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Texas Instruments SR-20 Scientific Calculator

The SR-20 was the first scientific (though I consider it a bit of a stretch to call this machine a true scientific) calculator made for mass market by Texas Instruments. It is an AC-only powered desktop machine. This particular instance of the SR-20 appears to have been made in the late-1973 timeframe, based on the 7317 and 7327 date codes on the LSI IC's. This machine is a rather rare find, with apparently not very many having been produced likely because AC/battery-powered scientific handheld machines came along in about the same timeframe, making a desktop, AC-only machine much less marketable. Another factor that may have contributed to this machines relative rarity could be that TI "cheaped out" on the keyboard, using a flakey 'spring metal' contact-type keyboard. These keyboards were prone to bounce even when new, as well as providing a rather poor keyboard 'feel', which may have led to a lot of returns and warranty problems.

Inside the TI SR-20

The SR-20 is an IC-based machine, using a TI chipset made up of 2 LSI IC's, a TMS 0202, and a TMS 0304. The entire guts of the machine are located on a single board, which is connected to the keyboard via individual metal strips soldered between the boards. A couple of other small-scale integration chips (TI 7400-series) are also used, perhaps in the clock generation circuitry. The display device, a Burroughs Panaplex II panel containing 14 elements, is connected directly to the main board, and driven by discrete transistor driver circuitry. The display is organized (from left to right) as a single element for sign of the mantissa, 10 digits for mantissa, then a sign element for the exponent, and two digits for exponent. The display digits are quite small, but very legible. The machine has a rather complicated power supply, appearing to be a combination linear supply with IC-based voltage regulators for logic supply, and a discrete transistor-based switching supply (running off of one of the logic supplies) to generate the high-voltage potential for the Panaplex display. As mentioned before, the keyboard is a very cheaply made affair, with small spring metal 'spirals' which compress when a key is pressed, making contact with etched traces on the keyboard circuit board. These spring metal contacts serve both as the active switching element, as well as the 'return spring' for the key. The problem with these type of switches is that they are very prone to intermittent contact, especially if any dust or dirt gets into the keyswitch assembly. The high likelyhood of bounce (IE: double, or triple entry of a single keypress), plus the generally 'cheap' feel of this keyboard type, makes the whole machine feel as if it is lacking in quality. This particular example of the SR-20 is in extremely good condition, likely very rarely used. Even in the exceptional condition this machine is in, the keyboard tends to bounce occasionally.

A Detailed View of the Burroughs Panaplex II Display Element

The SR-20 is a rather limited scientific machine, with some features which are unusual, and others that are omitted which seem to be obviously needed. An example of this is the inclusion of a factorial (x!) function, but the obvious omission of a memory function. Why the designers opted to not include a memory function on a machine such as this is a mystery to me. Another example is the inclusion of the key to recall the constant 'e'. e by itself isn't a terribly useful number, however an ex function, along with an LogeX function can be used to perform many derived functions, such as raising arbitrary numbers to arbitrary powers, etc.

The machine provides the standard four-functions using the algebraic entry methodology. The machine operates with full-floating decimal, and automatically shifts to scientific notation when the result is too large to display in standard notation. The machine has a range of -9.999999999x1099 to 9.999999999x1099. Along with the factorial function mentioned above, the machine also provides one-key access to reciprocal (1/x), square root, square (x2), and integer (discard fractional part of number, leaving only the whole part of the number) functions. The machine has two built-in constant keys, one to recall the value of Pi, and the other to recall the value of e. The "EE" key allows entry of the exponent part of a number in scientific notation. The "CD" key clears the display, and, of course, the "C" key clears the machine. A change sign key (+/-) changes the sign of the mantissa or exponent, depending on which part of the number is being entered.

The machine indicates error conditions by flashing the display at an approximately twice per second rate. Overflow is indicated by a flashing display showing 9.999999999x1099. A flashing display locks the keyboard, requiring the "C" key to be pressed to clear the error condition. The machine is quite good at detecting error conditions, with errors generated for square root with negative argument, divide by zero, and factorial with negative argument.

SR-20 Keyboard Layout

The SR-20 is a rather fast machine, with the result for 'all 9's' divided by 1 taking just a blink of an eye. The scientific functions run quickly, with square root function averaging about 1/4 second, and with the factorial taking a period of time determined by the size of the operand, with the longest (69!) taking about 3 seconds, and the shortest factorial (0! = 1) taking virtually no time at all. During the time the calculations are in progress, the display is blanked with one decimal point flickering to indicate that the machine is busy.

I remember playing with one of these machines when they first came out in the calculator section of a large (and now defunct) department store years ago. This machine introduced me to the factorial function, which I'd not seen before. My mother didn't fret that I had disappeared for a long time while she was shopping there...she always knew where I could be found, and in those days, a youngster was not at risk being left without parental supervision. After tinkering around with the factorial function for a while, it became clear what it was doing, but why such a function would be considered useful was a mystery to me at the time. But, it was fun to plug in different numbers and watch the machine churn away at generating the result.

Text and images Copyright ©1997-2019, Rick Bensene.