Texas Instruments SR-20 Scientific Calculator

The SR-20 was the first scientific (though I consider it a bit of a stretch to truly call this machine a full scientific calculator) electronic calculator made for the mass market by Texas Instruments. It is an AC-only desktop calculator. 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 integrated circuits. This machine is a rather rare find, with apparently not very many having been produced likely because rechargeable battery-powered scientific handheld machines came along in about the same timeframe, making a desktop AC-only calculator less marketable. The SR-20 is also fairly limited in the functions it provides, and lacks a memory register, both of which would have contributed to lackluster sales of the machine. Another factor that may have contributed to this machines relative rarity could be that TI cheaped out on the keyboard, using a spring metal contact-type keyboard. These keyboards were prone to bounce (registering more than one digit with a single keypress) 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 large-scale integration (LSI) IC-based machine, using a TI chipset made up of two IC's; the 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, likely in the clock generation circuitry.

The Two Texas Instruments LSI Brains of the TI SR-20

The display device, a Burroughs Panaplex II panel containing 14 digit 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 (1/2-inch tall0, but very legible. The machine uses 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 cheaply made. The keyboard uses spring metal spirals that 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, either due to contact corrosion from ambient humidity, as well as contamination from dust and dirt that gets into the keyswitch assembly. The high likelyhood of bounce, 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 even after the keyboard had been treated with a contact cleaner.

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 e^x function, along with an Log_eX 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 π, 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 SR-20 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 logically 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 quite 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 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.