Function

Hi ! 

I have a  PLC S7-300  in  a instalation with 120 signals ( 120 digital inputs) . I have a problem with one signal that falls for few miliseconds. In that time ste instalation stops and i have a lot more signals that are falling . I need to know  exactly witch signal is falling first  (even for few miliseconds)

How can I do that ?

Thanks!

Best regrds !

Hello,

You can check diagnostic buffer where you will find the information of failuer/interrupt. Check you PLC logic OB1 if OB82 & OB83 are present. If not call those compile project save & download. In next time when you face problem, please check diagnostic buffer for more information.

pepe1

Hi ! 

I have a  PLC S7-300  in  a instalation with 120 signals ( 120 digital inputs) . I have a problem with one signal that falls for few miliseconds. In that time ste instalation stops and i have a lot more signals that are falling . I need to know  exactly witch signal is falling first  (even for few miliseconds)

How can I do that ?

Thanks!

Best regrds !

 Hi, 

If you want to find out the "sequence of event" you may find Address Trending topic useful. 

I hope I could help!

The problem is not with the plc. The problem is with a contact from the field, an I need a solution that I can see on HMI the first input that falls.  The solution with trend on PG field

Thanks!

Best regards.

Hi, 

You can set up a trend for sure, but the  cycle times in the PLC are a bottleneck. 

If you want to the "First Out", you could try to build "Latch" for each signal.

Also you can try to use the System time and Hardware interrupt to be more precise. 

Think about it!

Hello pepe,

for first if I good understand giving this idea, in case of falling signals you need for first know that true signal inputs, logically. That situation must be monitored from right known time - f.ex. Start of process, you must know where to start monitor signals. Then you prepare falling edge triger of signals, but conditioned probably also via some memory bit to not happen monitor again after first signal falls down. In your prepared memory you then will have your needed first registered signal. That's only my first idea, implementation is up to you my friend. Little hint if it can be helpful, you can work with IB0,... address system, or IW, as you need in case of test big memory areas. also attention can be paied to reality of right identification of fall signal due CPU scan.

pepe1

I have a  PLC S7-300  in  a instalation with 120 signals ( 120 digital inputs) . I have a problem with one signal that falls for few miliseconds.

Does your installation support hardware interrupts? If so, set it up for firing at falling edge and handle events in HW Interrupt OB. The adventage is that you can timestamp the events with resolution of 1 ms.

If not, you will have to program detection of falling edges by yourself. The resolution drops down to the scan time.

Once event is detected you can use WR_USMSG (sfc52) to store it in the diagnostic buffer. Or you can program buffer by yourself.