Rung question

ceilingwalker

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• Feb 27, 2011

• #1

Hello. I have a rung from a program attached. The program is protected so I'm not able to show more. I'm trying to understand what the programmer was doing with this instruction. What I can say is that this is a temperature control program and this specific rung is the set point for one of the tanks, that is set by the operator, via the HMI. If you may have an idea, I appreciate your time.

 

bernie_carlton

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• Feb 27, 2011

• #2

It's possible the control on the HMI was not able to limit the entry to 2000 or below or the programmer was being especially paranoid (which in safety terms isn't bad).

 

Tharon

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• Feb 27, 2011

• #3

I do this when later I'm doing math that will overflow if the number is greater than a certain value. Even if the HMI I am using has a limit to the entry, it stops someone from being online and changing the value to above the limit I want.

The rung limits the value in N152:0 to 2000 or lower. If it's over 2000, it puts 2000 into N152:0

 

ceilingwalker

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• Feb 27, 2011

• #4

Thank you, that does make a lot of sense now.

 

Mickey

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• Feb 27, 2011

• #5

IMHO its always a good idea to check values from HIM's or SCADA's for limits.
Even if the HMI or SCADA has it's own limit features.

 

shoelesscraig

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• Feb 27, 2011

• #6

Mickey said:

IMHO its always a good idea to check values from HIM's or SCADA's for limits.
Even if the HMI or SCADA has it's own limit features.

Click to expand...

Agreed

 

Kent Hostetler

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• Feb 27, 2011

• #7

Mickey said:

IMHO its always a good idea to check values from HIM's or SCADA's for limits.
Even if the HMI or SCADA has it's own limit features.

Click to expand...

This is true, and I see it as absolutely necessary for control setpoints.

 

Tharon

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• Feb 27, 2011

• #8

Not only for control setpoints. Even when an HMI is not involved, when doing ramping math in ladder, it lets you limit the result to within a range you want to work with, and/or so you don't overflow and fault the processor out.