Controlling Siemens F-DO from user program

[boneless]

Hi all,


I received a request from a customer, and I am doubting if this is the correct way of doing things.


We have a Siemens 1500F PLC, with full size S7 300 safety modules (ET200M).



The client would like to be able open/close a (fail-safe close) valve (connected to a F-DO) from the user program. Is that advisable?


I guess as long as we cannot open the valve when safety is not present, the safety function is warranted?


Thanks!

 

[JesperMP]

Just to rephrase it a little bit:

You want to add a control that turns the F-DO off.
For the F-DO to turn on, all safeties must be in place and reset.

It should be possible. Here is a quote from the user manual for Simatic Safety (link to chapter: https://support.industry.siemens.com/cs/mdm/54110126?c=107369942155&lc=en-DE)

Use the ---( S )---: Set output (STEP 7 Safety V15.1), ---( R )---: Reset output (STEP 7 Safety V15.1) or SR: Set/reset flip-flop (STEP 7 Safety V15.1) instructions, for example, with tags from the standard user program to allow a motor to be switched off, but not switched on.

 

[boneless]

Yes correct, but I also want to turn the F-DO as long as safety is okay.


I will check out the link, thank you!

 

[mk42]

The way that may make sense is to have the command tag in the standard logic, and then to read that tag in the safety. You can use standard bits in the safety code as long as you aren't using them to jumper your safety logic.

OK in the safety code:

   Safety OK     User Command          F-DO

-------||-----------||------------------()----

BAD in the safety code:

Safety OK                  F-DO
-------||--------------------()----
             |
User Command |
-------||----|

 

[boneless]

Thank you both.


I was just worried about using non-safety tags in safety system. The link from Jesper clarifies a lot, it is sometimes hard to find the correct documentation from Siemens .

 

[mk42]

Thank you both.

I was just worried about using non-safety tags in safety system. The link from Jesper clarifies a lot, it is sometimes hard to find the correct documentation from Siemens .

Something else to keep in mind:

If you use a standard tag in the safety program, if anything changes about that standard tag, the safety program will need to recompiled, and possibly re-downloaded. This could mean that simply adding a tag to a standard DB (with another tag read from the safety) could cause you to need to stop your PLC when you download, because the safety needed to go along for the ride.

The best practice outlined in the documentation (not sure which manual Jesper linked, it won't load for me) is to have a DB where you buffer all the tags between standard and safety. That way, only the tags from that DB are used in the safety code, and no other changes will affect it.