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Goals and Symptoms
The purpose of this resolution is to explain why a resistor is added across switching devices connected to a 140DSI35300 Supervised Input Module.
re: Modicon Quantum Automation Series Hardware reference Guide, p/n 840USE10000
Facts and Changes
If the field connection is good, when the switching device connected to a module input is open circuit, this shunt resistor completes the circuit and provides a small current sensed by the module. This current indicates that field wiring is not interrupted and a complete circuit exists.
Causes and Fixes
The resistor selection is sized so a minimum input current of 0.15 mA is maintained. If input current drops below that, the module will signal a broken field connection in the appropriate bits of I/O mapped Broken Wire registers #3 and #4.