The Lektronic's Cousin, the Casio 121-B
Image Courtesy Andy Anderson

Remington didn't design its own calculators, instead relying on OEM relationships with Japanese calculator maker Casio. This Sperry Remington machine is no exception, being identical (other than keyboard and case coloring) to Casio's Model 121-B/AS-B. This calculator was built in early 1971, with serial number 312435. The Lektronic, along with its Casio counterpart, was a follow-on to the Casio 121-A/AS-A calculators, replacing the discrete transistor display driving circuitry of the 121-A/AS-A with integrated circuit display drivers, and adding an accumulating 'total' register.

Inside the "Lektronic"

The machine is built on two tightly stacked circuit boards interconnected by hand-wired jumpers. The top circuit board (visible above) is home mostly to display multiplexing and driving functions, along with keyboard encoding, and master clock generation.

The main logic board, sandwiched below the display board, makes up the calculating engine of the machine, consisting of a number of Small- and Medium-Scale Integration IC's. The small-scale devices contain simple gates and flip-flops used for logic and sequencing functions. The medium-scale devices consist of shift registers (used for register storage), and the main adder circuit. The adder IC, a Hitachi HD3112, puts all of the circuitry that performs BCD (Binary-Coded Decimal) addition on one chip. In earlier Casio calculators such as the Commodore AL-1000 (made by Casio for Commodore), the functions contained on this single IC required an entire 12x7-inch circuit board full of discrete components.

The "Lektronic" is a very simple-minded machine, performing only the four basic math functions. It has no capability to deal with input of decimal points (notice that there is no decimal point key on the keyboard!). The Nixie displays have decimal points in them, but they are only used for displaying the decimal point in quotients, and then, sometimes a non-intuitive method is required to know the result. For example, in operations which result in only a fractional result, (e.g.: with 0 as the whole-number part of the result), the decimal point will position itself such that the user starts at the decimal point, working to the right, and when the right end of the display is hit, the user must 'wrap around' to the left end of the display to get the answer. e.g. 1 divided by 10 results in a display of '100000000000.' In another example, 1 divided by 1000 results in '1000000000.00', which really means .001. The machine also represents negative numbers as their 10's compliment, e.g. -1 is displayed as 999999999999. Pressing the [-=] key will compliment the number on the display. Overall, the machine can require a bit of thought when in use because of its limitations. The machine has no error or overflow indication. Dividing by zero results in the machine trying forever to position the divisor such that subtraction from the dividend can begin -- an endless venture which results in all of the decimal points flickering wildly, and the machine being non-responsive to anything but the [C] key. Addition and subtraction operations which cause overdraft simply toss the overflow, wrapping back around; e.g. 999999999999 + 1 = 000000000000. Multiplies which overflow give useless results. All operations perform as expected, with the [+=] and [-=] key adding to/subtracting from the number in the display, and multiply and divide generating a result by pressing the [+=] key. The machine calculates at a reasonable pace (considering its relatively slow master clock frequency of 45KHz), with the benchmark division of 999999999999 divided by 1 taking less than 1 second to perform. A small slider positioned in front of the display window allows the user to manually position the commas (groups of 3 digits) to make it easier to read numbers on the display. The machine does sport a memory register that automatically accumulates the sum of products during multiply operations. The [R] key on the keyboard recalls this register to the display, clearing the register after the recall.