Browse Prior Art Database

Multiple Key Detection in Noninterlock Keyboards

IP.com Disclosure Number: IPCOM000075182D
Original Publication Date: 1971-Aug-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 56K

Publishing Venue

IBM

Related People

Walters, NL: AUTHOR

Abstract

Described is a technique for the detection of multiple key depression in noninterlocked keyboards. The circuits illustrated require two simple threshold detectors and several resistors and work independent of the number of keys contained in the keyboard.

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Multiple Key Detection in Noninterlock Keyboards

Described is a technique for the detection of multiple key depression in noninterlocked keyboards. The circuits illustrated require two simple threshold detectors and several resistors and work independent of the number of keys contained in the keyboard.

Fig. 1 illustrates a typical keyboard configuration. Whenever a key is depressed, a column is connected to a row causing the transistor associated with the row to turn on and the column line to be pulled down to a voltage equal to the V(BE) of the row transistor. The column and row lines are wired to logic decode circuits, which provide outputs identifying the depressed key.

Fig. 2 illustrates how the typical keyboard has been modified to detect a multiple key. All the column pull-up resistors are connected to a common resistor which is connected to the supply voltage. The resistors are chosen such that the main current flows as before for a single key depression. A threshold detector illustrated in Fig. 3 senses the voltage developed across the common resistor RC. If a single key is closed, the voltage drop across RC is less than the threshold of the connector. However, if two or more keys are closed, the threshold will be exceeded and an error signal is generated.

An identical threshold detector senses when two or more rows are enabled. For the rows, resistors and isolating diodes have been added to the collectors of the row transistors. The outputs of the two th...