An optical amplitude demodulator for demodulating signals received from a fiber optic link. The demodulator comprises a plurality of optical sensors for detecting optical output from the fiber optic link, whereby each of the optical sensors has a different detection threshold. Generally, the plurality of optical sensors produce a plurality of digital outputs corresponding to the optical output level detected. A priority encoder encodes the digital outputs into a multi-bit digital signal. Additionally, each of the plurality of optical sensors has an associated optical filter, whereby each of the filters has a different level of opaqueness. Each filter filters received optical output prior to detection by the associated optical sensor.

A sensor characteristic adjustment circuit for adjusting the offset and the sensitivity of an output of a semiconductor sensor includes a serial-parallel converter for converting serial data bits into corresponding parallel data bits, a non-volatile memory for storing the parallel data bits, and digital-to-analog converters for converting the digital bits D.sub.1 through D.sub.m and D.sub.m+1 through D.sub.n into values corresponding to the offset and the sensitivity supplied to an adder and a voltage-controlled amplifier, respectively. Preferably, gates are inserted between the serial-parallel converter and the non-volatile memory to separate the non-volatile memory from the serial-parallel converter after the parallel data is written into the non-volatile memory.

A method and apparatus for notifying a receiving device of fault conditions in a sensing element or an instrument. The instrument generates an analog signal that has an amplitude which is representative of a variable. The analog signal amplitude has a range defined by a lower limit and an upper limit. The instrument transmits the analog signal to the receiving device. When the instrument detects a fault condition in itself or in the sensing element, the transmitter periodically changes the analog signal amplitude by a predetermined amplitude for as long as the fault condition exists. The predetermined amplitude and its polarity are such that the amplitude of the periodically changed analog signal lies within the range. The receiving device generates an alarm when it detects the pulses.

A temperature transmitter in a process control system includes a temperature sensor which senses temperature and provides a sensor output related to sensed temperature. An analog-to-digital converter coupled to the sensor output provides a digitized output related to the sensor output. A microprocessor receives the digitized output, compensates the digitized output and provides a temperature output. Comparison circuitry compares the sensor output to a first threshold and provides an inhibit signal to the microprocessor if the sensor is outside of the first threshold.

A first optical receiver and a second optical receiver are provided for receiving a calibrate command and a power switching signal, respectively, from a remote processor. A third receiver is provided for receiving an analog electrical signal from a transducer. A calibrator generates a reference signal in response to the calibrate command. A combiner mixes the electrical signal with the reference signal to form a calibrated signal. A converter converts the calibrated signal to an optical signal. A transmitter transmits the optical signal to the remote processor. A primary battery supplies power to the calibrator, the combiner, the converter, and the transmitter. An optically-activated switch supplies power to the calibrator, the combiner, the converter, and the transmitter in response to the power switching signal. An auxiliary battery supplies power continuously to the switch.