Seal-In Circuit Vs. Latch / Unlatch Sequence in PLC
Latch circuits and Seal-in circuits are types of electronics circuit that have two possible output states. The output is either true or false. They are mostly used as memory to store one bit of information.
In PLCs this can be done by Latch / Unlatch configuration or by a Seal-In sequence. They both have their advantages and disadvantages. The main difference between them arises when the PLC (or system) power is turned on and off:
After power OFF the latched output state stays the same as it was before the power was lost. The Seal-In circuit’s output resets to “FALSE”.
Latch/Unlatch circuits are less common in PLC programming and are generally avoided. You can see examples of the two circuits on the image on the right. The top configuration is seal-in and the bottom one is latch/unlatch.
To put a Latch symbol into Logix 5000, simply click on tab “Bit” and choose the icon wiht letter “L” inside it.
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