There are many forum topics / posts related to how timers work in Siemens s7-1200/1500 PLCs.

The F1-help on the various timers has at least one NOTE about how they are implemented according to the IEC spec (rather than making their own timers).

In my experience with IEC Timers, I find that the Timer instruction needs a reason to solve the Q output with something that can hold the state of the Q output.

I tend to use a NO contact with "IEC_TIMER_NAME_DB".Q  in parallel with the timer (for LAD/FBD) so the logic leading up to the timer is also leading up to the Q contact.

There should only be 1 reference to the Q output in the entire PLC program - unless you have a very specific reason to do so - and know exactly what you are doing (meaning, I never have more than one Q output reference for any IEC timer).

Rodrigo F.

Second screenshot (this solution compiles and works fine).

The Q output needs to be "interrogated", meaning the PLC logic needs to run across something that says "please solve the Q output".

I take that quite literally and definitely go overboard in how I make sure my logic works regardless of platform (S7-1200 / S7-1500) and what version of firmware and TIA Portal I'm using.

                                  SOME_TON_DB

                               +-------------+

---[logic preceding TON]---+---|IN          Q|-

                           |   |             |

                           |  -|..         ..|-

                           |   +-------------+

                           |

                           |  SOME_TON_DB.Q

                           +-------| |--------[other logic here]

This approach has never failed to operate as I expect - provided the TON is in a cycle OB and the network rung is always scanned (not jumped or skipped over).

Rodrigo F.

I did search around before posting, but couldn't really get a straight answer from any of the other posts (or, to be perfectly honest, this one either ).

But then, if understand correctly, the error has to do with what I'm doing with the output of the timer (Q)? Meaning, the only thing it  should be doing is directly outputting to a variable?

Thanks!

Rodrigo F.

unknown user

Network 1,This use of IEC timers / IEC counters is not possible in this target system.

Help for this message says: 

unknown user

The "Q" output may only be interconnected with one coil. 

My understanding is the input of another FB is not a coil.