AutoQuiz: What Ladder Logic Symbol Represents a Logically False Condition for a Discrete PLC Input?

AutoQuiz is edited by Joel Don, ISA’s social media community manager.

Today’s automation industry quiz question comes from ISA’s new course, PLC Fundamentals for the Technical Professional. This course focuses on the AutoQuiz-TC40-logically-true-conditionrole the programmable logic controller plays in the design of a control system and how proper selection, installation and maintenance can reduce operating costs and improve performance. It provides the technical expertise necessary to install, perform routine programming and maintenance and apply proper troubleshooting and configuration techniques.

Identify the instruction symbol shown in the post image.

a) normally closed
b) normally open
c) examine input closed
d) examine input open
e) none of the above

 

Click Here to Reveal the Answer

Here’s the answer to today’s quiz from the instructor for ISA’s course offering, PLC Fundamentals for the Technical Professional.

The symbol represents a logically false condition for a discrete input. When power is present at the addressed terminal the instruction becomes logically false. When power is not present at the address terminal the instruction becomes logically true. Examine Input (or If) Open is defined as follows:  Examines a bit for an off condition. Use an XIO instruction in ladder logic to determine if a bit is off. 1 = True.  0 = False.

The correct answer is D.

 

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