1432706 Keyboards HITACHI Ltd 13 July 1973 [21 July 1972] 33478/73 Heading G4H An electronic keyboard input circuit includes a plurality of key switches for passing, when operated, a corresponding plurality of timing pulse trains having different phases from one another, means for time-division-multiplexing the trains, and means for detecting which key switch has been operated, the detecting means output being encoded into binary form, which form is stored in memory means, there being means for preventing further binary coded signals from being stored in the memory means when a binary coded signal has been stored therein as a result of a key having been operated. A set of numeral keys are scanned sequentially (by clock signals), at the same time as a set of function keys, producing a time-significant pulse on one of two lines which are sampled in turn by respective sampling signals to feed a set of AND gates enabled in turn by the key scanning clock signals and one of them also by one of the two sampling signals. One of the AND gates produces an output which is encoded into binary form by OR gates and an AND gate enabled by said one of the two sampling signals. The encoded output is stored in flip-flops providing system output and feeding a NOR gate which provides a further enabling signal to the set of AND gates so that these are disabled after detection of one actuated key thus preventing erroneous output when more than one key is actuated simultaneously (since the keys are scanned in turn). A start pulse generator provides a further enabling input to the AND gates, to limit the output signal duration. The AND gate used in the encoding produces one "type" bit in the encoded output which distinguishes a numeral key from a function key. That one of the set of AND gates preceding the encoding which receives said one of the two sampling signals does so to result in the decimal point function being encoded with the "type" bit having the value used for numerals.
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