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S7-1200 Input Not Lighting up LED

Created by: jglmasongsong at: 11/15/2022 8:02 PM (4 Replies)

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11/15/2022 8:02 PM	Rate (0)
jglmasongsong Regular Member Joined: 5/28/2021 Last visit: 11/16/2022 Posts: 3 Rating: (0)	Hello all, I am reprogramming a PLC and added an analog input to our rack. I am not getting the input light to turn on from any of the digital inputs from either the 1214C DC/DC/RLY or the SM1221 DI16. I have measured the input and measured 24VDC but the lights will not turn on for either my PLC or my DI card. I have made sure that the input M- has been connected to a 0V as well as all of the cards basic power requirements. Bus bars are connected between all cards, as shown with the LED on all cards being green. PLC program has NC contact as the input, and once the relay outputs are turned on, a signal from the device being powered should return the 24VDC to the inputs. TLDR; Once outputs are turned on via HMI, devices being powered will send a 24VDC signal back to PLC, PLC and cards are no longer showing LED indicator that it is receiving an input, despite measuring 24VDC on input side. Any possible ideas? Picture of rack is shown. I will add a snippet of program for reference in a comment below. (Not sure how to add multiple attachments to the post) Thanks all Attachment PLC Rack.png (731 Downloads)
	Last edited by: jglmasongsong at: 11/15/2022 20:03:26 Last edited by: jglmasongsong at: 11/15/2022 20:05:15
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11/15/2022 8:04 PM	Rate (0)
jglmasongsong Regular Member Joined: 5/28/2021 Last visit: 11/16/2022 Posts: 3 Rating: (0)	Snippet of program is attached in this comment! Attachment Program snipper.png (672 Downloads)

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11/15/2022 9:01 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 9/12/2025 Posts: 16094 Rating: (2515)	I am guessing a wiring problem. I see you chose to use the L+ and M FROM the PLC's tiny input 24V source (using that for anything other than switches, relay contacts, and dry contacts is likely to rail that 24V source, shutting down any possibility of sensing any inputs. Might not be the problem. For a test, stop using the 24V coming out of the PLC and run a single 24V wire directly into one PLC Dig In. If the LED lights up, you know you have a wiring problem.
	science guy
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11/15/2022 9:08 PM	Rate (0)
jglmasongsong Regular Member Joined: 5/28/2021 Last visit: 11/16/2022 Posts: 3 Rating: (0)	Thanks Einstein, it is currently powering a fan of .24A, I will remove it and check it out.

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11/15/2022 9:27 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 9/12/2025 Posts: 16094 Rating: (2515)	I believe the 24V is only good for 0.3A, so using 0.24A for a fan is all it can realistically do, plus it's an inductive load, which I think the supply is only really meant to supply switch and relay contact wiring into PLC inputs. Check the spec's from the s7-1200 system manual for your specific model PLC. jglmasongsong Thanks Einstein, it is currently powering a fan of .24A, I will remove it and check it out.
	science guy
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This contribution was helpful to 1 thankful Users jglmasongsong

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