Ladder Logic and SCL Start-Stop Circuit Tester With Truth Table

Test a start/stop control circuit and see the Boolean expression and the truth table.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose the lamp circuit or the latching motor circuit, set the state of the E-stop, the stop button, and the start buttons (from the field and from SCADA), and see whether the output is on. The calculator also shows the Boolean expression and the full truth table of every combination.

In ladder logic contacts in series are AND, and contacts in parallel are OR. The E-stop and stop are wired as normally closed contacts so that a broken wire or a pressed button switches the output off. The two start buttons are in parallel, so either one starts it. The same logic is written in SCL as an IF statement.

The latching option adds the seal-in contact: the motor stays running after the start is released, until the stop or the E-stop opens. This is a tool for learning the logic. A real emergency stop circuit must be built to the safety standards with safety-rated components.

Formulas used

  • \text{LAMP} = E \cdot S \cdot (F + C)
  • M_{next} = E \cdot S \cdot (F + C + M)

Worked example

With the default numbers in the calculator above:

  1. Boolean expression: Lamp = E-stop AND Stop AND (Start field OR Start SCADA)
  2. The states you chose: E-stop closed, Stop closed, Start field pressed, Start SCADA not pressed
  3. Output: Lamp ON
  4. Truth table (E = E-stop closed, S = Stop closed, F = Start field, C = Start SCADA): 16 rows: the output is ON in 3 of them

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: Start pushbutton from the field Not pressed.

  1. Boolean expression: Lamp = E-stop AND Stop AND (Start field OR Start SCADA)
  2. The states you chose: E-stop closed, Stop closed, Start field not pressed, Start SCADA not pressed
  3. Output: Lamp OFF
  4. Truth table (E = E-stop closed, S = Stop closed, F = Start field, C = Start SCADA): 16 rows: the output is ON in 3 of them

Common questions

Why are E-stop and stop normally closed?

So a broken wire, a failed contact or a pressed button all open the circuit and switch the output off. That is a fail-safe design.

What does a seal-in contact do?

It is a contact from the motor output in parallel with the start button, which keeps the motor on after the start is released.

Is this enough for a safety circuit?

No. An emergency stop must meet the machine safety standards (AS 4024) and use safety-rated devices.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: IEC 61131-3 (programmable controllers: ladder diagram and structured text); AS 4024 (safety of machinery).

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