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PLC: Monitoring a Ladder Diagram

FAQ No. FAQ02892

Primary Contents

Question

When monitoring a ladder diagram, sometimes the output does not turn ON even though the input condition is ON, or the output turns ON even though the input condition is OFF. What is the reason for this?

Answer

A ladder program in which the same output bit is controlled by two different instructions can cause this type of problem. Even if the input condition is ON for one instruction, the bit may have been turned OFF by another instruction somewhere else in the program. Also, if the OUT NOT instruction is used, the status of the input condition and the output bit will be reversed causing the same thing (though in this case the operation is normal).

The way output bits are displayed in CX-Programmer FA Integrated Tool Package depends somewhat on the version, but versions higher than 2.1 display input conditions in the same was as the SYSMAC Support Software (SSS).

Output Status Monitor Display

Ver.1.0Ver.2.0Ver.2.0 or higher
ON
OFF
Same bit controlled by two different instructions.
This output bit is turned OFF, the other bits are turned ON.
Same bit controlled by two different instructions.
This output bit is turned ON, the other bits are turned OFF.

OUT NOT Status Monitor Display

Ver.1.0Ver.2.0Ver.2.0 or higher
ON
OFF
Same bit controlled by two different instructions.
This output bit is turned OFF, the other bits are turned ON.
Same bit controlled by two different instructions.
This output bit is turned ON, the other bits are turned OFF.