Automated warehouses use programmable logic controllers, or PLCs, to coordinate conveyors, sensors, motors, gates, and safety devices. Ladder logic is a common PLC programming language because it looks like electrical relay circuits and is easy for technicians to read. Contacts and coils are the basic building blocks that decide when outputs such as conveyor motors turn on or off.
Understanding them helps students connect digital logic to real machines moving packages through a logistics system.
A ladder rung is evaluated from left to right, using input conditions such as photoelectric sensors, start buttons, stop buttons, and emergency stops. Contacts represent conditions that can be true or false, while coils represent controlled outputs or internal memory bits. In a warehouse conveyor example, a motor coil may energize only when a start command is active, the emergency stop is clear, and a package sensor shows that the downstream zone is available.
This logic prevents collisions, controls flow, and makes the system safer and more reliable.
Key Facts
- A normally open contact is true when its assigned input bit is 1.
- A normally closed contact is true when its assigned input bit is 0.
- A coil turns its assigned output or memory bit on when the rung logic before it is true.
- Series contacts act like AND logic: Output = A AND B.
- Parallel branches act like OR logic: Output = A OR B.
- A basic seal-in circuit can be written as Motor = Stop_OK AND (Start OR Motor).
Vocabulary
- PLC
- A programmable logic controller is an industrial computer that reads inputs, runs a control program, and switches outputs to control machines.
- Contact
- A contact is a ladder logic instruction that tests whether an input, output, or memory bit has a required true or false state.
- Coil
- A coil is a ladder logic output instruction that writes a true or false value to an output or internal memory bit.
- Rung
- A rung is one horizontal line of ladder logic that contains conditions on the left and an output action on the right.
- Seal-in Circuit
- A seal-in circuit uses an output contact in parallel with a start input so a machine can remain on after the start button is released.
Common Mistakes to Avoid
- Treating a normally closed contact as the same thing as a physical normally closed switch is wrong because the ladder instruction tests the bit state, not only the hardware style.
- Forgetting the PLC scan order is wrong because inputs are read, logic is solved, and outputs are updated in a repeated cycle, which affects timing and troubleshooting.
- Putting all safety logic in ordinary program rungs is wrong because emergency stop and safety functions often require certified safety relays or safety PLC hardware.
- Using parallel contacts when series logic is needed is wrong because parallel branches create OR behavior, while series contacts require all conditions to be true.
Practice Questions
- 1 A conveyor motor coil M1 is controlled by Start = 1, Stop_OK = 1, and Downstream_Clear = 0 using the logic M1 = Start AND Stop_OK AND Downstream_Clear. What is the value of M1?
- 2 A package diverter coil D1 uses D1 = Sensor_A AND (Barcode_OK OR Manual_Override). If Sensor_A = 1, Barcode_OK = 0, and Manual_Override = 1, calculate D1.
- 3 In a warehouse conveyor, explain why an emergency stop condition is often represented with a normally closed ladder contact in series with the motor coil logic.