Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 43)

Fundamental Instruction Set
Chapter 5
5Ć6
Latch
Symbol: -(L)-
Purpose: This instruction tells the controller to set a specified memory bit. It
is used with the unlatch instruction.
Syntax: Programmed at the output side of the rung. This is a retentive
instruction. Retentive means that once the rung condition goes false, the latch
bit remains set until reset by an unlatch instruction.
Function: Controls a specific bit based on the rung condition. When the rung
conditions are:
True
The latch instruction sets a specified bit.
False
No action is taken.
NOTE: If power is lost, and back-up battery power is maintained, all latch bits
will remain on. when all power is off, all outputs associated with the latch bits
will be off.
Unlatch
Symbol: -(U)-
Purpose: This instruction tells the controller to reset a specified bit in memory.
It is used with the latch instruction.
Syntax: Programmed at the output side of the rung; used with the latch
instruction. This is a retentive instruction.
Function: Controls a specific bit based on the rung condition. When the rung
conditions are:
True
The unlatch instruction resets the specified bit.
False
No action is taken.
NOTE: The conditions for the unlatch instruction must be different than the
conditions that precede the latch instruction.
Latch and unlatch instructions should be used in pairs with each other. They
should also be used to control a store bit which controls an actual output.
So far you’ve only looked at rungs having a series of instructions. You use
branching instructions when you want several parallel sets of conditions to
make an output action possible. A program with branching says, “If this set of
conditions is true, or if that set of conditions is true, perform the following
action.” Branching allows two or more paths to reach the same output
destination.
Branching Instructions