Overview

In Siemens SIMATIC Manager (Step 7), implementing a "Hold ON" or latching logic in Ladder Logic (LAD) is a fundamental technique for maintaining an output state after a momentary input trigger. This is commonly achieved using a combination of contacts and coils arranged in a specific configuration.

🔄 Basic Hold ON Logic in Ladder Logic

To create a simple latching circuit, you can use a normally open (NO) contact for the input trigger and a normally closed (NC) contact for the reset condition. The output coil is placed in parallel with the input contact, forming a self-holding loop.

Example:

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|----[ I1 ]----( Q1 )----|
|                       |
|----[ Q1 ]----[ I2 ]----|

• I1 is the input trigger (e.g., a push button).

• Q1 is the output coil (e.g., a relay or motor).

• I2 is the reset input (e.g., another push button).

Operation:

1. When I1 is activated, Q1 is energized, turning on the output.

2. Once Q1 is energized, the parallel branch [ Q1 ] keeps the coil energized even if I1 is deactivated.

3. To turn off Q1, I2 must be activated, breaking the circuit and de-energizing the output.

This configuration ensures that the output remains on until the reset condition (I2) is met.