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CHAPTER6 FUNCTIONS
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6.3 Latch Function
6 - 9
(6) Device data latch method and influence on the scan time
Data latch processing is performed during END processing.
For this reason, the scan time increases.
Consider an influence on the scan time when latching devices. (
Section 10.1.2)
To shorten the scan time due to latch, minimize the number of latch points (latch (1) setting, latch (2) setting,
and latch relay (L)) as much as possible.
The number of latch points can be reduced by performing the following.
• Move data to be latched to the file register.
• Store device data that is less frequently updated in the standard ROM with the SP.DEVST instruction.
(The device data stored in the standard ROM can be read with the S(P).DEVLD instruction.)
QCPU Programming Manual (Common Instructions)
(7) Device data latch clear
Tabl e6. 2 shows the status of device data when the latch clear operation is performed.
*1: For the clearing method, refer to Section 3.7.
Data in the file register (R or ZR) will not be cleared by the latch clear operation.
To clear data in the file register (R or ZR), perform data clear operation by a sequence program or GX Developer.
( Section 9.7.3)
(8) Precautions
(a) When a local device or initial device value is specified
Device data cannot be latched even if the device has been latch-specified,
(b) Use of battery
Device data in the latch range are held with the battery installed to the CPU module.
• Even for the boot operation, the battery is required to latch device data.
• Note that if the battery connector is disconnected from the connector of the CPU module while
the power supply for the programmable controller is off, device data in the latch range will not
be held and will become undefined.
Table6.2 Status when the latch clear operation is performed
Latch setting Status of data
Device data without latch setting Cleared
Device data in the "Latch (1)" range Cleared
Device data in the "Latch (2)" range
Held
*1