Difficult PLC problem for me. Start stop button

[BenTheLogic]

Hi, guys
I know you have to program an emergency stop button as normally open button and it's for safety. But a stop button is a normally close button. How can you program a normally close as normally open? Do you still need to close the stop button at the beginning if you've already programmed a stop button as normally close.


Can you program a start button as normally close? In that case, wouldn't it be that you don't even Need to press the start button to run a motor, it would start by itself.

Ok. It's been a little bit confusing now. What I want to know is that if you have a normally open button and can you program that as normally close. And if you can do that, do you need to press that button to actually close the circuit.

 

[Thealanator]

I think everyone who has done PLC programming has felt this confusion at one time or another.

You are actually programming a PLC input (if high then... if low then...). You must consider the input device separately, (voltage presented to input when pressed, voltage presented to input when not pressed, etc).

Now you have to consider what you want this combination to do.

This "confusion" forces you to slow down and think through the process, which is a good thing in my mind.

 

[xnuke]

You select a programming NO or NC contact based on whether or not voltage is present on the input, and what you need the instruction to be (i.e., True or False) to execute the program.

For example, when using physical relay ladder logic, your NC stop switch has a status of "True" until it is pressed. This allows the remainder of the ladder rung to determine the logical result. After pressing the stop switch, it becomes "False" and the rung loses power, i.e., it makes the whole rung become logically "False."

When this is connected to a PLC input instead of a relay ladder, the input has a status of "Voltage Present" until the switch is pressed. After pressing the switch, the input status becomes "Voltage Absent."

Inside the PLC ladder logic, you want the instruction to have the same "True" status when the stop switch is not pressed as it did in the relay ladder logic because the PLC logic should exactly mimic the relay logic.

Since a NO contact instruction in the PLC linked to the input address is "True" when a "Voltage Present" status is read by the program and you want a "True" status associated with the stop switch, you program the physical NC stop switch with a NO contact instruction. This allows the remainder of the program rung to determine the logical result.

When the switch is pressed and the status of the input changes to "Voltage Absent," the NO contact instruction linked to that input address becomes "False" and the whole PLC ladder rung becomes logically "False."

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Thealanator]

Another way to do it:

Use an e-stop switch with two sets of contacts.
Use the "NC/closed on mushroom up" contacts to interrupt control power to the starter.
Use the "NO/open on mushroom up" contacts to signal the PLC that an e-stop switch has been pushed.

 

[BenTheLogic]

So does it mean that I can program a N/C as normally open into PLC. And once I run the program, the n/o button will change to n/c instantly. And if I program a n/c as normally clOse, then it will change to normally open once I run the program. Is that right??

 

[xnuke]

That isn't quite right, but it's close. You can program any input switch type (NC or NO) as any instruction type (NC or NO). The NO instruction type doesn't "change to n/c instantly" when the program is run, but it is evaluated as "True" or "Closed" any time there is voltage on its associated input terminal, and it is evaluated as "False" or "Open" any time there is no voltage on its associated input terminal.

Since you're using a NC stop switch wired to the input, it will apply voltage to the input any time it isn't pressed, and therefore if you use a NO instruction assigned to the input address it will be evaluated as "True," if you use an NC instruction it will be evaluated as "False." When it is pressed, the evaluated status will change, i.e., an NO instruction will become "False" and an NC instruction will become "True" since there will no longer be voltage on the input.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[DaveMeharg]

As indicated by others you need to think about how the E-stop is wired to the input of the PLC. The PLC processor will scan or read all its inputs and then look at the ladder logic in the PLC. If the input has voltage on it then the PLC when it scans it places a logic 1 in its internal memory for this input. If it has not voltage on the input then its a logic 0. Then when the processor goes to solve the PLC logic it examns the ladder logic rung. If for example you wired the E-stop so its normally closed and opens when pressed. Then normally the PLC input is going to be a logic 1. If in your ladder run you use a normally open contact then when the PLC gets to this part of the logic its goint to look in its memory to see whether the input is a logic 1 or logic 0. It its a logic 1 it will consider this contact as closed. If however you programmed a normally closed contact then it will look at the memory and see that its a logic 1 and consider this contact as open. When programming ladder logic some people prefer to call normally closed contact in the ladder logic as an "examine off" contact because it will only be true when the logic input is "off" or 0. They will also call a normally open contact "examine on" because it will only be true when the input is "on" or a "1".

To be safe you should always hard wired the E-Stop to directly kill the power to the motor starter or device. As suggested by someone else you could also wire another set of contacts on the E-stop button to the PLC. If you want to be very conservative you could wire the PLC input from a normally closed contact on the E-stop. Then in the program you would use a "examine on" or normally open contact. When you viewed this "examine on" contact in the PLC you would see that it is normally in the true state as long as there is voltage on the input. It would drop out when the E-stop is pressed or if the wire fell off the E-stop.