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The Monroe 820 CRT-display calculator was introduced in August of 1969. It is a basic four-function electronic desktop calculator. It operates with fixed or floating decimal with settings for 2, 3, or 4 digits behind the decimal for fixed mode. Numeric entry uses floating decimal, and leading zero suppression is performed to make the display more readable. Overflows cause the display to flash, an audible signal to sound (I'm sure that the audible warning was not popular in office environments), and the numeric and function keys to be electronically locked out. Pressing the [C] key will clear the machine and unlock the overflow condition. Addition and subtraction operate arithmetically, and multiplication and division operate algebraicly, with the [=] key finishing off multiplication and division problems. The "diamond" (subtotal) and [*] (total) keys copy the accumulator register to the entry register, and in the case of the [*] key, the accumulator is then cleared.
The machine is rather unique in that it uses a small CRT display as its display. The display presents two lines of 14 digits. The top line displays the accumulator register (where results of arithmetic operations are placed), and the bottom line is the entry register, where input from the keyboard is entered. Digits are rendered on the CRT display in a seven-segment fashion. The logic of the machine is implemented by small-scale DTL integrated circuits in dual-inline plastic packages, combined with a fairly sizable complement of discrete components that are used in the display generation circuitry.