side note, there is a very important historical twist to this story. One of Cintra's design engineers was a bright young man named Michael J. Cochran[5/21/1941-12/2/2018]. Cochran started out in his career in electronics in his teens, repairing TVs and Radios in a small shop near his home. He later ended up working for RCA doing missile systems testing, and then worked for a time on monitoring systems that were part of NASA's Gemini space program test flights. Later, he ended up working at Fairchild Semiconductor, and while there finely honed his digital logic design skills, applying his talents to work on integrated circuit technology. Sometime in the latter part of 1967, Cochran was approached by a some former Fairchild employees that he had known, who had left to go work for Cintra. At that time, Cintra was in the early stages of the design of its electronic calculator. Cochran was intrigued by what they had to say, and was eventually convinced to defect from Fairchild and come to Cintra, where he became a critical force in the development of the Cintra calculator logic design and microcode. The calculator, which became the Cintra 909 Scientist, came to market in the latter part of 1969. Sadly, due to depressed 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(TI) and was offered a lucrative position leading the development of large-scale integrated circuits for electronic calculators. Given his experience in the development of Cintra's calculator, it seemed a good fit for him, and, knowing of the situation at Cintra, decided it was time to leave. In March of 1971, Cochran 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 architecture that would fit on a single-chip that could serve as the basis for any number of calculators with different capabilities based on microcode contained in the chip . Within 4 months, Cochran had made major contributions to finalizing the design of the architecture, as well as writing the microcode to program the chip to operate as a basic four-function calculator. It was just a few months later, in April, 1972, that Texas Instruments introduced three new, low-cost calculators, including two desktop machines, the TI-3000($79.95), and the TI-3500($89.95), as well as a stylish handheld, LED-display, rechargeable battery-powered four-function calculator it called "Datamath" at a MSRP of $149. Thus began Texas Instruments' meteoric rise in the calculator industry. Historians mark the development of what became the TMS 0100-series "calculator on a chip" as the first commercial microprocessor on a chip, since it had all of the components necessary to be a general-purpose (albeit minimal) computer on a chip. Cochran and Gary Boone jointly are listed as inventors of the first commercial microprocessor on a chip (Not the Intel 4004 as many people believe) in US Patent 4,074,351. The architecture became the foundation for Texas Instruments' calculators for years to come, and resulted in Texas Instruments becoming a major force in the calculator marketplace that still thrives to this day.

The result of almost two years of intense engineering and development effort were the Tektronix Model 21 and Model 31 calculators, both of which were introduced to the marketplace on August 2, 1973. At introduction, the based Model 21 was priced at $1,850, and the Model 31 started at $2,850. These prices were considerably lower than comparable calculators from Tek's competitors. Tektronix purposefully priced the machines aggressively in order to get their foot further in the door of the calculator marketplace that they had a small foothold on due to their continued sale of the Cintra 909 and 911 calculators and accessories under the Tektronix brand name, which added some credence to the machines because of Tektronix' stellar reputation for extremely high-quality instruments.

Tek 31 with top cover removed

Unfortunately, the 21 and 31, while capable calculators, had a hard time competing with other well-established high-end programmable calculators from Hewlett Packard and Wang Labs, among others, despite their attractive pricing. As pointed out earlier, timing in the business of high technology is very important. Along with poor timing, a combination of other forces combined to make Tek's venture into the high-end calculator market a disappointment to the company's management. First, Tek's development effort took too long, giving competitors even more time to advance their lines of high end machines. Along with getting into the market somewhat late, Tektronix discovered that its salespeople did not really know how to sell calculators. The Tektronix sales force was extremely skilled at selling Tek's best-in-class test and measurement equipment, however, calculators just didn't fit their sales expertise. HP, Wang Labs, 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, despite intense efforts to train the sales force how to sell the Tek machines versus the competition. 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 for low and mid-range calculators, and also a dose of world-wide economic malaise. This combination of factors resulted in disappointing sales figures for the new Tektronix calculators. Over time, the situation did not really change that much despite aggressive marketing campaigns and even price reductions. By late 1975, management realized that the return on the investment in the calculator business unit simply wasn't going to occur, and the calculator division was shut down. Tektronix' 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, called the Tektronix 4051, which put Tektronix squarely into the computer marketplace, marketing a stand-alone desktop computer that provided advanced programming features with a built in BASIC interpreter and, most importantly, high-resolution graphics capabilities, at a price that put it within reach of small-to-medium sized businesses, educational institutions and even well-to-do individuals who wanted their own personal computer before the term truly existed. The 4051 falls into the category of being one of the earliest "personal computers", which gives the 4051 some serious, yet unrecognized, historical credentials.

