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Casio AS-L Calculator

Updated 1/23/2012

The Casio AS-L is the first Casio machine to use true LSI (Large-Scale Integration) technology. When I first saw pictures of this machine, its resemblance to the Casio 121-A and Remington Lektronic made me think, "I've already got two machines like this one...I don't need another." On the other hand, there were some intriguing differences between the pictures of the AS-L and the 121-A and Lektronic that I had, that, when one became available, I decided to acquire it. When it arrived, it was clear that I had made a wise choice. Even though both the AS-L and 121-A were produced at nearly the same time, they have significant differences in functionality and technology. The AS-L is a much more refined design, with a highly integrated (for the time) four-chip microcoded CPU versus the small- and medium-scale hardwired logic of the 121-A. The AS-L provides a full function calculator, as opposed to the rather crippled implementations on the 121-A and Lektronic.

It is clear that the Casio AS-L is a somewhat later design than the 121-A, with this AS-L perhaps being an early production machine in the AS-L production run, and the 121-A being a late production machine in the 121-A run. This would account for the similarity in the dating of the machines, though the AS-L is a more technologically advanced machine.

Internally, the difference between the AS-L and the 121-A(also known as the Casio AS-A) are fairly dramatic, with the AS-L being a much more highly-integrated machine than the 121-A. Functionally, the AS-L adds a memory register, as well as the ability to operate in fixed and full floating decimal mode. The AS-L also properly deals with negative numbers, properly handles overflow and error conditions, and also provides round-off/truncate functionality. All of these improvements over the 121-A are due to the fact that the AS-L, with its higher levels of integration, uses a microcoded CPU with ROM-based code that guides the operation of the machine. In contrast, the earlier technology Casio 121-A has to rely on hard-wired less-highly integrated logic for its functionality.

Internal view of Casio AS-L

The machine is built in a similar fashion to the 121-A, though, though the use of LSI circuitry and integrated circuit Nixie drivers, the entirety of the circuitry of the AS-L resides on a single circuit board, opposed to a two card stack in the 121-A.

Closer view of the early LSI IC Chipset of the Casio AS-L

The AS-L, as mentioned before, uses a four-chip chipset to make up 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 general control logic, the HD3212 contains the Arithmetic-Logic Unit, and the HD3214 contains the shift registers that make up the working registers of the machine. A fifth large-scale device (HN3203) is a ROM (Read-Only Memory) that contains the microcoded program that runs the machine. All of these devices are made with ceramic packages with metal lids to cover the chip itself. Along with the main brain chips, there are a number of other plastic-packaged IC's in the machine which provide glue and display drive functions. These include three oddly-packaged Toshiba T4097 Nixie tube driver devices, a NEC uPD116(BCD to 1-of-10 decoder for the Nixie tubes), a NEC uPD13(Buffer/Driver), seven Hitachi HD3227(Shift Register) 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 Nixie tubes and a circuit board with the connections for the Nixie tubes. The display subassembly connects to the main board via hand-wired interconnects. Each Nixie tube contains the 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 which 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 Casio 121-A. 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 must be pressed simultaneously. This is a bit awkward, and takes some getting used to. One negative of this method 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 121-A, does a decent job of detecting overflow and invalid operations, though it doesn't have any positive indication of such event occurring. When the machine overflows or an invalid operation (such as division by zero) is performed, the display is cleared to all zeroes, and the keyboard is locked, 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. The machine will overflow in multiplication or division operations if any part of the operation has a digit in the 12th position of the display. Some examples which should not, but do cause overflow:

999999999999 X .1
999999999999 ÷ 10
99999999999 X 10

Why these operations cause an overflow is a mystery to me, as all of these operations have results that fall within the range of the machine. Another quirk that I observed 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.

Keyboard Construction

The AS-L, like the 121-A and Lektronic, 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 can be set at 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 is rated at 7.5W at 120VAC.


Text and images Copyright ©1997-2012, Rick Bensene.