Mitsubishi Electric MELSEC iQ-R Series Quick Start Manual page 63

4.
Double-click the 0th bit of Y0.
The 0th bit of Y0 is changed from '0' to '1', and the output (Y) is output forcibly.
3
An operation test by forced write to a buffer memory can be executed in the same manner.
3 FREQUENTLY-USED FUNCTIONS
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3.2 Monitoring Module Status and Testing Operations