RsLogix 500 - Latches OTL and OTU

In this lesson you will learn the other two basic output instructions for bits, the OTL Output Latch, and the OTU Output Unlatch. 

You will continue working with the program you ended up with in RsLogix 500 - Seal In Rung and PLC Scan Explanation lesson.

1.  Open up the files in RsLogix 500 and drag a new rung in between rung 1 and rung 2.

latch1.png

 

2.  Drag a second new rung in between rung 1 and rung 2.

latch2.png

 

3.  Below the icons you have been using for the instructions so far, you will see a variety of tabs.  Click on the bit tab.

latch3.png

 

4.  Now you can see the OTL and OTU instruction which look almost the same as the OTE instruction but have a "L" and a "U" inside of them.  Using those and the XIC instruction you already know, create the following instructions on the two rungs you have just added.  Download your program and go online.

latch4a.png

 

5.  With neither button pressed, both rungs are FALSE.  From earlier lessons you learned that just because a rung is FALSE, it doesn't mean that it does nothing.  The OTL and OTU instructions are the notable exceptions (though there are others) to this rule.  If an OTL or an OTU is false, it does not manipulate any of the data tables, it simply moves on to the next step.

latch4a.png

 

6.   When the Yellow button is pressed, the XIC sees a "1" and it is evaluated as TRUE.  A TRUE OTL instruction writes a "1" in the corresponding data box.  It is an OTE without the ability to write a "0" in a data box.  So in this case it writes a "1" in O:0/3's data box.

latch5a.png

 

7.  When you release the Yellow button and the OTL turns FALSE, O:0/3 stays ON.  It isn't because the OTL is holding it on, it is simply because there is nothing to write a "0" into the O:0/3 data box.

latch6b.png

 

8. When the Blue button is pressed, the XIC sees a "1" and it is evaluated as TRUE.  A TRUE OTU instruction writes a "0" in the corresponding data box.  So in this case it writes a "0" in O:0/3's data box.

latch7a.png

Conclusion

That is the basics of latching and unlatching outputs.

Continue to the Rslogix 500 - Online Edits lesson.

Information