Browse Prior Art Database

Keyboard Scanning

IP.com Disclosure Number: IPCOM000094407D
Original Publication Date: 1966-Oct-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 2 page(s) / 47K

Publishing Venue

IBM

Related People

Beausoleil, WF: AUTHOR [+2]

Abstract

This circuit scans a keyboard in a data processing system. A three out of eight code is used to specify the key activated. A request is sent to the central processing unit CPU as soon as a key is depressed.

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Keyboard Scanning

This circuit scans a keyboard in a data processing system.

A three out of eight code is used to specify the key activated. A request is sent to the central processing unit CPU as soon as a key is depressed.

Each key of the keyboard is connected to three out of eight lines through diodes D1, D2, and D3. With this three out of eight code, fifty-six different combinations are available. Besides, each key is connected through a diode D to a read line 0. This read line is integrated through an RC circuit to eliminate bounces. Its output becomes the request signal which is made available to CPU through a scanner. The scanner finding this line in a request state activates a gate line associated with the particular keyboard scanned. The gate line signal is used to fire crosspoint A and B.

When fired, crosspoint A gives the zero state to the CPU request. This insures that only one message is sent to the CPU for a single depression of a key. Crosspoint B allows crosspoints S1... S8 to be fed by a +V voltage. When the three out of eight lines are activated, by depression of a key, the three corresponding crosspoints S fire and transmit the information to the CPU logic through transistors T1...T8. Reset is performed through transistor Tr by turning off the corresponding transistors T which are turned on.

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