All of the PLC come along with Latch and Unlatch in built. These instructions have been given different names by various manufacturers. Siemens PLCs have (S) for Set and (R) for Reset, while Allen Bradley PLCs have -(L)- for Latching and -(U)- for Unlatching commands. Likewise, KEEP instruction is used for latching in Omron PLCs. So, let's define what is latch and what is unlatch.
The definition given by professionals, in PLC programming, a latch is a way to hold onto a state until an explicit command modifies it. In essence, a latch stays active even if the initial activating condition is removed once it has been set. The unlatch command, on the other hand, flips the latch's status and deactivates it until it is triggered once more.
There are two major types of using Latch and Unlatch. By using the manufacturer's given instructions or Sealing also known as without using instructions. In this scenario, I'm using Omron's Sysmac Studio.
As shown the figure, the first photo, I'm using Latch and Unlatch with instruction. When BInput_1 is triggered, BOutput_2 will be On. Even when i released BInput_1, BOutput_2 won't change the status. Nevertheless, BOutput_2 will be Off if BInput_2 is triggered. That is how Latch and Unlatch works by using Instruction.
In second photo, I've only used one Rung. BInput_1 and BOutput_2 have been used as OR gate. Along with them, BInput_2 has been used as AND gate. In Initial status, BInput_1 and BOutput_2 are normally opened contact and on the other hand, BInput_2 is normally closed contact. When BInput_1 is triggered, BOutput_2 will be On. However, when BInput_1 is released, BOutput_2 will maintain that it will keep going in On status. BOutput_2 will be Off if the BInput_2 becomes from normally closed contact to normally opened contact. That is also known as Latch and Unlatch an Output by Sealing.
Reference : Sanfoundry, Indmallautomation
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