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Time Duration Detector and Encoding Apparatus for Low Cost Switching Systems

IP.com Disclosure Number: IPCOM000087255D
Original Publication Date: 1977-Jan-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 4 page(s) / 66K

Publishing Venue

IBM

Related People

Cannon, JW: AUTHOR

Abstract

A detection method is described for low-cost switching systems that achieves the same reliability as that accomplished by more complex switching designs. A solution is offered to the problem of multiple interrupts from elastic diaphragm switch (EDS) keyboards, such as for engine-room systems in maritime applications. It is also adaptable to existing collision-avoidance system keyboards in maritime applications.

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Time Duration Detector and Encoding Apparatus for Low Cost Switching Systems

A detection method is described for low-cost switching systems that achieves the same reliability as that accomplished by more complex switching designs. A solution is offered to the problem of multiple interrupts from elastic diaphragm switch (EDS) keyboards, such as for engine-room systems in maritime applications. It is also adaptable to existing collision-avoidance system keyboards in maritime applications.

Due to the inherent characteristics of single-pole single-throw (SPST) switches, most systems designers will avoid this switch-type in order to maintain a high level of system reliability and a high degree of user confidence. The switches normally selected are single-pole double-throw (SPDT) or double-pole double-throw (DPDT). These switches are more complex and expensive, and must be installed with a considerably higher wiring cost.

The types of errors associated with SPST switches are known universally as switch bounce problems. Another term used to describe the same phenomena is the human jitter factor (i.e., a shaky human finger used to actuate the switch.) The end result is that a wide variety of errors can occur within the controller itself producing lost, false or multiple interrupts and/or improper encoding of interrupts.

When design considerations require the use of SPST switches, a typical approach to solving the problem is to run the switch output through an RC filter network and then through a hysteresis device before entering the controller's logic. This method works to some degree. However, due to component size and tolerance restrictions, the roll-off characteristics associated with RC networks, and the wide range of inputs associated with differing human stimuli, this method (or any other method for that matter) never produces completely satisfactory results. There always seems to be an overlap in accepting all the desired inputs and rejecting all undesired inputs. A method is described that will much more accurately accept only the desired inputs and reject all others resulting in an almost infallible detection system.

The described detector takes advantage of the fact that mechanical switches can only be actuated at certain maximum frequencies. For example, a single human finger is restricted to approximately six depressions per second and actual depression time on an EDS keyboard is approximately 35 m.s. minimum. Therefore, one can assume that any components of a switch output that do not meet a minimum continuously on time of 35 m.s. and a minimum continuously off time of 125 m.s. (160 m.s. - 35 m.s.) can be rejected as an invalid input and consequently ignored. It is primarily the on-time stipulation that rejects the glitches and the off-time stipulation that rejects multiple entries due to human jitter and/or slow switch release times. This same philosophy can be applied to mechanically operated switches, except that the detec...