350
4.2.4 Latch relay (L)
The latch relay (L) is a device for auxiliary relays that can be latched inside the CPU module.
Latch relay data are retained by batteries in the CPU module during power failure.
Operation results (on/off information) immediately before the following will be also retained.
• Powering off and then on the CPU module
• Resetting the CPU module
(1) Latch relay clear
The latch relay is turned off by the latch clear operation. ( Page 124, Section 3.3 (7))
However, the latch relay set in "Latch (2) Start/End" in the Device tab of the PLC parameter dialog box cannot be
turned off even by a latch clear operation.
(2) Allowable number of normally open or closed contacts
There are no restrictions on the number of normally open or closed contacts used in a program, as long as the
program capacity is not exceeded.
Scan time is prolonged when the latch relay (L) is used. Reducing the points of latch relay (L) can reduce the prolonging
scan time. ( Page 477, Appendix 3.2 (6))
(3) Method for external output
The output (Y) is used to output sequence program operation results to external devices.
● If latch is not required, use the internal relay (M). ( Page 349, Section 4.2.3)
● The latch clear invalid area is set in the Device setting of PLC parameter. ( Page 123, Section 3.3 (4))
X0
L0
T0
Y20
X1
L0
L100
X2 L0
L2047
SET L0
K20
Latch relay (L0) can be set to on only for
use inside the CPU module, and not for
output to the outside.
No restrictions on the quantity used.
M0 is set to on when L0 turns on from off.
The on/off information of L0 is output from the
output module to the external device.
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