Tektronix Model 31 Electronic Calculator

This is a rather unusual calculator, somewhat bridging the gap between a calculator and a computer. Many people do not know it, but Tektronix, famous as a maker of very high quality and high performance electronics test and measurement equipment (especially oscilloscopes), spent a short time in the high-end electronic calculator marketplace in the early to mid-70's. In reality, Tek didn't create the calculator business itself, but instead acquired the technology and people from another company, called Cintra, located in Sunnyvale, California.

Cintra made high-end scientific calculators in the late '60's through May of 1971, when Tek purchased the assets of the company. Before Tek purchased them, Cintra was falling upon hard times due to competition from HP, Wang, and others, along with some early production problems with their calculators that resulted in some bad press. Tek's purchase of Cintra was a response to Hewlett Packard's (HP has always been a major competitor of Tektronix, and there's always been a strong rivalry between the two companies) very successful line of high-end calculators.

Tek 31 with top cover removed

When Tektronix purchased Cintra, they rebadged Cintra's existing line of calculators, (the Cintra 909 Scientist, and the 911 Statistician) and sold them under the Tektronix brand for a period of time while Tek put Cintra's and it's own engineers to task to design machines capable of competing in the high-end calculator business. The result of the development effort were the Tek 21 and 31 calculators, both of which were introduced to the marketplace in July of 1973.

Unfortunately, the 21 and 31, while capable instruments, had a hard time competing with other well-established programmable calculators from competitors Hewlett Packard and Wang. As it turned out, a combination of forces combined to make Tek's venture into the high-end calculator market less than Tek's management (and investors) expected. First off, Tektronix salespeople did not really know how to sell calculators. The Tektronix sales force was extremely skilled in selling test and measurement equipment, and calculators just didn't fit their sales expertise. HP and Wang had well-seasoned calculator sales experts out in the trenches that could generally out-savvy the relatively green Tektronix sales folks. Along with that, the Tektronix got out of the gate a little late. By the time the Tek 31 hit the market Wang had well-established its 700 and 600-series calculators, HP was selling it's 9800-series calculators like hotcakes, and Compucorp (and OEM Compucorp customer Monroe) had been selling LSI-chipset technology-based high-end programmable calculators for a couple of years. Tek's timing wasn't the best, either, because at about the time that the machines were introduced, the calculator market was beginning to experience its first major shakeout, with many companies falling by the wayside between 1973 and 1975. This combination of factors made it hard to justify the existence of Tek's calculator division, and by 1975, Tek realized that that their return on investment simply wasn't there, and disbanded their calculator business.

Before the calculator business was disbanded, attempts were made to sell off the rather massive remaining inventory of un-sold machines at drastically reduced prices. This met with little response from the marketplace, with savvy customers realizing that something was amiss. 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 it was purchased for $7.00.

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

The machine exhibited here is a Tektronix Model 31 calculator, circa 1973, which was the big brother of the somewhat less feature-laden Model 21. Both machines were programmable, had both a display and 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 also has an expansion bus which allowed external devices such as test equipment, printers, plotters, paper tape readers and punchers, graphic display, and other peripherals.

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 via a daisy-chain cable arrangement (similar, but not identical to GPIB/IEEE-488) to the calculator. 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, it was possible to interface 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 modified Tektronix Model 4010 "DVST" (Direct View Storage Terminal) 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 Tektronix' famous 4051 Graphic Computer System...one of the first true "personal computers".

Tektronix Model 21 and 31 Calculators

The technology in the machines is a combination of MOS/LSI circuitry and TTL (7400-series). It appears that both the Model 31 and Model 21 share a common "calculator board", which contains 15 custom-designed (by Tektronix) 40-pin MOS LSI (Large Scale Integration) IC's manufactured to Tektronix' specs by AMI. Along with the LSI's, a whole slew of 7400-series small-scale TTL IC's glue everything together. This board contains the calculating brains of the machines. The programmability of the calculators is where the Model 21 and Model 31 differ. The Model 21 is somewhat limited in the number of steps and complexity of programs that it can handle, and has much less complex programmer circuity. The programming capabilities of the Model 21 are very similar to the earlier generation Cintra/Tektronix 909 and 911 calculators. The Model 31 supports a much larger memory space (using Intel 2102 or 2602 1Kx1 Static RAM chips for storage) which is expandable, and can be (via internal jumpers) divided between program storage space and memory register storage. The calculator exhibited here is configured such that 5120 steps are available for program storage, and there are 640 memory registers available. 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 DMA (Direct Memory Access) data or program steps directly into 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's, which appear to be ROMs used for sequencing or microprogramming of the programmer logic. 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 of the Model 21 provides the 256-step program storage and sequencing logic to allow learning and playback of keypresses, as well as interfacing for the magnetic card reader.

The Calculator Board

The Programmer and Memory Boards

The calculator functions of the both the Model 21 and 31 provide answers to a total of 12 significant digits, with ten digits displayed, and two digits which are not displayed acting as guard digits. The machine can operate in scientific notation, with ten digits (plus a digit for negative sign) displayed for mantissa, and exponent from -99 to +99. The display is made up of plug-in modules which contain two or three digits.