FX PLCs do not have a simple switch for clearing/resetting latched memory. While this may not be a problem, it does need to be taken into consideration when creating PLC programs because, unlike modular PLCs such as the Q-Series, FX PLCs have default settings for retentive/latched device words, bit memory, timers, etc.

Design Constraints

All FX PLCs have retentive or latched word and bit devices as stated above. In a FX3U PLC for example, there are 2 types of retentive or latched bit (M) devices as can be seen from the extract below, in the FX PLC Programming manual.

There are also 2 ranges of latched data registers as can be seen from the extract below:

When a FX PLC is stopped and started, the latched data is not cleared/reset and there is no simple hardware switch to perform this clear/reset function either.

Should it be necessary to clear/reset latched data, the special data register M8032 must be set ON and then OFF again. This can be achieved by using the tools within PLC programming software (GX Developer and/or GX IEC Developer) or from a user screen on an attached HMI if present.

GX Developer example

GX IEC Developer example:

Further Information

For further information, consult the PLC programming manual.

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