The Tektronix Model E31

Before Tektronix' calculator business was disbanded, some attempts were made to recoup as much investment as possible. The sales price of the calculators was cut dramatically to try to stimulate sales. A special low-cost version of the Model 31 was introduced in May of 1975, designated the E31, which omitted the I/O interfacing capability to reduce cost for calculator buyers that didn't need I/O capabilities. These actions elicited little response from the marketplace. Once the calculator division shut down, the remaining stock of Tek calculators was at first put up for sale through the Tektronix company store, with the calculators selling for pennies on the dollar to Tektronix employees. What didn't sell through the "Country Store" (as the Tek company store was called) were sold en-masse at essentially volume scrap prices to scrap dealers, who unceremoniously parted out the machines for metals reclaiming. It is rumored that hundreds of brand-new Tek 21 and 31 calculators ended up being scrapped.

The exhibited Tektronix 31 calculator was apparently a lease return (note the Intel property tag at the right front of the machine), and showed up at the Tektronix Country Store in 1979, where the curator purchased it for $7.00, a price determined by the weight of the device, a common practice for surplus sold to employees at the Tektronix company store.

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

The exhibited Model 31 was built sometime in late 1973, based on date codes on components in the machine. The Model 31 was the big brother to the Model 21. Both machines were programmable, had a ten plus two digit seven-segment gas-discharge display made from individual two and three-digit modules, offered an optional 16-column thermal dot-matrix printer, and provided for off-line storage of programs and data via a magnetic card in the Model 21, and a magnetic tape cartridge in the Model 31. The Model 31 had dramatically improved programming capability versus the Model 21, and also provides an expansion bus that allows external devices such as test & measurement instruments, printers, plotters, paper tape readers and punches, and a graphic display system to be connected to the calculator.

The exhibited calculator is outfitted with the Tektronix 31/53 Instrumentation System (introduced in March, 1974) that connects a modified Tektronix TM-500 instrument mainframe containing a Tektronix Model 153 Instrumentation Interface as well as modified Tektronix DM501 Digital Multi-meter and DC503 Counter/Timer instruments. Through the calculator's I/O capabilities, it is possible to trigger the DC503 to make a measurement, as well as to be able to read the content of the display of both instruments. This system allows the calculator to become part of a simple automated measurement system capable of measuring voltages and frequencies/time periods through a program on the Model 31 calculator. Typically this kind of capability was only available in very expensive minicomputer-based automated test systems. An exhibit for this instrumentation system will eventually be prepared that will provide much more detail about the 31/53 Instrumentation System.

An example of a Tek 31/53 Instrumentation System

Tektronix published a detailed guide for interfacing external devices to the Model 31. The interface was such that just about anything could be interfaced to these calculators with fairly simple TTL 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 early GPIB). This I/O interfacing design was based on the Cintra 909's "REMOTE" capability, which was Cintra's method for interfacing their calculator to external devices.

Another unusual option package for the Tek 31 was an interface to a Tektronix graphics computer terminal to make what Tek claimed as the "first interactive graphic calculator". The package was called the Tektronix 31/10 Graphic Calculator System, which included an interface that allowed the Model 31 to communicate with a Tektronix 4010 "DVST" (Direct View Storage Tube) high-resolution graphic terminal. With this arrangement, it was possible to use the calculator as an interactive number cruncher, displaying the results of calculations in text and graphical form on the 4010 terminal. This product set ended up being a precursor to the prior-mentioned Tektronix 4051 Graphics Computer System.

