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Old November 14th, 2017, 02:52 PM   #1
rayflores
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Siemens S7-200 Output Indicator LEDs

Does anyone know if the Output Indicator LEDs are controlled directly from the logic or is the LED somehow connected to the actual relay so that if the relay coil is bad, the LED would not turn ON?
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Old November 14th, 2017, 03:01 PM   #2
kalabdel
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[quote=rayflores;758962]Does anyone know if the Output Indicator LEDs are controlled directly from the logic or is the LED somehow connected to the actual relay so that if the relay coil is bad, the LED would not turn ON?[/quote

I will *assume* it's similar to all the PLCs I'm familiar with and state that the LED doesn't have a direct connection to the relay output and that you can have an LED on ad a faulty relay.
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Old November 15th, 2017, 05:04 AM   #3
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I can tell you that a client of mine had an S7-300 digital output module that had a saturated transistor output that stayed on all the time, and the LED was on independent of the logic status of the output. And the digital inputs always followed the electrical state of the input, not the logical status (for example, when forcing an input ON, the LED is OFF).

Sorry I can't answer about the S7-200, but someone might come across this question about the S7-300.
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Old November 15th, 2017, 06:30 AM   #4
geniusintraining
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Depends on the model, I know transistors outputs if wired wrong can be on if the logic is not true, I would think if wired wrong or shorted out then yes the LED could be on if the logic was false
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Old November 15th, 2017, 08:57 AM   #5
dwoodlock
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I believe the LED is somehow tied into the output current. I've seen in the past that if you were to apply voltage to it, that the LED will come on although no logic is making it go high.

Last edited by dwoodlock; November 15th, 2017 at 09:01 AM.
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Old November 15th, 2017, 05:55 PM   #6
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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
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Old November 16th, 2017, 12:39 AM   #7
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ASF, that's basically the long version of the same story I had.

The "stuck on" output controlled the application of glue in a packaging machine, and the glue just kept on coming out. They turned off a breaker to stop it. Like you, I changed the output, and everything was fine. But then a few months later, that new output did the same thing, so I moved the output again, but told them this time to replace the module. This was a 24 VDC output module, however.
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