What Is
Ladder Logic?
PLC Programming Explained (2026)

What Is Ladder Logic?

Ladder logic is a graphical programming language used to program Programmable Logic Controllers (PLCs). It was developed in the 1970s as a way for electricians and engineers to program PLCs using a visual language that resembled the electrical relay diagrams they already knew how to read.

The name comes from its appearance: the program looks like a ladder lying on its side. Two vertical lines (called "rails") represent the power supply - the left rail is the live line, the right rail is the neutral or ground. Between the rails are horizontal lines called "rungs," each representing a logical condition or action. Electricity (or more accurately, logical power flow) travels from the left rail through the rung conditions to the right rail, turning outputs on or off depending on whether the conditions on each rung are met.

History and Why It Looks Like a Ladder

Before PLCs existed, industrial control systems were built from physical electrical relays - electromechanical switches that opened and closed based on electrical signals. Electricians drew these relay systems as ladder diagrams, with power rails on each side and relay contacts and coils on the rungs. When Modicon introduced the first commercial PLC in 1969, they designed the programming language to look like these familiar ladder diagrams so that electricians could transition to PLC programming without learning a new mental model. The language stuck, and today ladder logic remains one of the most widely used PLC programming languages.

How Electricians Relate to It

An experienced electrician who reads relay schematics can typically learn ladder logic in a few days. The XIC (Examine If Closed) contact in ladder logic is equivalent to a normally open relay contact in a relay schematic. The XIO (Examine If Open) contact is equivalent to a normally closed relay contact. The OTE (Output Energize) coil is equivalent to a relay coil. The logic is the same - only the medium changed from physical hardware to software.

Basic Ladder Logic Elements

Every ladder logic program is built from a small set of fundamental elements. Master these and you can read and write the majority of oil and gas PLC programs.

Normally Open Contact (NO) - XIC

A normally open contact passes logical power when its associated bit (tag) is TRUE (1). In Allen-Bradley ladder logic, this is the XIC instruction. In Siemens TIA Portal, it is the Normally Open contact in LAD. Think of it as a switch: when the bit is 1, the switch is closed, and power flows through. When the bit is 0, the switch is open, and power stops.

Example in oil and gas: A high pressure switch (PSH) has its output wired to a PLC input. When pressure exceeds the setpoint, the PSH closes, the PLC input bit goes TRUE, and any XIC contact referencing that bit passes power. If that contact is in series with an emergency shutdown valve (ESV) coil rung, the ESV energizes and closes the valve.

Normally Closed Contact (NC) - XIO

A normally closed contact passes power when its associated bit is FALSE (0) - the opposite of XIC. It is the Examine If Open instruction in Allen-Bradley. Think of it as a switch that is normally closed and opens when the bit turns on.

Example: A motor overload relay has a fault bit that goes TRUE when the motor draws too much current. A XIO (normally closed) contact on this fault bit is placed in series with the motor start output rung. Normally (when there is no fault), the XIO passes power and the motor can run. When an overload occurs, the bit goes TRUE, the XIO opens, power stops flowing to the motor output, and the motor stops.

Output Coil - OTE

The output coil (OTE - Output Energize) is placed at the right end of a rung. When the rung has power flowing through it (all conditions on the rung are met), the coil bit is set TRUE. When power stops flowing, the coil bit goes FALSE. The coil bit may control a physical output (a relay that energizes a motor starter) or an internal memory bit used in other rungs.

Simple Examples with Diagrams

The simplest possible ladder logic rung: a single XIC contact driving an OTE coil.

This rung reads: "When the STOP_PB bit is TRUE, turn on PUMP_RUN." Not very useful in practice - the pump would only run while someone holds the stop pushbutton, which is backwards. Real motor control uses latching logic:

This reads: "Turn on PUMP_RUN if either the start button is pressed OR the pump is already running - but only if the stop button is not pressed." The PUMP_RUN XIC contact in parallel with the START_PB is called a "seal-in" or "latch" contact. It holds the pump running after the start button is released, until the stop button is pressed.

Timers and Counters

Timers and counters are essential elements in oil and gas PLC programs. They are available as function blocks (instructions) in all major PLC platforms.

TON - Timer On Delay

The TON instruction starts a timer when its input rung goes TRUE. After the programmed preset time, the timer's Done bit (DN) goes TRUE. If the input rung goes FALSE before the preset is reached, the accumulated time resets to zero.

