Importance of latch and unlatch logic in PLC ladders

The output latch (OTL) instruction is a retentive output instruction that is used to maintain, or latch, an output. If this output is turned on, it will stay on even if the status of the output logic that caused the output to energize becomes false. The OTL instruction remains in a latched on condition until an unlatch instruction (OTU) with the same referenced tag is energized. The output latches are used in programs where the value of the output is maintained constant even if the plant shut down due to power failure.

  • The OTL instruction will write a 1 to its address when true.

  • When the OTL goes false, the output address will remain a 1.

  • This is true even if the processor powers down and then back up.

  • The output address will remain a 1 until reset to 0 by the unlatch instruction.

  • If the output address is off, both the latch and unlatch instructions are not intensified, but once the bit is turned on, you will see both the latch and unlatch intensified even though both inputs are shut off.