The machine uses a mix of round metal can-type IC's (8 pin), and dual inline package (plastic) IC's (14-pin), totaling 60 IC devices. There are also a large number of transistors and diodes in the 1121, but the machine is clearly an IC-based machine, not a hybrid (a few IC's, but the majority of the logic made up of transistor/diode logic elements) machine like the Burroughs C3350. The logic of the machine is spread across four circuit boards that plug into a hand-wired backplane, with individual wire connections which come from the backplane to a separate circuit board to which the Nixie tube display elements are connected.

The Commodore 1121 is a 12-digit, four function electronic calculator with memory. The calculator uses Nixie tube displays, with each Nixie containing the digits zero through nine and a decimal point. The calculator uses a curious mix of floating and fixed decimal which I haven't quite figured out yet. It appears that when the machine is first cleared, if the decimal point key is pressed, the decimal point will start at the right-most position, and each time the key is pressed, it will move one digit position to the left. If the "ADD =" key is pressed at this point, the decimal point is set at this location for future calculations. This makes it necessary in cases where fractional-part only numbers are being entered, to first press "0." to take the calculator out of 'decimal point location' mode and into numeric entry mode. I've not been 100% successful in making this work consistently -- perhaps I've got the key sequences not down correctly, but the general notion seems to apply. If no decimal point location is set, the machine behaves as if it is a floating decimal machine.

The Insides of the Commodore 1121

The 1121 operates conventionally for a machine of the time, but chooses somewhat different legends for the key caps, and has memory register functionality that is a little different than most. The "basic four" function keys are labeled "ADD =" for addition, "SUBT =" for subtraction, "ENTER X" for multiplication, and "ENTER /" (with the / replaced with a division symbol, which I don't know how to render in HTML) for division. Adding and subtracting work as an adding machine, with the result accumulating in the display. Multiplication and division work by entering the first number of the problem, pressing the appropriate function key, then entering the second number, and pressing the "ADD =" key. The "SUBT =" key can be pressed, and the result will be negated.

The Four Logic Boards in the Commodore 1121

The memory function of the 1121 is rather interesting. There are two keys to place/accumulate results in the memory register. These are the "STORE +" and "STORE -" keys. Pressing one of these keys with a number in the display adds or subtracts the number in the display to/from the content of the memory register, and places the result back in the memory register without affecting the content of the display. A third function key, "CALL STORE", places the content of the memory register into the display but doesn't 'enter' it, meaning that you can continue digit entry after pressing the "CALL STORE" key. There is no indicator to tell the user that the memory register has a non- zero content, and there is no explicit memory register clear key. To clear the memory register, one must do a "CALL STORE", then, if the number on the display is positive, press the "STORE -" key to offset the number in the store, or "STORE +" if the number on the display is negative. The memory register is volatile, meaning it loses its content between power cycles, but the memory is retained through clearing the machine. The machine has two 'clear' functions, which also are labeled rather differently. The "CLEAR REG ONLY" key appears to clear the registers of the machine (except the memory register), and the "CLEAR DISP ONLY" key just clears the display in case of entry errors.

A Closer View of one of the Commodore 1121 Logic Boards

The 1121 uses an interesting mix of IC technology. All of the IC's in the machine are made by NEC, and are in the uPD01xx series. As mentioned before, there is a mix of can-type and DIP type packages used in the machine. These devices are early MOS IC's, which required more external discrete components than later DTL and TTL logic. To minimize the discrete component count, a special hybrid device was used with many of the IC's. This hybrid circuit module is located adjacent to most of the IC's.

Can-type IC Package with Hybrid

The Commodore 1121 uses a conventional keyboard with plastic keys with moulded in legends for the digit keys, and keycaps with plastic covers and paper legends for the function keys. The power switch is a bright red push-on/push-off button at the left bottom of the keyboard panel. A "ROUND OFF" key is located next to the power key, and appears to provide a rounding function when the calculator is in fixed decimal point mode. It doesn't seem to have any effect when the machine is in floating decimal point mode. Three small white plastic sliders are located near the display window to allow the user to mark off locations of where commas would be for easier recording of numbers in the display.