PLCs and control logic

Why this matters

A PLC (programmable logic controller) is an industrial computer built to withstand heat, dust and electrical noise and to run continuously for years. Almost every factory machine is controlled by a PLC, and so is the drone battery-swap station. Technicians and engineers who can read and write PLC logic are therefore in demand in both industry and drone work.

The PLC scan cycle

A PLC does not respond the instant an input changes; it works in repeating cycles. According to the Siemens S7-1500 manual, each cycle has four steps:

1. Read all inputs into the process image
2. Run the program from top to bottom using that image
3. Write the results to the outputs
4. Do system housekeeping and start the next cycle

As a result, an input that changes mid-cycle is seen in the next cycle, so the worst-case response time is about two cycles, and a signal shorter than one cycle may not be seen at all.

Ladder logic

Ladder logic writes programs as rungs of a ladder, like a relay wiring diagram. Per Rockwell’s description:

• A normally open (NO) contact is true when its bit is 1
• A normally closed (NC) contact is true when its bit is 0
• A coil is an output
• Series contacts give AND; parallel contacts give OR

In the seal-in start–stop circuit of Figure 2, pressing START turns the motor on, and the parallel M contact keeps it on after the button is released, until STOP or E-STOP, both wired normally closed, is pressed. The reason: if a stop button’s wire breaks, the circuit stops by itself, which is safer than a normally open wiring where a broken wire means the stop button no longer works.

An on-delay timer (TON) makes its output true when its input has been true continuously for the preset time, used to filter bouncing signals or wait for an object to settle.

DT = 0.1


def inputs_at(t):
    start = 0.5 <= t < 0.7 or 6.0 <= t < 6.2           # short button presses
    stop_ok = not (8.0 <= t < 8.2)                        # NC: True when not pressed
    estop_ok = not (4.0 <= t < 4.5)
    battery = 2.0 <= t < 5.5 or 6.5 <= t < 7.0
    return start, stop_ok, estop_ok, battery


state = {"motor": False, "ton": 0.0, "grip": False}
previous = None
for k in range(100):
    t = round(k * DT, 1)
    start, stop_ok, estop_ok, battery = inputs_at(t)                              # 1 read inputs
    motor = (start or state["motor"]) and stop_ok and estop_ok                     # 2 rung 1
    state["ton"] = state["ton"] + DT if (battery and motor) else 0.0               #   rung 2 (TON)
    grip = state["ton"] >= 1.0 - 1e-9
    state["motor"], state["grip"] = motor, grip                                    # 3 write outputs
    if (motor, grip) != previous:
        print(f"t = {t:4.1f} s  motor {'ON ' if motor else 'OFF'}  gripper {'ON' if grip else 'OFF'}")
        previous = (motor, grip)

t =  0.0 s  motor OFF  gripper OFF
t =  0.5 s  motor ON   gripper OFF
t =  2.9 s  motor ON   gripper ON
t =  4.0 s  motor OFF  gripper OFF
t =  6.0 s  motor ON   gripper OFF
t =  8.0 s  motor OFF  gripper OFF

IEC 61131-3 languages and safety

IEC 61131-3 Edition 4 (2025) defines the PLC programming languages Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), which resembles Pascal, and Sequential Function Chart (SFC) for step-by-step sequences. The former Instruction List language has been removed in the latest edition. The lab uses CODESYS Development System, which is free and includes a simulated PLC on Windows.

Module lab

Common mistakes

Summary

• A PLC works in cycles: read inputs, run the program, write outputs, repeat
• Ladder logic uses NO/NC contacts and coils; series is AND, parallel is OR
• A seal-in circuit latches the motor; a TON timer waits for an input to stay true
• IEC 61131-3 has LD, FBD, ST and SFC, and emergency stops must follow safety standards

Check your understanding

1. A PLC has a 10 ms scan cycle. What is the approximate worst-case response time?
2. Contacts A and B are in series. When is the coil energised?
3. Why is the STOP button wired normally closed?
4. A TON timer is set to 2 s. Its input is true for 1.5 s then drops. What is the output?
5. What is a Category 0 stop under IEC 60204-1?