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Relay Logic vs. Ladder Logic Symbology


Relay Logic vs. Ladder Logic Symbology

Tech Tip – PLCs

Taking another lesson from our students in our PLC-Allen Bradley and Siemens classes, today’s Tech Tip addresses the confusion many technicians face when it comes to Symbology and PLCs.  Technicians, coming from over 100 years of relay logic, are taught that when a contact is normally open, the resulting output is “off” and thus when a contact closes, the output is “on”.

The confusion arises between the differences between Relay Logic and Ladder Logic.  PLCs can do things differently than simple relays, and thus reading their Ladder Logic with the same interpretation of Relay Logic may not produce the same results.  A fact, most think the symbols in a PLC are backwards!

Try answering this question as an example:

The best description of how this instruction functions [ \ ] in a PLC program is:

A.) Normally open contact

B.) Normally closed contact

C.) A logically true condition when an input is in its low state (open)

D.) A logically true condition when an input is in its high state (closed)

Did you get the correct answer?   If you did, it may be because you are more used to PLCs than relay logic.  The answer is (C) – A logically true condition when an input is in its low state (open). However, for relay logic this would be a normally closed contact.

However, in the Allen Bradley (the #1 selling PLC in the USA) they refer to the contacts as (XIO) and (XIC), whereas Siemens refers to the same thing as “normally open” \ “normally closed”. Siemens is the European competitor to Allen Bradley and the #1 selling PLC outside of North America.

It may have been easier if Siemens would change its terminology to Allen Bradley’s and stop using the phrase “normally open” \ “normally closed”.  This phrase, this terminology, does not match the below in PLC Ladder Logic and will get you in trouble.

How to stay out of trouble?  Use the following words and thought process:

Chart 1
Clear as mud?  Here’s a more practical example to help illustrate this concept where you have an E-STOP included in the circuit: Where (XIC) means (examine if CLOSED) meaning IF the E-STOP button is CLOSED it is in a RUN condition.

What will be the condition of the output device wired to terminal Q27.5 after the next scan?

A.) The devices is “ENABLED”

B.) The devices is “DISABLED”

Logix Language

The correct answer is (B) – The devices is “DISABLED”. This is because terminal I13.1 is true if open. However the ladder logic -]/[- if read like relay logic, appears to be a normally closed contact, however, in ladder logic it states True if OPEN, however the physical limit switch is shown CLOSED.  Don’t do that to yourself.

Chart 234567

Said another way…

A physical relay contact [   ] labeled “1CR” says “IF” 1CR is ON then the contact is closed. Hence “true if energized (on)”

A physical relay contact [ / ] labeled “1CR” says “IF” 1CR is OFF the contact is closed. Hence “true if de-energized (off). In a ladder [   ] wants to see the item closed, energized, on, and [ \ ] want to see the item open or off.

You see? It is not backwards, just the terminology is different. Another way to think about it is:

Chart 1234555

Let us know if you have any questions about this or other PLC topics and we’ll do our best to answer them. To discuss how you can implement PLC Hands-On Training into your company, call (855) 712-7353 or CONTACT NTT TRAINING today!

You can also of course demystify PLCs and HMIs in our popular courses where the training is over 75% hands-on:


Post Author: J Gordon Fleming – NTT Training Automation Instructor & Author of “The World According to Flem”


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