What type of logic is critical for maintaining the state of outputs in a PLC?

Seal-in logic is essential for maintaining the state of outputs in a programmable logic controller (PLC) system because it enables the outputs to remain activated even after the initial input signal that triggered them has ceased. This is particularly important in applications where a device must stay ON or maintain its status until a specific condition is met to turn it OFF, such as in motor control circuits or lighting control.

In a seal-in logic configuration, the output is connected back to the logic that controls it, effectively “sealing” the output in its ON state. When the input signal occurs, the output energizes; if the input signal subsequently drops, the output remains energized due to the feedback provided through the seal-in circuit. This maintains the continuity of the desired state, allowing for reliable operation in various automation scenarios.

This use of seal-in logic not only prevents rapid cycling of outputs, which could lead to wear and tear on physical components, but also provides a simple way to create a user interface that responds to user inputs or external events consistently. Overall, seal-in logic is fundamental in building stable and reliable control systems within PLC environments.