Ladder Logic Fundamentals: Contacts, Coils, and Program Scan

What Is a PLC and Why Electricians Need to Understand It

A Programmable Logic Controller (PLC) is an industrial computer that controls machines and processes by reading input signals, executing a stored program, and switching output devices on or off. In modern construction and industrial facilities, PLCs replace banks of relays, timers, and counters with a single, reprogrammable unit. As an electrician, you will wire sensors and actuators to PLC terminals and troubleshoot control systems, so you must understand how the PLC thinks.

The PLC Scan Cycle

A PLC does not run its program continuously the way a personal computer multitasks. Instead, it repeats a fixed loop called the scan cycle thousands of times per second. Every scan has three main phases.

Phase 1 — Input Scan. The PLC reads every physical input terminal (push buttons, limit switches, sensors, float switches, etc.) and copies their ON or OFF status into a block of memory called the input image table. The program never reads the physical wires directly; it reads only this snapshot taken at the start of the scan.

Phase 2 — Program Scan. The PLC evaluates the ladder logic program from the top rung to the bottom rung, left to right on each rung. It uses the values stored in the input image table and any internal memory bits to decide which output coils should be energized. Results are written to the output image table in memory — not yet to the physical wires.

Phase 3 — Output Scan. The PLC copies the output image table to the actual output terminals, switching real-world devices (motor starters, solenoids, pilot lights) on or off. Then the scan begins again.

This three-phase cycle typically completes in less than 10 milliseconds for small programs. The key insight is that inputs are frozen during program execution, and outputs change only at the end of each scan. This predictable, sequential behavior is what makes PLCs reliable for safety-critical control.

Ladder Logic: Reading the Diagram

Ladder logic gets its name from its appearance: two vertical power rails (like the sides of a ladder) connected by horizontal rungs. Each rung represents one control expression. Current flows left to right through the rung; if the logical path is complete, the output at the right end is energized.

Contacts

A contact is the ladder logic symbol that represents an input condition being tested.

• Normally Open (NO) contact — drawn as two vertical lines with a gap: `| |`. When the associated input bit is a 1 (ON), the contact passes current (closes). When the bit is 0 (OFF), the contact blocks current. This mirrors a physical NO push button: pressing it completes the circuit.
• Normally Closed (NC) contact — drawn as two vertical lines with a diagonal slash: `|/|`. When the associated input bit is 0 (OFF), the contact passes current. When the bit is 1 (ON), the contact blocks current. This mirrors a physical NC contact: it passes current until operated.

Remember: the word "normally" refers to the un-operated, de-energized state of the device in the real world.

Output Coils

An output coil — drawn as a circle or parentheses `( )` at the right end of a rung — represents the result. When the rung path is logically complete (true), the coil bit is set to 1 and the corresponding physical output terminal is energized at the output scan. When the path is incomplete (false), the coil bit is 0 and the output is de-energized.

Seal-In (Latch) Circuit

A fundamental motor control concept in ladder logic is the seal-in circuit, also called a memory rung. A NO contact tied to the same address as the output coil is placed in parallel with the start push-button contact. Once the start button is pressed and the coil energizes, the seal-in contact closes and keeps the rung true even after the start button is released. A NC contact wired in series (representing the stop button) breaks the rung when pressed, de-energizing the coil and opening the seal-in contact so the circuit does not re-latch. This is the ladder logic equivalent of the classic three-wire control circuit you learned in relay logic.

Safety Considerations

PLC panels contain live voltage — often 120 V AC control circuits and 480 V AC power circuits in the same enclosure. Always apply Lockout/Tagout (LOTO) procedures per OSHA standards and your site energy-control program before opening any panel or touching terminals. Verify absence of voltage with a properly rated meter. Wear appropriate PPE: safety glasses and voltage-rated gloves rated for the circuit voltage you may contact. Even when a PLC program is in RUN mode, the outputs can switch unexpectedly if an input changes state, making attached machinery dangerous. Before editing or force-testing any rung on a live system, place the PLC in PROGRAM mode, communicate with all affected personnel, and follow your site's management-of-change procedure. Never bypass or defeat safety interlocks in ladder logic without written authorization and a formal risk assessment.

Putting It Together: Reading a Control Narrative

A control narrative is a plain-language description of how a circuit must behave. Translating it into ladder logic is a core skill. Work through it systematically: identify every input device and assign it a PLC address and contact type (NO or NC), identify every output and assign it a coil address, then draw the rung connections that satisfy the stated logic. For start/stop motor circuits, always include the seal-in contact and verify your stop logic uses a NC contact in series.