Fault detection method and apparatus

1. An input module for electrically isolating an input signal received via a field sensor to be transmitted to a plurality of processors, in which

the input module comprises a plurality of optical coupler/controller circuits; and

each optical coupler/controller comprises

a light emitting diode drive sub-circuit driving a light emitting diode; and

an input sub-circuit; and in which

a sensor voltage representing said input signal and a supply voltage are connected to each of said input sub-circuits; and

each of said light emitting diode drive sub-circuits are connected in series between the supply voltage and ground and each of said input sub-circuit are connected in parallel to drive each light emitting diode drive sub-circuit.

2. An input module according to claim 1 , in which a sensor current indicative of a sensor value generated by said field sensor is used to generate said supply voltage as well as said sensor voltage.

3. An input module according to claim 1 , in which said light emitting diode is coupled to a feedback photodiode and to an output photodiode.

4. An input module according to claim 3 , in which the feedback photodiode connects to a control amplifier to control a drive transistor which drives the light emitting diode.

5. An input module according to claim 1 , in which the light emitting diode drive sub-circuit further comprises one or more Zener diodes in parallel with said light emitting diode.

6. An input module according to claim 1 , in which the supply voltage is connected to the input sub-circuit via a series of resistors.