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Old November 4th, 2017, 01:16 AM   #1
Brandon_K
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Why do "Switch in STOP" tags exist?

This has always eaten away at me ever since I started programming PLC's.

Simply, why do "Switch in STOP" tags exist? If the PLC is in stop, then it's not running any logic, as such it's silly to reference the tag. Of course, you could use it as a NC contact, but that contact will always be closed if the PLC is running, so why bother in the first place?

As someone who programs for retail-type applications, I would love to be able to make use of it so I can have a red indicator lamp turn on if someone is messing about and switches the PLC to stop.
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Old November 4th, 2017, 02:16 AM   #2
Saffa
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Tags can still be read by a SCADA system or other controller, so can be useful for alarming or HMI purposes.

If you want to drive a red lamp if the PLC is not executing code then wire the lamp to a relays normally closed contact. Drive the relay from your PLC with a "always on" bit. If the PLC fails or stops, the relay turns off and the lamp turns on.
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Old November 4th, 2017, 07:11 AM   #3
kwade
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As Saffa says... since the STOP stops your programming, not the PLC FW or comms, anything that can read from the PLC can read the Stop switch state - take action based on that state.
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Old November 5th, 2017, 03:40 PM   #4
ASF
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I saw an application once where the PanelView Plus was configured to display an alarm if the MicroLogix PLC was not in run mode.

The only thing was, they hadn't directly addressed the S bits themselves. They'd written ladder logic in the PLC something like:
Code:
|   PLC_RunMode            B3:0/0
|------|/|------------------(  )--|
...and then tied the HMI alarm to the B3:0/0 bit. I laughed so hard the project manager thought that the 13-hour days had finally caught up with me and sent me mad.

But, as others have suggested, if you DON'T do something as dumb as the above, and instead address the S bits directly, you can have a HMI display the status of the PLC even if it's not in run mode.

Remember, the PLC is still doing things when it's not in run mode. It still services communication requests, still updates status bits, the only thing it doesn't do is execute the user code.
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Old November 5th, 2017, 05:11 PM   #5
Brandon_K
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Thanks guys. I never gave thought to the usability in a HMI or SCADA.
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