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Old April 21st, 2017, 08:28 AM   #76
ndzied1
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Ron thanks for that. I must say I have never run into the issue you mentioned but see how it can be a problem. My Structured Text is all based on CoDeSys which does not have the [:=] operator.
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Old April 21st, 2017, 08:37 AM   #77
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Originally Posted by ndzied1 View Post
Ron thanks for that. I must say I have never run into the issue you mentioned but see how it can be a problem. My Structured Text is all based on CoDeSys which does not have the [:=] operator.
Agree, I've never bumped into something like that in the Siemens world, either. You don't need to worry about everything being retentive, you can choose what is or isn't. Chalk it up to needing understand the specifics of your system vs the generic language.

Great tip, Ron!
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Old April 21st, 2017, 11:01 AM   #78
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Most of these examples are completely academic and non-realistic.

If you intend to start and stop a motor with push buttons you have a lot more to think about.

For instance for a direct start motor you have a circuit breaker, a contactor, overload relay and likely an additional relay (if your PLC output has a transistor output).

Done right you have inputs to your PLC from the circuit breaker, from the contactor and from the overload relay. Because you need to know if the circuit breaker has tripped, if the motor is overloaded or if there is a problem with the contactor.

In a lot of cases you also have an isolator switch so the motor can turned off locally and locked in is off position for maintenance. You also have an input from the isolator switch to your PLC.

You might also have a sensor that senses the rotation of the motor or conveyor or whatever it is driving.

And then you probably have an emergency stop and if it's a machine you might have guards, light curtains and what not that would prevent the motor from running.

If it's some kind of process system you might have process dependent interlocks as well.

Safety issues for humans like emergency stops are controlled by safety circuits like safety relays or safety PLCs. There might also be process safety systems involved if you deal with chemical industry, oil and gas or similar.

Besides this you also have to figure out what state you want the system to be at power startup, after an emergency stop etc etc like mentioned in earlier posts.

In reality when you have a PLC you often control start and stop automatically as well so any push buttons might be just for manual control under certain operating conditions. Also physical push buttons might coexist with HMI and SCADA systems so start and stop commands might originate from multiple sources.

The best option in those case is to NOT use a start and a stop push button and instead use a start/stop button (toggle function) that's illuminated so it shows if the motor is running or not. It takes less real estate and feedback of motor status is important.

To discuss how one line of ladder or structured text looks like to start a motor is like looking at the problem through a keyhole where you see about 5% of the whole picture.

Last edited by Pete.S.; April 21st, 2017 at 11:11 AM.
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Old April 21st, 2017, 11:14 AM   #79
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well, to be honest, I was trying hard to come up with a short simple example that would clearly illustrate a specific facet of the PLC code's operation which few people know about - and which is seldom if ever discussed ...

I apologize - and I'm sorry to have wasted your time ... I'll try to contact a moderator and have my ridiculous post removed - before anyone else is forced to waste their time in reading it ...

best regards ...
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Old April 21st, 2017, 11:17 AM   #80
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Originally Posted by Ron Beaufort View Post
well, I was trying to come up with a short simple example that would clearly illustrate a facet of operation which few people know about - and which is seldom if ever discussed ...

I apologize - and I'm sorry to have wasted your time ... I'll try to contact a moderator and have my ridiculous post removed - before anyone else has to waste their time in reading it ...

best regards ...
Dripping with irony but I wasn't really talking about your post. You emphasized an important issue - status after PLC in program - which we could extend to status after power on, e-stop etc.

I'm just saying we need to look at the whole picture and every issue. Start and stop of a motor in real life isn't solved by one line of ladder logic or structured text.

Last edited by Pete.S.; April 21st, 2017 at 11:19 AM.
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Old April 21st, 2017, 11:52 AM   #81
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Originally Posted by Ron Beaufort View Post
I apologize - and I'm sorry to have wasted your time ... I'll try to contact a moderator and have my ridiculous post removed - before anyone else is forced to waste their time in reading it ....
Boy I have wasted a lot of time on this site but it was never reading one of your post... thank you for taking the time and explaining the way you do PLEASE keep up the great help Ron
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