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Hello rsdoran

Can u help me on this ? If you have a rung say in any PLC which contains these series of No and NC and the Op of the rung fails ...then how can you detect the exact contact (which no/nc ) causing the failure.. I need this to set up text messages for the operators to check that particular reason for the fault instead of checking the whole system, can u drop a hint ?

Anshuman

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I am placing this in the main forum because you will get better answers that way.

I all depends on the situation and components involved. Normally you develop code to provide an alarm when a function does not occur. More details may require more information.

Hello Anshuman,

It sounds like you are using this to report to a HMI, then alarm out on their screen

If you dont have a lot of these you could just make a seperate rung with each contact setting a bit...there are many ways that this could be done, you could also use 'bitwise' then the sum of the word would set the alarm

what type of PLC do you have? some have better ways then others

What you are asking about is called functional alarming.

Here is a simple example:

  PUMP_START    PUMP_STOP   TANK1     TANK 2

    BUTTON        BUTTON    LEVEL     LEVEL      PUMP 

-----] [----+------] [-------] [-------] [-------(  )-

            |

            |

            |

     PUMP   |

-----] [----+



      TANK 1                                   TANK 1

      LEVEL                               LOW LEVEL ALARM

-------]/[--------------------------------------(  )-

 

      TANK 2                                   TANK 2

      LEVEL                               LOW LEVEL ALARM

-------]/[--------------------------------------(  )-

The two low level alarm bits are used to display a message or other visible annunciator on the HMI so that the operator can see why the pump won't run. There isn't any magic to it, you just have to program the alarms with the necessary logic for them to function as you want. The PLC program "knows" exactly why its not turning on an output, you just have to give it the tools to communicate that reason to the operator.

Thankyou everyone for posting the replies.

I guess I shall post the entire application for you to understand exactly what I need, that would be way better .

-Anshuman

It depends on the state of the input for example low level might not mean it's an alarm, it could be if it stays low for a period of time then it's an alarm.
the other one is in step logic, again if a rung should be true but it is not, it doesn't mean it's in alarm it may be that it's not required.
You need to configure your alarms with logic for example for a machine to start it has to be in a particular position, however it will not always be in that position, only when required to start.
So you use a form of logic for the start condition & the start button, if the output is not true then set alarm bit
example
AN "down Position PX" if machine not in down position
AN "Running" & not running
A "Start Button" & start button pressed
= "Alarm bit 1 (Not in down position)" then equals alarm bit (or Set if alarm to hold)
By using combinational logic some with alarm timers you should be able to cater for most alarms or operator messages, to be honest sometimes over 50% of the logic in a program is not used to control but display information or alarms on an HMI or scada.