Ladder Diagrams: Contacts, Coils, and Scan Logic

Read contacts, coils and parallel branches in a conceptual seal-in rung. Distinguish Boolean instructions from wiring and PLC execution.

Open the Ladder Diagram Generator to draft the visual, then use the topic-specific checks below to review it.

Original AI-generated ladder diagram teaching reference. Simplified for explanation; review labels and relationships before use.

Describe logic before drawing symbols

Write the intended Boolean behavior in words. In the example, Stop_OK must be true and either Start or the existing Run state must be true. Use the ladder diagram generator to illustrate that expression, then inspect the exact branch topology.

Distinguish contact instructions from wiring

A logical contact tests a value in the program. A physical normally closed stop circuit can produce a true Stop_OK bit when healthy. Keep electrical wiring, input polarity and instruction behavior separate so the drawing does not invert the intended stop condition.

Review scan and startup assumptions

A seal-in example is incomplete without knowing initialization, I/O updates and what happens after a power cycle. A diagram can explain the concept but cannot validate a PLC program. Use the target vendor tools and an appropriate test environment for executable logic.

Reference key

Mistakes to check

Treating a logical contact as a wire

The same visual convention may be read differently in programming and electrical schematics. Label the instruction and the meaning of its Boolean input.

Leaving the branch endpoints ambiguous

A parallel Run contact must bypass Start only, not the stop condition. Check both junctions carefully.

Using a teaching rung as a safety circuit

Standard control logic is not a safety-rated emergency-stop design. Hardware architecture and safety validation require the applicable process.

Trace the seal-in expression

With Stop_OK false, both parallel paths are blocked by the series condition and the output should be false in this simplified model. With Stop_OK true, Start can establish Run; the parallel Run test can then maintain it when Start becomes false. This describes Boolean behavior only. Actual task execution, retentive state, output refresh and restart policy depend on the PLC program and platform.

Common questions

What does the ladder diagram generator create?

It drafts a conceptual PLC ladder visual from described Boolean conditions, including contacts, branches and coils.

Will it generate a runnable PLC project?

No. It does not create or validate a vendor-specific program or controller configuration.

Does a normally open instruction mean a physical normally open switch?

Not necessarily. The instruction tests a program value; physical wiring and input polarity must be documented separately.

Can I use the example as an emergency-stop design?

No. The seal-in rung is a teaching example and does not provide a safety-rated architecture.

Sources and editorial notes

• PLCopen: IEC 61131-3
• Wikipedia: Ladder logic

Prepared by the AcaDraw product team on November 14, 2026. The reference illustration is original AI-generated artwork. It is a simplified explanatory model, not an experimental result, production specification or independently reviewed analysis.