wire break determining

The suggestion to add 2 inputs for your digital sensor is a very good one.  If your digital sensor(s) have both NO and NC contacts, wire both to PLC inputs and do a parity comparison.  If both are in the same state, you have a broken sensor or broken wire, or bad PLC input.

Either way, your process is not acting properly, and you can pause or stop it as dictated by your process needs.

Another approach is to wire your digital sensor to a simple DC relay with 1NO/1NC contact.  Wire each contact to the PLC.  The cable from that relay mounted in the field near the sensor back to the PLC input should allow you to monitor if it is broken.

Alternatively, you can wire that PLC input to an analog input with a resistance bridge set up similar to what has been suggested.  You would then have 2 valid voltage values during normal operation, and 2 invalid values in event of short and open circuit situations.

This last option is overkill for most setups.

If you need wire-break detection for a digital input, you most likely need 2 sensors with 2 cables routed 2 different ways so if one cable is cut, the other cable is still ok and working.

It may be helpful if you better described why you need to know.

Another option may be to change the sensor type to analog, or to a safety rated sensor with appropriate safety interface.

Tekabi

Hello dear friends, 

I need to determine wire break of my digital sensors that connected to a DI module of a s7-1200. the DI module does not have this possibility.

May you please tell me that haw can i do that in s7-1200 automation system?

Best regards

Tekabi 

 

Here's a diagram of what I think you are asking: 

              CBL

24V--+---| |---/-------[I0.0]

     | sensor  |

     +---|/|---/-------[I0.1]

If CBL is disconnected:

  I0.0 = FALSE

  I0.1 = FALSE

  PARITY_ERROR = NOT (I0.0 XOR I0.1)

  [or better]
  PARITY_ERROR = (I0.0 XNOR I0.1)

To me, it sure seems like the only time PARITY_ERROR is true is when the cable is disconnected (or damaged), sensor failure, or PLC dig in failure. 

FT82

Hi 0_Einstein

I have just read your response about the wire break detection on digital input signal.

Regarding the part here below

The suggestion to add 2 inputs for your digital sensor is a very good one.  If your digital sensor(s) have both NO and NC contacts, wire both to PLC inputs and do a parity comparison.  If both are in the same state, you have a broken sensor or broken wire, or bad PLC input.

This solution can be used to detect a single wire break, if the NO or NC wire is broken...but it's not useful to detect if entire sensor wire is broken (if for example the cable is disconnected from sensor, this condition cannot be detected from the solution proposed).

Please correct me if I'm wrong.