Oil and gas example: After a high pressure alarm activates (input goes TRUE), the TON timer starts. After 10 seconds (preset = 10,000ms), the DN bit goes TRUE and activates the emergency shutdown valve output. The 10-second delay allows for pressure transients to settle without causing nuisance shutdowns, while still protecting equipment from sustained over-pressure conditions.

TOF - Timer Off Delay

The TOF instruction starts timing when its input rung goes FALSE (after having been TRUE). This is used for "cooldown" sequences where something needs to continue running after the main process stops.

Oil and gas example: After a compressor shuts down (input goes FALSE), a TOF timer runs the cooling fans for an additional 5 minutes. When the timer expires, the cooling fan output turns off.

CTU - Count Up

The CTU instruction increments an accumulated value each time its input transitions from FALSE to TRUE (a rising edge). When the accumulated value reaches the preset, the Done bit goes TRUE.

Oil and gas example: A rod pump jack unit uses a proximity switch on the walking beam to count strokes. Each stroke increments the CTU counter. After 100 strokes (preset = 100), the DN bit triggers a data log entry and resets the counter to track the next 100-stroke interval for pump efficiency calculations.

Ladder Logic in Oil and Gas

The following examples show how ladder logic applies to real oil and gas control scenarios. For the full context of how PLCs and SCADA work together, see our PLC programming for oil and gas guide.

High Pressure Shutdown Example

A wellhead high-high pressure protection rung:

The GRT (Greater Than) comparison instruction returns TRUE when tubing pressure exceeds 1,200 PSI. The OTL (latch) instruction sets HIPPS_LATCH TRUE and holds it even after pressure drops back below the setpoint. An operator must physically press a reset button to clear the latch - ensuring someone acknowledges and investigates the over-pressure condition before restarting the well.

Motor Start Sequence Example

A safe motor start sequence checks permissive conditions before allowing the motor to start:

The motor can only start if all permissives are satisfied: no existing faults, adequate lubrication pressure. Each condition is in series - all must be TRUE simultaneously for power to flow. This is the "series AND" logic fundamental to all safety interlock programming.

Tank Level Control Example

A transfer pump controlled by tank level:

The GRT (Greater Than) and LES (Less Than) comparison instructions provide hysteresis - the pump starts at 80% and stops at 20%, preventing rapid start/stop cycling as the level fluctuates near a single setpoint.

Ladder Logic vs Other Languages

In practice, most oil and gas PLC programs use ladder logic for the majority of the code - digital safety logic, permissives, and status bits - with function blocks for PID control loops and structured text for calculations. A single TIA Portal or Studio 5000 project can use all languages simultaneously within different program sections. And when a project calls for production-ready code rather than practice rungs, our PLC programming and panel fabrication services cover both platforms.

How to Learn Ladder Logic

Free Simulators

The easiest way to start learning ladder logic is with a software simulator that runs on your PC without needing actual PLC hardware:

• Connected Components Workbench (CCW): Free from Rockwell Automation. Programs Allen-Bradley Micro800 series PLCs and includes a basic simulator. Good starting point for Allen-Bradley syntax.
• Codesys: A free IEC 61131-3 development environment used by many European PLC brands. Supports all five languages including ladder logic. Has a built-in soft PLC for simulation.
• TIA Portal Trial: Siemens offers time-limited trial licenses for TIA Portal. Includes the S7-PLCSIM software for PC-based simulation of S7-1200/1500 programs.

Recommended Resources

• RealPars YouTube channel: High-quality video tutorials on ladder logic, TIA Portal, and Studio 5000 with industrial examples. Free and beginner-friendly.
• Rockwell Automation Training: Rockwell offers self-paced online training through their PartnerNetwork Training program. Studio 5000 Logix Designer courses cover ladder logic through motion control.
• Siemens Industry Online Support: Siemens provides extensive TIA Portal documentation, application examples, and video tutorials through their SIOS portal.

Getting Started with Allen-Bradley and Siemens

For oil and gas in Texas, learning Allen-Bradley Studio 5000 first is a practical path, as it is widely deployed in the region and well represented in local job postings. After learning the ladder logic fundamentals in Studio 5000, transitioning to Siemens TIA Portal takes days rather than weeks because the underlying concepts are identical. See our dedicated guides: Allen-Bradley PLC programming guide and Siemens PLC programming guide.

Frequently Asked Questions