Tektronix Model 21 and 31 Calculators

The technology in the Tek 21 and 31 calculators is a combination of custom and off-the-shelf MOS (Metal-Oxide Semiconductor) LSI (Large-Scale Integration) integrated circuits and TTL (common 7400-series logic) integrated circuits. Both the Model 31 and Model 21 share a common calculator board which contains fifteen LSI ICs that effectively re-implement the logic of the Cintra/Tektronix 909 Scientist calculator. The LSI chips consist of (Tek Part Number in parenthesis) the following devices: Adder-Subtracter (156-0243); Working Register Storage(W, X, Y and Z registers, 165-0238x2); Function Decoding (156-0239, 156-0240, 156-0242); Microcode Routine Address ROM (156-0235); Mask-Programmed Microcode ROMs (4), 156-0231 through 156-0234); Timing Generation and Constant Storage ROM (156-0237); K Register Storage (156-0236x2); and A & C Operation Control ROM (156-0241). These chips were fabricated by AMI (American Micro-systems, Inc.) as full custom proprietary parts exclusively for Tektronix. AMI was the leader in the fabrication of custom MOS LSI integrated circuits at the time, and had fabricated calculator chips for Smith Corona Marchant (SCM) and Computer Design Corporation. Along with the LSI's on the calculator board for the Tek 21 and 31, 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, as well as providing the interfacing between the LSI devices on the calculator board and other parts of the calculator. The programming facility on both the Model 21 and Model 31 calculators is implemented in a separate circuit board, containing 7400-Series TTL devices, with the only MOS devices being read-only memory devices (ROM) that contains microcode for sequencing the programmer.

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. In base form, the Tek 21 can store 128 program steps, with optional additions to bring the total to 256 or 512 steps. A single conditional test function is provided, and branching is provided to eight fixed locations within the program memory. Excluding the above test and branch features, programming capabilities of the Model 21 are the same as Cintra's earlier 909 and 911 calculators, The Model 21's built-in learn-mode programmer can hold a sequence of up to a maximum of 512 steps, with each step storing one key press on the calculator keyboard.

The Model 21 provides a single conditional skip function that tests the value in the display, and if it is greater than or equal to zero, it will skip the following program step and continue execution. If the condition is not met (e.g., the display contains a number less than zero), the step following the >=0 test will be executed. The program memory in the Tek 21 is divided logically into eight equally-sized regions, with the starting point of each region branched to by pressing a key associated with the region, e.g., [F₀] branches to the beginning of program storage, while pressing [F₇] branches to the beginning of the last region of program steps. In the base Model 21 with a program memory of 128 steps, each region contains 16 steps, therefore each of the [F_x] keys would branch to the beginning of each of the 16-step regions. While this provides a useful extension to the Cintra 909/912 calculator's simple linear program capability, it can be wasteful of program steps since it is only possible to branch to the beginning of a region. The Tek 21 also provides a STOP instruction which allows the program to stop in order for the operator to input values into the program.

Another difference between the Tek 21 and the Cintra 909's programming capability is that the Tek 21 has the ability to load and store keystroke sequences from/to a magnetic card, whereas the base Cintra 909 and 911 calculators could only have steps entered into program memory from the keyboard. When the calculator is turned off, the content of program and memory registers is lost, and, in the case of the 909 and 911, must be re-entered by hand if needed again. With the magnetic card reader on the Tek 21, the program memory can be stored and read-back via the magnetic card device, making it a simple matter to load a specific program into memory at any time. Each re-usable magnetic card can store up to 256 program steps. The magnetic card reader is quite fast, reading in an entire 256 step program in just over one second.

The Tektronix 31 programmer board essentially implements the combination of the earlier Cintra 909/911 calculator combined with the optional Cintra 926 Programmer add-on, although substantially improving upon the capabilities of the combination. All of this capability is built into the Model 31's cabinet, including the magnetic cartridge tape drive that, incidentally, was the same as used in the Cintra 926 Programmer. Along with the additional programming features added to the Tek 31 Programmer, it also provides access to vastly more memory than the 926 had available. The memory can be used to store program steps and provide 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 calculator's memory board.

On some 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. This switch allowed the user to select how the memory in the calculator is partitioned. The left-most (STEPS) position selects more program step memory at the expense of storage register space; the center position selects a 50/50 split between program step and register memory; and the right-most position (REGISTERS) selects a preference for more memory registers over program steps. The actual partitioning of the memory in the "STEPS" and "REGISTERS" positions of the switch is selected by internal jumpers within the calculator. The function of this switch was not programmable, so it was not possible to change the partitioning of the calculator's memory within a program. When the setting of the switch is changed, the calculator must to be reset or power-cycled in order for the new partitioning to take effect.

The programmer boards in the Model 21 and Model 31 are made up entirely of 7400-series small and medium-scale TTL IC's. The Model 31's programmer board has 4 MOS integrated circuit read-only memory (ROM) devices containing microcode that sequences the operation of the programmer. The Model 21's programmer board is called the f(x) board. The f(x) board implements an extended version of the [DEFINE f(x)] and [f(x)] keys of the 909/911 calculators. The [f(x)] board provides 128 or (optionally) a total of 256 or 512-step program step storage and the sequencing logic to allow learning and playback of key presses, as well as interfacing for the magnetic card reader and optional printer.

