10.
10 - 5
OTHER FUNCTIONS
10.3 Latch Function
When the PLC power is turned ON, the CPU module is reset using the RUN/STOP key
switch, or a instantaneous power failure lasting longer than the allowable momentary
power interruption time occurs, the all device values in the QnACPU are cleared, and the
default values are set in the devices (Bit devices: OFF, word devices: 0).
The latch function retains the data in the devices when performing these operations.
The operations in the program are the same whether or not the latch function is used.
(1) Application of the latch function
The latch function can be used when continuing the control to retain data such as
production quantities, numbers of defects, addresses, etc., even if a instantaneous
power failure longer than the allowable time occurs.
(2) Devices that can be latched
(a) The following devices can be latched.
1) Latch relay
2) Link relay
3) Annunciator
4) Edge relay
5) Timer
6) Retentive timer
7) Counter
8) Data register
9) Link register
(b) The latch range is set on the "Device" in the parameter mode of GPP function.
In latch range setting, it is possible to set a range within which the latch clear key
is effective (Latch (1) Start) and a range within which the key is not effective
(Latch (2) Start). For details on device latch ranges for each device, refer to the
QnACPU Programming Manual (Fundamentals).
POINT
Even if a latch designation is set for a device, the device will not be latched if a
local device designation or device initial value designation is made.
POINT
The devices data in the latch range are retained by the battery (A6BAT) installed
in the CPU module.
(1) Even if a sequence program is written to a memory card and ROM operation
is performed, the battery is required for the latch function.
(2) If the battery connector is disconnected from the CPU module connector while
the PLC power is OFF, the devices data in the latch range is lost.
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