Introduction to Seal Circuits
Seals and latches allow you to momentary press a button to energize an output. The output stays on until you press another button to shut the output off. There are some dangers associated with the latches that we will also discuss. If you are not familiar with basic ladder logic, please read this post before continuing.
When I think of seals, I think of a standard motor starter circuit on a schematic. There is one button to start the motor, and another button to stop the motor. You logic might vary depending on how the switches are wired. Here, just for clarification in logic, we will assume that both the start and stop buttons are normally open.
Let’s take a look at a basic seal in ladder logic.
First, let’s convert this logic into plain English: If we are NOT stopping the motor, AND (the operator presses the start button OR the motor is already running) then the output is energized.
Once the start button is pressed, the output will come on. Since the XIC below the start button is looking at the output, it will go true as well. Once the operator releases the start button, we still have logical continuity. When the operator presses the stop button, there is no longer logical continuity to the output, so motor.1 will shut off. Therefore, the XIC instruction for motor.1 will no longer be true. When the operator releases the stop button, button.0 will have logical continuity again, but there is no path for the output to energize. When the operator presses the start button again, motor.1 is energized, and will seal.
Let’s look at what happens when the operator presses the start button:
Now, let’s see what happens when the operator releases the start button:
You can see that even though the operator released the start button, there is still a path for the output to remain energized. Next, let’s see what happens when the operator presses the STOP button.
When the operator presses the stop button, the first instruction is no longer true. Therefore, the output shuts off, and the seal breaks. When the operator lets up on the stop button, there is still no path for the output to come on again. The output will remain off until the operator presses the start button again.
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— Ricky Bryce