another EZ question about how MicroLogix scans

Skidood · Apr 3, 2017

Hi, I was told that in a MicroLogix program, all the rungs and included subroutines in Ladder 2 will be scanned from top to bottom, and only when the last rung has been scanned will any outputs actually change state if needed, and their state will be based on the last scanned instructions during the scan. Is this true?
I have a situation where (halfway through Ladder 2) a certain output is unlatched based on a couple inputs that may or may not be active, and then at the bottom of ladder 2, there is a rung that may command this output ON based on a different input. This instruction must take precedence for safety reasons.
Thx in advance.

gclshortt · Apr 3, 2017

The RS500 series is a synchronous scan. This means that I/O and Communications are updated only once during the program cycle. Therefore unless changed in the program, they will remain the same throughout the program scan.
The program is scanned left to right, top to bottom. The result of the previous rung is available for the next rung.
In your situation the last rung will always override any previous setting.

Regards,

GaryS · Apr 3, 2017

If you need to read or write the I/O faster than once a scan then you could use
The Immediate Input or the Immediate Output

OkiePC · Apr 3, 2017

Skidood said:
Hi, I was told that in a MicroLogix program, all the rungs and included subroutines in Ladder 2 will be scanned from top to bottom, and only when the last rung has been scanned will any outputs actually change state if needed, and their state will be based on the last scanned instructions during the scan. Is this true?
I have a situation where (halfway through Ladder 2) a certain output is unlatched based on a couple inputs that may or may not be active, and then at the bottom of ladder 2, there is a rung that may command this output ON based on a different input. This instruction must take precedence for safety reasons.
Thx in advance.

What type of instruction is this last one?

If it is an OTE, then it will always control the contents of its address. If the rung conditions are false, it will write a "0", if the rung conditions are true it will write a "1". Basically, that means it will render the previous "unlatch" instruction useless.

dmargineau · Apr 3, 2017

Skidood said:
Hi, I was told that in a MicroLogix program, all the rungs and included subroutines in Ladder 2 will be scanned from top to bottom, and only when the last rung has been scanned will any outputs actually change state if needed, and their state will be based on the last scanned instructions during the scan. Is this true?
I have a situation where (halfway through Ladder 2) a certain output is unlatched based on a couple inputs that may or may not be active, and then at the bottom of ladder 2, there is a rung that may command this output ON based on a different input. This instruction must take precedence for safety reasons.
Thx in advance.

The PLC Rule #1 is "Last one wins!"...That is the last rung addressing any OTE makes the decision that will be implemented.
Since any PLC always Scans Inputs -> Processes Logic Program -> Sets Outputs -> Scans Inputs -> Processes Logic Program -> and so on it doesn't matter what former logic decides if there is latter one which decides something else.
Scan the "safety form logic" rung last if it has more priority.

another EZ question about how MicroLogix scans

Skidood

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gclshortt

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GaryS

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OkiePC

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dmargineau

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