Slave offline dropped PLC out of run mode

[davidbgtx]

If a slave is powered off and dst51 has a "3" for the value, should the CPU drop out of run mode (which is desireable)?
and if so can I clear dst51 and/or put cpu back into run mode from the program logic (say from my HMI)so the operator can reset without opening the panel?
I turned power back on to slave and cycled power to CPU, but cpu did not go back into run mode (switch set to term)

[Evilbeard]

If a slave is powered off and dst51 has a "3" for the value, should the CPU drop out of run mode (which is desireable)?
and if so can I clear dst51 and/or put cpu back into run mode from the program logic (say from my HMI)so the operator can reset without opening the panel?
I turned power back on to slave and cycled power to CPU, but cpu did not go back into run mode (switch set to term)

Not with the program.

[franji1]

The default behavior is for missing I/O to cause a FATAL error (typically desired, as you stated).  However, if you want to switch it back to RUN, you have to use Designer w/Mode switch in Term position, or possibly powercycle the PLC with the Mode switch being in RUN position, but this has its own set of concerns (e.g. cannot change mode via Designer software until you move the switch back to TERM).

Another possibility is to change the missing I/O condition to be NON-FATAL error.  This can be done as part of your System Configuration.  HOWEVER, YOU ARE RESPONSIBLE FOR WRITING LOGIC THAT DETECTS THIS ERROR CONDITION AND HAVE YOUR PROGRAM BEHAVE ACCORDINGLY.

The PLC will stay in RUN mode and continue to run WITH FAILED I/O.  I do not know if this is acceptable for your application.  Even if it is, you have to realize that there can be different causes for failed I/O, but the PLC cannot distinguish between a bad comm cable, failed Ethernet switch, or bad hardware, or ...

Regardless, failed I/O is a critical situation and must be handled properly and safely.

Remember that minimizing or eliminating the failed I/O event would be the safest/best solution.

[davidbgtx]

^Thanks to all.

[BobO]

You can put the switch in RUN to get the behavior you are wanting, but TERM remembers the last state. No way to affect RUN/STOP from an HMI...not sure how I feel about that.

[Mike Nash]

You might be interested in this thread on AD regarding a similar situation.

https://forum.automationdirect.com/forum/general-applications/104135-domore-occasionally-starts-in-program-mode

[Evilbeard]

You can put the switch in RUN to get the behavior you are wanting, but TERM remembers the last state. No way to affect RUN/STOP from an HMI...not sure how I feel about that.

We're far too conservative as Engineers. Let's put the top down and put the petal to the metal sometimes!

[Controls Guy]

Anything safety-critical should be hardwired anyway, right?

[BobO]

Anything safety-critical should be hardwired anyway, right?

Please.

[davidbgtx]

The slave is only a washer to clean wire on the fabricating line (not safety critical), sooo I set slave to not drop cpu out of run and cpu is allowed to boot into run without slave online. BUT I do program line to stop and alarm if slave goes offline. It all started because maintenance didn't want to walk to other end of the line to shut down power to the panel with cpu, to clean the washer, only killing main power on the washer. The things I do to save someone else a few steps once a week. Thanks everybody

[Evilbeard]

The slave is only a washer to clean wire on the fabricating line (not safety critical), sooo I set slave to not drop cpu out of run and cpu is allowed to boot into run without slave online. BUT I do program line to stop and alarm if slave goes offline. It all started because maintenance didn't want to walk to other end of the line to shut down power to the panel with cpu, to clean the washer, only killing main power on the washer. The things I do to save someone else a few steps once a week. Thanks everybody

Savages.

[BobO]

The slave is only a washer to clean wire on the fabricating line (not safety critical), sooo I set slave to not drop cpu out of run and cpu is allowed to boot into run without slave online. BUT I do program line to stop and alarm if slave goes offline. It all started because maintenance didn't want to walk to other end of the line to shut down power to the panel with cpu, to clean the washer, only killing main power on the washer. The things I do to save someone else a few steps once a week. Thanks everybody

Savages.

Ambitiously lazy. The hallmark of a good engineer.

[deep6ixed]

As the maintenance supervisor in our plant, we actually incorporate laziness into our safety designs now.  We had a compliance issue with lockouts where operators wouldn't lock a part of a machine out because the lockout point for a section of the machine was 150ft away from the area where they had to work and it took longer to walk down to lock out then it took to do the job. 

Several meetings later, after the only real discussions were about how to write people up for this, I simply asked if I could add another lockout to where the operator station was (and where the power for the other section of the machine came from).

$100 switch and 100% compliance now...  Now all the safety guys ask questions about things like this with every safety upgrade.