Just recently I was overseas in a plant with an S7-300 PLC. One of those "wild frontier" countries that make you second guess everything you know about everything, because wierd s***
just keeps happening.
It wasn't even related to what I was supposed to be doing, but at some point I noticed that a pump was running when it really shouldn't have been. I started digging back through the drawings until I found that yes, the PLC output was on, which turned on this relay, which turned on this other relay, which turned on this contactor, which started the motor. SO I went back to the control room and looked for what drove that output.
In the code, the output was off.
I walked back out to the control panel, and the LED on the output card was most definitely on.
I walked back to the control room, and set up a flashing bit on the next (spare) output, to reassure myself that I was
definitely looking at the right card. I was.
I walked back to the control room and cross referenced that bit to see if it was used somewhere else. It wasn't. I cross referenced the whole byte, the whole word, and the whole double word. Not used anywhere except this one rung. I opened that hardware configuration, and did a live status monitor of that card at the hardware level. The output was off. I walked back out to the control panel. The output was most definitely on.
I dragged out a laptop and set myself up right in front of the PLC, and did a status monitor of the hardware. Step 7 assured me that the output was most definitely off. The LED on the card (and the pump running away merrily across the room) assured me that Step 7 was full of s***, and that the output was most definitely on. I forced the output on. The LED stayed on, and the pump kept running. I removed the force, and the LED on the output card turned off, and the pump stopped. And then, about two seconds later, the LED slowly started fading back into life, and a couple of seconds after that, I heard the relay in the next panel click in and the pump start back up.
Later on, I had to do a download to this PLC, and that output stayed on the whole time. Of course, by this stage, I'd relocated the wiring to a spare output and all was good again.
This was an S7-300 relay output card, and the fact that it slowly faded on leads me to believe that the LED is effectively in parallel with the coil of the internal relay, and that the transistor driving that internal relay has failed.
Once again, that doesn't necessarily prove anything when it comes to an S7-200, but I feel better for having told that story