The EL-811 is a four-function, 8 digit floating point calculator with memory. The machine actually has a capacity of 16 digits for single multiplication operations, with a '<->' key that toggles the display between the upper and lower eight digits of a result that exceeds the eight digit display capacity of the machine. As with the EL-8, the display of the EL-811 is provided by Itron eight-segment vacuum fluorescent display tubes. The display is composed of eight numeral tubes, and a special ninth tube located at the right end of the display panel that is used to indicate sign, memory status, constant mode, and display status (a "<->" indicator that lights when the least significant eight digits of a result that exceeds eight digits are being displayed).

The machine uses two Rockwell-made LSI (Large Scale Integration) devices in 'zig-zag' packages. The devices are mounted on a small circuit board that plugs into an edge connector which connects the main logic to the keyboard assembly.

The display system is a separate module that plugs into the main logic board via another edge connector. The Itron display tubes are wired directly to a small circuit board which interconnects the multiplexed displays. The display tubes are surrounded by a mounting frame that provides some protection for the delicate glass tubes, as well as aligning the tubes.

This machine was built in the days (early 1972) where calculators were still expensive enough that repair was a viable option if something went wrong with the machine. Given the highly modular construction of the machine repair would be a much easier (and less costly) proposition than other less-modular designs. Along with the two LSI devices, a total of four small-scale devices complete the complement of IC's that make up the machine. Three of these dual-inline package devices make up the display drive circuitry, and the last device appears to be related to generating the master clock that orchestrates the timing of the machine.

The calculator is powered by a rechargeable NiCd (Nickel Cadmium) five cell (7.2V) battery pack (Part number EL-84) for power when running without the AC adapter. The external power supply/charger module (Part number EL-81) plugs into a connector on the back panel of the machine for operation from AC power, or for charging the NiCd batteries. A circuit board that connects into the rest of the calculator circuitry via a plug-in connector provides power conversion functions such as creating the high-voltage necessary to drive the vacuum-fluorescent display from the 7.6 volts provided by the Nickel Cadmium battery pack.

The EL-811 dispensed with the dual function multiply/divide key that I'm sure was a source of confusion for users of the EL-8. The additional real estate required for a separate multiply and divide function key, along with keys for control of the 16-digit display mode and memory register, caused the footprint of the EL-811 to actually be just a bit larger than that of the EL-8, but the height of the EL-811 is almost one-half that of the EL-8.

The EL-811 provides a memory register, improving the usefulness of the machine for more complex calculations requiring temporary storage of intermediate results. The memory register acts as an accumulator, with the 'M+' key adding the content of the display into the memory register. When the memory register is non-zero, an "M" indicator lights in the right-most tube of the display. A dual-function 'MR/CM' key serves to recall the memory register to the display on the first press, and a second press clears the memory register without disturbing the display. The "C" key also serves dual purpose, with the first press clearing any entry made thus far, and the second press clearing the machine with the exception of the memory register. Lastly, a "Change Sign" key is included to make it easier to work with mixed-sign calculations.

The EL-811 benefits from improvements in technology in terms of its error detection. The EL-8 gets rather confused when commanded to divide by zero, while the EL-811 properly detects this condition and flags an error. The EL-811 also flags an error condition on any overflow situation. The error state is indicated by all of the numeric display tubes lighting up with both zero and the decimal point lit at the same time, and locking the keyboard. Pressing the "C" key once will clear the error condition. The EL-811 also benefits in the speed department as compared to the EL-8, with virtually instant results given to 'difficult' calculations, such as 99999999 X 99999999.