Abstract: An intrinsically safe telephone as provided for use in hazardous areas. The telephone includes a single line instrument located in the hazardous area which, in turn, is coupled by a two-wire transmission path to a shunt diode barrier and a telephone line interface located in a safe environment. The telephone line interface reduces power of incoming ringing and audio signals. The telephone line interface further includes an off-hook indicating circuit which senses when the single line instrument is off-hook. The intrinsically safe telephone is useful in all hazardous environments as classified by the National Electrical Code and requires no explosion-proof housing.

Abstract: A watchdog device for monitoring control signals from a computer being used to control an operating system, is disclosed; said device having sufficiently low operating power requirements so that it is compatible with the power requirements of a series connecte LED indicator. The device is thereby capable of displaying a visual indication of its output status by activating an LED indicator which may be rack mounted or integrated into the device. In addition, the timer disclosed provides an adjustable timer interval which allows its adaptation to applications having widely differing time-out requirements. Depending on the requirements of the application, the adjustability is accomplished using a variable resistor, capacitor, or inductor for short timer intervals, or an oscillator driven frequency divider circuit for long timer intervals.