PLC Programming: What a Ladder Diagram Is and How a Scan Cycle Works

45 min read Training Guide

Ladder logic, scan cycles, and the inputs-outputs model every controls technician starts with.

Table of contents

What the work looks like

A PLC (programmable logic controller) is a rugged industrial computer that reads inputs (sensors, pushbuttons, switches), runs a program, and writes outputs (contactor coils, solenoids, pilot lights). The common brands in North America are Allen-Bradley (Rockwell) on CompactLogix and ControlLogix, Siemens on S7-1200 and S7-1500, and Mitsubishi and Omron on packaging lines. You program them using ladder logic (LD), function block diagram (FBD), or structured text (ST). Ladder is still the foundation and the right place to start.

A scan cycle: the PLC reads all inputs into an input image table, executes the logic top-to-bottom and left-to-right, writes the output image to the physical outputs, runs housekeeping (communications, diagnostics), and starts the next scan. A typical scan is 1 to 20 milliseconds depending on program size.

Job titles: PLC Programmer, Controls Technician, Automation Engineer, Controls Engineer. Entry rates $28 to $45 per hour. Senior integrators and controls engineers make $80,000 to $140,000 salaried.

Safety and tools

The dangerous part of PLC work is not the PLC (24VDC I/O is safe to touch); it is the field devices the PLC controls. A PLC output can start a conveyor, fire a solenoid, or close a contactor that energizes a 480V motor. Before you bypass a safety interlock or force an output, check what is downstream and confirm no one is inside the machine. Lockout/tagout applies.

Tools: a laptop with the vendor's programming software (Studio 5000, TIA Portal, GX Works3), the correct communication cable (Ethernet for modern PLCs, USB for CompactLogix), and a multimeter to verify I/O signals. Most shops keep a manual with wiring diagrams that map every input and output address to terminal numbers.

Your first exercise

Write (on paper) a ladder diagram for a start-stop circuit in PLC logic:

  • Input I:0/0 = start pushbutton (normally open).
  • Input I:0/1 = stop pushbutton (normally closed, wired to read as 1 when not pressed).
  • Input I:0/2 = overload contact (normally closed).
  • Output O:0/0 = motor contactor.

Rung 1: [ I:0/0 OR O:0/0 ] AND I:0/1 AND I:0/2 = O:0/0.

This is the exact software equivalent of the hardwired three-wire start-stop. The seal-in is O:0/0 (the output sealing around the start button input). If you understand why that rung works, you understand the foundation of ladder logic.

Where to go next

Build on PLC Programming with Motor Controls and Starters (Introduction to Motor Controls), Reading Electrical Schematics (Introduction to Schematic Reading), Industrial Electrical Systems, HMI Programming, and Industrial Networking (EtherNet/IP, Modbus TCP, Profinet). Safety: NFPA 70E Arc Flash Safety and Lockout/Tagout.