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Can A Single Sensor Output Be Connected To Multiple PLCs Input Units In Parallel?


The conditions for setting up this type of connection are as follows:

  • The sum of the total input currents of the Input Units must be below the maximum switching capacity of the transistor in the sensor output.
  • The leakage current from the transistor in sensors output may prevent the inputs to the Input Units from turning OFF. You must be sure that the inputs will turn OFF when sensors output turns OFF.
    As a condition for this, the input impedance of the Input Unit and the leakage current can be used to calculate the voltage when the sensor output is OFF. You must be sure that this voltage is less than the OFF voltage of the Input Unit.  
  • Sensors signal must be long enough to be detected by the Input Units. The ON/OFF signal from Sensors must be longer than the PLCs' scan time.

The following formula must be satisfied by the above conditions.

Sum of all leakage currents < Maximum switching capacity of transistor in sensors output

The leakage currents of the pulse output for the transistor in the sensor pulse output when it is turned OFF will not cause the PLC's input circuit to turn OFF (i.e., satisfies the following condition).
Leakage current of output transistor when OFF x Input impedance of input circuit on PLCs < Voltage at which PLC's input turns OFF

The PLCs must be able to detect the sensor’s ON/OFF signal.
Shorter of ON or OFF signal for sensor/pulse output > Length of one PLC scan


 


 




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Created 2008-10-30
Modified 2009-01-22
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