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Casio AS-L Calculator

Updated 3/1/2020

The Casio AS-L is the first Casio calculator to use Japanese-made LSI (Large-Scale Integration) technology. The AS-L is the last of the group of vintage calculators that Casio produced using the horizontal form-factor, with the keyboard beside the display rather then below it. There were two earlier machines made by Casio that used this arrangement, the Casio 121-A/AS-A and the 121-B/AS-B calculators. Some much later Casio calculators returned to this horizontal form-factor, but these calculators are far outside the scope of The Old Calculator Museum.

Oddly, though the Casio AS-L uses significantly more-advanced technology, this machine was manufactured at nearly the same time as the museum's Casio 121-A, a calculator made with much lower levels of integration. This is because the AS-L was manufactured near the beginning of production, while the museum's 121-A was manufactured during latter part of the production lifetime of the 121-A/AS-A. In order to get as much product lifetime out of a calculator that cost a significant amount of money to develop, production of older machines would continue for a time after they were no longer "current" technology, with reductions in sales price and more significant volume purchase discounts put in place to wring as much profit out of a product before it was no longer viable in the marketplace. This led to production overlaps, where older machines were still being produced while newer machines were also in production.

The AS-L is a much more refined design, with a highly integrated (for the time) four-chip microcoded arithmetic processor versus the small- and medium-scale hardwired logic of the 121-A/AS-L. The AS-L provides a full function calculator, as opposed to the functional compromises made on the earlier 121-A/AS-S and 121-B/AS-B calculators.

As compared to the 121-A/AS-S, the AS-L adds an accumulator-style memory register, as well as the ability to operate in fixed and full floating decimal mode. The AS-L also properly supports operations with negative numbers, properly handles overflow and error conditions, as well as round-off/truncate functionality. All of these improvements over the earlier 121-A/AS-A and 121-B/AS-B are due to the use of Large Scale Integration chips, and a microcoded architecture that provides a much more flexible environment for implementing additional features, with the only limitation being the amount of ROM (read-only memory) that contains the microcode that implements the functionality of the calculator.

Internal view of Casio AS-L

The AS-L is mechanically constructed in a similar fashion to the 121-A/AS-A and 121-B/AS-B, but because of the use of LSI circuitry and integrated circuit Nixie tube driver circuitry, the entirety of the logic of the AS-L resides on a single circuit board, as opposed to a two card stack in the 121-A/AS-A and 121-B/AS-B calculators.

Closer view of the early LSI IC Chip set of the Casio AS-L

The AS-L uses a four-chip chip set that comprises the "CPU" of the machine. These chips are made by Hitachi, and have part numbers HD3210, HD3211, HD3212, and HD3214. The HD3210 handles keyboard management; the HD3211 contains the main timing and microcode sequencing logic; the HD3212 is the ALU (Arithmetic-Logic Unit) that performs basic binary arithmetic and logic operations; and the HD3214 contains three 64-bit shift registers that provide the working registers of the calculator. A fifth large-scale device, also made by Hitachi, is the HN3203 ROM that contains the microcoded firmware that defines the functionality of the calculator. All of these devices are packaged in dual-inline ceramic carriers with metal lids to cover the chip itself. Along with the large-scale chips, there are a number of other plastic-packaged IC's in the machine which provide glue logic and display drive functions. These include three unusually-packaged Toshiba T4097 Nixie tube driver devices; a NEC µPD116(binary to 1-of-10 decoder for the Nixie tubes), a NEC µPD13(Buffer/Driver), seven Hitachi HD3227(Four Flip-Flops) IC's, and one Toshiba T1191(Keyboard Encoder). The twelve-digit Nixie tube array is located in a small subassembly, with a metal support structure for the Hitachi CD-71 Nixie tubes and a circuit board with the connections for the Nixie tubes. The display subassembly connects to the main board via individual hand-wired interconnects. Each Nixie tube contains approximately 1/2-inch tall digits, zero through nine, along with a right-hand decimal point. Situated at the left end of the display panel is a small incandescent lamp that lights through an orange-colored lens in the shape of a '-' symbol to indicate a negative number in the display.

The Display Subsystem

The AS-L operates much the same as the earlier horizontal form-factor calculators. Addition and subtraction operate adding-machine style, with each number entered and followed by [=+] to add the number to the accumulator, or [=-] to subtract. Multiplication and division work as expected, with the first number entered, followed by the function key, followed by the second number, then the [=+] key is pressed to calculate the result. Multiplication and division can be done with a constant by pressing the function key twice after entering the constant value, for example, to calculate successive powers of two starting at 21, one would press the [2] key, then press the [X] key twice to enter the 2 as a constant, then press the [=+] key successively to see each result in the series. The AS-L has a single memory register which operates as an accumulator. The [M+] key adds the content of the display into the memory accumulator, and the [M-] key subtracts the number in the display from the memory accumulator. The blue [TOTAL] key under the Nixie display panel recalls the content of the memory accumulator to the display.

Casio AS-L Keyboard Layout

The [CLEAR] key is used to clear erroneous keyboard entries only, functioning more like a clear entry key. In order to clear the whole machine (including the memory register), or to reset the machine in the event of an overflow or error condition, both the [TOTAL] and [CLEAR] keys are pressed simultaneously. This is a bit awkward, and takes some practice to get both keys pressed at exactly the same time. One down-side of this method to reset the calculators is that if there is a valid number in the memory register, and an operation occurs which overflows the calculator, the only way to unlock the machine from the overflow condition is the do a "ALL CLEAR" (pressing both [CLEAR] and [TOTAL] at the same time), which, as a by-product, clears the memory register. The AS-L, in contrast to the earlier calculators, does a good job of detecting overflow and invalid operations, though it doesn't have any direct indication of such events occurring. When the machine overflows or an invalid operation (such as division by zero) is performed, the display is cleared to all zeroes, and keyboard entry is ignored, requiring a "ALL CLEAR" operation to restore the machine to normal. While the AS-L does not have nearly as many quirks as the 121-A, I did notice a few oddities. One quirk is that when the machine is in floating point mode, operations that result in answers that have more than 11 digits behind the decimal point will cause the machine to overflow rather than just truncating the extra digits. (E.g., constant multiplication of 1.001 will overflow on the third cycle). Performing the same calculation with the machine in fixed point mode works properly.

The AS-L will overflow in multiplication or division operations in situations the would not normally be expected to cause an overflow. The reason for this is that one digit position in the working register is used as a counter digit during multiplication and division operations, meaning that there can only be a maximum of 11 digits in a product, and the divisor must contain less than 11 digits. This restriction is noted in the user manual and marketing material published for the Casio AS-L.

Keyboard Construction

The AS-L, like the 121-A/AS-A, uses a high-quality keyboard made of magnetic reed switches. In addition to the main keyboard, three switches share the keyboard panel. These switches include the main power switch, a slide switch for selecting floating decimal or fixed decimal mode with round-off or truncation [labeled "F" (for floating decimal mode), "5/4" (for fixed decimal mode with round-off, and "CUT" (for fixed decimal mode with truncation], and another slide switch for setting the decimal point location when in fixed decimal mode. Fixed decimal point location can be set to 0, 1, 2, 3, 4, or 5 digits behind the decimal point. The keyboard connects to the main board via hand-wired connections from the switches to an edge-card connection that plugs into the left end of the main board.

The AS-L uses a transistor-regulated linear power supply. A small transformer takes up the upper left corner of the machine, with a small circuit board located underneath the main board on which some of the power supply components are found. The total power consumption of the machine 7.5 Watts at 120VAC.

Text and images Copyright ©2020, Rick Bensene.