Watchdog between 2pcs S7-1200 over Ethernet?

Hi,

What is the easiest way to create a Watchdog between 2pcs S7-1200 over Ethernet?

Put/Get example from siemens:


https://cache.industry.siemens.com/..._S7_communication_Sequencer_TIA_Portal_en.pdf

Downside of Put/Get is it is one sided, so unless you document well inside the program, the data just sort of magically appears, or is used for no good reason. We often don't recommend put/get for this reason, but, to be honest, it kinda just works (as long as the connection info is valid).



I-Device is pretty straightforward to set up, and would be faster depending on how fast the watchdog needs to be (based on Profinet, could communicate every couple milliseconds if you want). Upside over put/get in many circumstances is that it has a bit more diagnostics, but for a watchdog, i guess the data itself is all the diagnostics you really need.

As mk42 pointed out, I-Device is the easiest and most robust. For the watchdog what I like to do is send either the clock byte MB0 (if it is setup) or send the millisecond clock. This way you know it should change every scan. I prefer this over the heartbeat approach since it takes two scans minimum for the heartbeat.

Here's a good link from DMC about I-Device
https://www.dmcinfo.com/latest-thinking/blog/id/8907/siemens-plc-communication-with-i-device

Otherwise here is one from Siemens.
https://cache.industry.siemens.com/...00558/v1/109741600_config_SharedDevice_en.pdf

mk42 said:

Downside of Put/Get is it is one sided, so unless you document well inside the program, the data just sort of magically appears, or is used for no good reason. We often don't recommend put/get for this reason, but, to be honest, it kinda just works (as long as the connection info is valid).

Click to expand...

The other big downside (though not very likely to happen) is that they always work even if the partner PLC is in STOP.

Although others use the pulsing bit, I prefer the rolling counter incrementing the received value and checking that it was incremented on the other end.