Checking for loss of MPI connection between S7-300 and WinCC


Thread Starter

D. Turner

I'm using a Siemens S7-300 PLC connected to a laptop with WinCC and a CP-5511 interface card on an MPI connection. I'm looking for a way to place the machine in a safe condition if the connection between the PLC and HMI is broken (the operator and the machine may be some distance apart).
Thank you.

I have used WinCC to set a bit in the S7-300 at 2 second intervals and I program the PLC to turn it off after it is on for 1 second. This heartbeat would then tell you that WinCC is down if it is not set. Contact WinCC tech support on how to do this.

Zan Von Flue

I looked through the siemens website and didn't find anything but a example for the 400 series.
A easy way would be to have the win cc invert a bit in the plc at some rate. the plc just watches it(watchdog). if the bit stops changing problem. for the hmi side i would either do the same (plc side) or use the clock flag byte that can be activated in the system manager. However it could be the msb is to quick.

There is no way the S7-300 PLC can itself detect an absence of communications. The only sure method is to have some code in the PLC set a bit, and then have WinCC read it and reset it, and the PLC read it and set it ... etc. If the bit in the PLC stays in one state or another for more than a given time period (say, a few seconds, depending on to the scale of the project and the number of nodes/tags etc) you could assume that the WinCC PC is no longer communicating to the CPU.
Thank you all very much. It sounds like a good way to do it. I will give it a shot. Thanks again.