The programmer board in the Model 31 is considerably more complex than the Tek 21's programmer board, providing substantially more powerful programming capabilities. The Model 31 programmer provides multiple decision making instructions, branching by absolute program step number or by labels, and also a subroutines capability to allow commonly-used functions within a program to be coded as a sub-program. It also provides programmable access to the magnetic cartridge tape, providing the ability to load or store data on a tape through a program, as well as the ability to create programs that have more steps than can fit in the program memory by performing a chaining operation to read more program steps in from the tape cartridge and execute them. With the optional Option 10 alphanumeric thermal printer, the programmer provides the ability for a program to write arbitrary alphanumeric strings to the printer, allowing the printer to serve as a prompting device for input data, as well as for descriptive labeling of output.

Memory Allocation Selection Switch on Later Model 31 Calculators

The base memory configuration of the Tek 31 provides storage for 512 program steps and 64 memory registers. The maximum memory configuration allows for 8192 program steps and 256 memory registers, a data and program storage capacity that rivaled some small computer systems of the time. The calculator exhibited here is configured with 5120 steps available for program storage, and 640 memory registers. The memory chips used for the program and memory register storage are Intel 2102(or various manufacturer's equivalent) 1,024-bit Static RAM chips.

The capability of the magnetic cartridge tape drive in the Tek 31 was quite advanced. Each continuous-loop cartridge can hold up to 6,000 program steps. Data on the cartridges is organized in up to six addressable blocks of program steps or memory registers. It is possible to access the cartridge tape through programmed operations, allowing for chaining of programs, as well as allowing storing and recalling blocks of memory registers on the tape. The cartridges have a removable rubber plug, that when in place, write-protects the cartridge to prevent accidental erasure of important programs or data.

The Model 31 also provides another very computer-like capability that allows external devices to utilize a Direct Memory Access (DMA) facility to transfer memory register content or program steps from an external device directly to/from the calculator's memory. This capacity could become the basis for addition of complex high-speed I/O devices such as floppy and hard disk drives, high-speed magnetic tape, and other devices that require a faster I/O rate that can be attained through the regular programmed I/O capabilities of the calculator. Unfortunately, Tektronix never seemed to take advantage of this capability, at least as for as formal products went, though there are stories that this DMA capability was utilized by the US Department of Defense (a major customer for Tektronix 31 calculators), to make high-speed measurements of "events".

Block Diagram of Tek 31 Architecture

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

The Programmer and Memory Boards

The calculator board performs computations to a total of 12 significant digits, with ten digits displayed and two digits 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 significant digits displayed for the mantissa, and an exponent from -99 to +99.

Tek 31 Display in Program Mode showing Error Code E3

The display uses seven-segment planar gas-discharge display modules manufactured by Sperry. 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 tendency for leakage of the internal gas mixture over long periods of time, rendering the module 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 don'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 significant failure rate over time. Unfortunately, these chips (manufactured by Sprague, who exited the microelectronics market many years ago) are very difficult to find today, and direct replacement devices are available. The display segments glow a bright orange when energized, though a red filter in front of the display makes the digits appear red to the user. If an error condition exists, the display flashes on and off at 1 Hz rate (1 on/off cycle per second), and a numeric error code is displayed in the exponent digits that the user can use to determine what kind of error occurred.

The Display Board

The Tek 21 and 31 calculators provides a large array of built-in math operations, including: standard and hyperbolic trig functions (with arguments in degrees or radians); factorial; natural and base-10 logarithms; e^x; x^y; squaring; square root; integerize; and various summing functions for statistical calculations. The calculator uses algebraic logic, with parenthesis keys allowing entry of calculations exactly as they would be written on paper, within limits. Parenthesis cannot be nested, with only one level of parenthesis pending at any point in time, which to some degree seems to defeat the purpose of having them in the first place. The reason for this is that the aptly-named "Hierarchy Control" function of the Cintra 909's architecture was left out of the Tektronix chip set, likely for cost reasons. This particular functional unit within the Cintra 909/911 calculators was among the most complex units, and may have been deemed too complex or expensive to bother with implementing in integrated circuit form. 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 a very important part of Irwin Wunderman's (founder of Cintra) vision behind the Cintra calculators.

Memory functions include a ten register constant (K) storage area, and access to up to 99 numeric memory registers on the Tek 21, and up to 1000 numeric storage registers (depending on the amount of memory installed in the calculator and how it is divided between program and data storage).

