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(3) Effect on the scan time
Data latch processing is performed during END processing therefore the scan time increases. Consider the effect
on the scan time when latching devices. ( Page 304, Appendix 2.1)
To minimize the extended scan time, reduce the number of latch points (latch (1) setting, latch (2) setting, and latch relay (L))
as many as possible. The number of latch points can be reduced by the following.
• Move data to be latched to the file register.
• Store the device data to the standard ROM using the SP.DEVST instruction if the data are less frequently updated.
(The device data stored in the standard ROM can be read with the S(P).DEVLD instruction.)
MELSEC-Q/L Programming Manual (Common Instruction)
(4) Latch data clear
Perform a remote latch clear operation. ( Page 111, Section 3.13.4)
[ONLINE] [Remote Operation] [Latch Clear]
Note that any of the following operation is required to clear the latch data and file registers that are set as the latch
clear disabled data (Latch (2) Start/End).
• Clear from the display unit. ( Page 196, Section 4.2.2)
• Reset with the RST instruction.
• Transfer K0 with the MOV or FMOV instruction. ( MELSEC-Q/L Programming Manual (Common
Instruction))
• Select "Clear Device's whole Memory" and/or "Clear all File Registers".
[ONLINE] [PLC Memory Operation] [Clear PLC Memory...] [Clear Device's whole Memory/
Clear all File Registers]
(5) Precautions
• The latch setting does not take effect if the device is specified as a local device or the initial device value is
specified for the device.
• A battery is required to retain the device data that are set in the latch range. If the battery connecter is
removed during power-off, the device data within the latch range are discarded and undefined values may
replace. (A battery is required also in the boot operation to latch devices.)
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