Signal line fault monitoring circuit - monitors voltages across resistors coupling auxiliary voltage source to each line

摘要

The signal line monitoring circuit uses an auxiliary voltage source of opposite polarity to the voltage supply and coupled to each of the signal lines via an identical protection resistance. These have a relatively small ohmic value in comparison to the resistance connected in parallel with the switching contacts incorporated in the signal lines. The voltage drop across each protection resistance is monitored to verify the state of the lines. The voltage across each protection resistance is fed to an assigned threshold switch followed by an electronic switch coupled via a resistor to one terminal of a further voltage source and via a third electronic switch to the other terminal of this voltage source. A comparator provides a fault signal in response to the voltages at the junction between each first electronic switch and the associated resistor.