The Rear Panel of the Tek 31
Note ROM Slot at Upper Left, and I/O Slot at Lower Left Board

The Model 31 calculator was a spring-loaded flapper door covering a slot at the right rear top of the cabinet when viewed from the front of the calculator that is a place to plug in a ROM cartridge. The ROM cartridge could contain the effective equivalent of program steps to create a custom application that could be made available by simply plugging the ROM cartridge into this slot. The Tek 31 keyboard has an array of 24 keys on the left end of the keyboard panel that are used for various purposes, which include access to the advanced math functions, and entering alphanumeric charaters. These 24 keys could also be assigned as user-definable keys that would branch to specific places in program memory. Above and below these keys are what looks like a hold-down method for securing an key overlay that would fit over the keys for specific user-defined functions. User defined functions could be used in normal programs, but also in the ROM packs, which would provide an easy way to allow a user to interact with the program in the ROM. The notion was that ROM packs with pre-coded applications could be provided, along with a printed key overlay, that would identify the various functions within the ROM allow applications to be "canned" for ease-of-use by a customer. The top part of the overlay securing mechanism is hinged like a door, so that it can flip to up allow easy insertion and removal of key overlays, and also holds the overlay securely in place when pressed back down. Beneath the finger hold to lift the top part of the key overlay securing mechanism is a plastic button that connects to a switch. This switch is normally left un-depressed when no key overlay is in place, but if door is closed with a key overlay in place, the button is depressed, allowing the calculator to sense the presence of the key overlay. When a key overlay is sensed, the normal functioning of the 24 keys is disabled, and they are re-assigned to labeled branch points in program memory. This way the keys can be customized to branch to a point in a program to allow each key to provide a function as part of the program. Tektronix provided key overlays that had areas over each key that the user could use to write-in a designator for the function the key performs. When a ROM is inserted in the ROM slot, the allocation of the function keys is yet again re-defined such that entry points within the ROM cartridge will be accessed when a each of the 24 keys is pressed and an appropriate key overlay identifying the function within the ROM is in place. Tektronix offered a service whereby users could send in a program tape or listing and Tektronix would burn the program into a ROM and return the user a ROM cartridge with their program in it. It is known that a number of third-party program developers utilized ROM cartridges in order to provide custom applications such as complex land surveying and structural calculations.

Programming functions available with the Model 31 are extensive. Programming mode is entered by pressing the [LEARN] key on the keyboard. Successive key presses are learned (stored) as 8-bit codes in memory, with some instructions (such as GOTO, or memory register access) requiring multiple key presses 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 branches by program step address or label and a GOTO display instruction that branch to the address in the four least-significant digits on the display. There are also conditional branches based on display content (>=0, =0, <0). Branches can also be made if an error condition exists. There is also a flag bit which can be set or cleared programmatically, and a branch taken if the flag is set. The programmer has a basic subroutine facility, where where a section of code can begin with a label, and a subroutine branch to the label will cause the program steps after the label to be executed. When the branch to the label is executed, the return address is stored in a single register. At the end of the subroutine, the content of the return address register can be recalled to the display, and then a "GOTO Display" command executed, which will cause the program to branch back to the main program. Since a single register contains the return address, subroutine nesting can be a rather challenging to implement since calling another subroutine within a subroutine will over-write the original return address, requiring some programming gymnastics to work around. Similar calculators from competitors such as Hewlett Packard and Wang Laboratories provided a push-down stack to hold return addresses, making subroutine nesting trivial on these machines as opposed to the Tek 31.

Sample of Program Listing Printout from Tek 31

The optional printer shared by the Model 21 and Model 31 calculators is a very quiet and fast (2 1/2 line-per-second) thermal dot-matrix printer, designed and manufactured by Tektronix, 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 only print numeric data, while the printer on the Tek 31, while sharing the same 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 that 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.

The Tek 31 seems to run at a fairly decent pace, but can take a little while to perform some operations such as trig or factorial functions. It is assumed that the built-in math functions, which are the same on the Tek 21 and Tek 31 calculators, execute at the same speed since they share the same calculating logic. The 21/31 calculators are definitely not as fast as their bipolar logic predecessors, which are among the fastest electronic calculators ever produced. The 21 and 31 have a 'BUSY' indicator that lights while the machine is performing calculation functions. Some specification information 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. Trigonometric operations on the Tek 31 take between 70 to 160 milliseconds to perform, with the Cintra 909 taking approx. 100mS.