In modern industrial automation, PLC symbols are essential for designing, programming, operating, and troubleshooting automated systems. Whether working with a manufacturing line, chemical process plant, water treatment facility, or semiconductor production system, engineers and technicians rely heavily on standardized symbols to interpret control logic and electrical diagrams quickly and accurately.

A PLC Symbols Chart serves as a visual reference that organizes the symbols used in programmable logic controller (PLC) systems. These symbols represent components such as inputs, outputs, sensors, timers, relays, communication modules, power supplies, and ladder logic elements. Rather than writing long technical descriptions, engineers use symbols to simplify drawings and improve communication across teams, contractors, and equipment manufacturers.

PLC symbols appear in multiple types of documentation, including electrical schematics, ladder diagrams, wiring drawings, HMI designs, and control panel layouts. Understanding these symbols is critical because even a small misunderstanding in a PLC drawing can lead to wiring errors, programming issues, equipment failures, or production downtime.

Using a well-organized PLC Symbols Chart provides several benefits:

• Faster reading of PLC diagrams
• Improved troubleshooting efficiency
• Easier system maintenance and upgrades
• Better communication between automation professionals
• Reduced engineering and installation errors

Many industries depend on PLC symbols every day, including:

• Manufacturing automation
• Oil and gas processing
• Chemical and petrochemical plants
• Food and beverage production
• HVAC control systems
• Water and wastewater treatment
• Semiconductor manufacturing
• Robotics and machine building

This guide explores the complete PLC Symbols Chart, including PLC input symbols, output symbols, communication symbols, ladder logic symbols, standards, and practical applications used in industrial automation systems.

1. What Is a PLC Symbol?

A PLC symbol is a graphical representation used to identify a device, function, instruction, or hardware component inside a PLC control system. Instead of describing components using lengthy text, engineers use standardized symbols to create diagrams that are easier to read, interpret, and maintain.

In a PLC system, symbols can represent physical field devices such as sensors, switches, motors, and valves, as well as internal software functions such as timers, counters, contacts, and logic coils.

For example:

• A push button may be represented by a simple switch symbol.
• A motor starter may use a contactor or relay symbol.
• A timer function in ladder logic has its own dedicated PLC programming symbol.
• A proximity sensor can be shown using an industrial sensor symbol.

By using symbols consistently, engineers can understand a control system layout without reading extensive explanations.

Purpose of PLC Symbols in Automation Systems

The main purpose of PLC symbols is to create a common engineering language for automation design and operation.

PLC systems often involve multiple stakeholders, including:

• Control engineers
• Electrical engineers
• Maintenance technicians
• Machine builders
• Panel fabricators
• System integrators
• Plant operators

Because these teams may work across different locations or countries, symbols help establish a universal visual communication method.

PLC symbols help engineers:

Simplify complex control systems

Industrial automation systems can contain hundreds or thousands of devices. Symbols make large systems easier to organize and visualize.

Improve drawing clarity

Diagrams become cleaner and more readable when standardized symbols replace long text descriptions.

Speed up troubleshooting

Maintenance teams can rapidly identify devices and control functions using familiar symbols.

Reduce installation mistakes

Clear symbol usage minimizes confusion during panel assembly, field wiring, and commissioning.

Support standardized engineering documentation

PLC projects frequently require documentation packages containing wiring diagrams, logic drawings, and control panel layouts. Symbols ensure consistency throughout these documents.

PLC Symbols vs Standard Electrical Symbols

Although PLC symbols and electrical symbols share similarities, they are not exactly the same.

Standard electrical symbols generally focus on power distribution and electrical equipment such as:

• Circuit breakers
• Transformers
• Motors
• Switches
• Fuses
• Grounding systems

PLC symbols, however, concentrate more on industrial control and automation functions.

PLC-related symbols often include:

• Digital inputs
• Analog inputs
• Digital outputs
• Analog outputs
• PLC CPUs
• Communication modules
• Ladder logic contacts
• Timers and counters
• Internal memory bits

Many PLC drawings combine both electrical symbols and automation symbols because industrial control systems require both power distribution and logic control.

Components Commonly Represented by PLC Symbols

A typical PLC system includes numerous components represented using standardized symbols.

Common PLC symbol categories include:

Input Devices

Input symbols represent devices that send signals to the PLC.

Examples include:

• Push buttons
• Selector switches
• Limit switches
• Proximity sensors
• Pressure transmitters
• Temperature sensors
• Flow switches

Output Devices

Output symbols represent equipment controlled by the PLC.

Examples include:

• Solenoid valves
• Motors
• Alarm buzzers
• Indicator lights
• Contactors
• Heaters
• Pumps

PLC Hardware Components

PLC hardware symbols identify the control equipment itself.

Examples include:

• PLC CPU modules
• Input/output modules
• Power supply units
• Terminal blocks
• Communication cards
• Expansion racks

Logic Programming Symbols

These symbols are used in PLC ladder logic programming.

Examples include:

• Normally open contacts
• Normally closed contacts
• Output coils
• Timers
• Counters
• Set/reset instructions
• Comparators

Understanding these symbols forms the foundation for reading PLC diagrams and developing industrial automation systems.

2. PLC Symbols Chart Basics

A PLC Symbols Chart is a structured reference table that organizes the graphical symbols used throughout PLC systems. It helps engineers, programmers, and maintenance personnel quickly identify automation components and understand how they interact inside a control system.

Without a PLC Symbols Chart, interpreting electrical drawings and ladder logic diagrams would become significantly more difficult, especially in large industrial projects involving hundreds of field devices and control functions.

PLC symbols are typically grouped into categories based on their functions within the automation system.

How to Read a PLC Symbols Chart

Reading a PLC Symbols Chart involves more than simply recognizing shapes. Engineers must understand the function, signal direction, and system role associated with each symbol.

A typical PLC symbol chart normally includes:

When reading a PLC Symbols Chart, engineers usually follow these steps:

Identify the symbol category

Determine whether the symbol belongs to:

• Input devices
• Output devices
• Communication systems
• Power systems
• Ladder logic functions
• PLC hardware

Understand signal direction

Determine whether the signal is entering or leaving the PLC.

Inputs → Signal enters PLC

Examples:

• Sensors
• Push buttons
• Switches

Outputs → Signal leaves PLC

Examples:

• Motors
• Solenoid valves
• Alarm devices

Read device tags and labels

Most PLC diagrams use identification tags such as:

• PB-101 = Push Button
• LS-201 = Limit Switch
• PT-301 = Pressure Transmitter
• M-401 = Motor

Tagging conventions help locate devices within the control system.

Cross-reference related drawings

PLC symbols often connect across multiple engineering documents including:

• Wiring diagrams
• Control schematics
• PLC panel layouts
• Ladder logic drawings
• HMI screen designs

Cross-referencing is an important skill when troubleshooting industrial systems.

Major Categories in a PLC Symbols Chart

PLC Symbols Charts generally organize symbols into several functional groups.

Input Symbols

Input symbols represent devices that provide information to the PLC.

These devices monitor field conditions and convert physical changes into electrical signals.

Common PLC input symbols include:

• Push buttons
• Selector switches
• Limit switches
• Proximity sensors
• Photoelectric sensors
• Temperature transmitters
• Pressure sensors
• Flow meters

Inputs allow the PLC to “observe” the process environment.

Output Symbols

Output symbols represent devices controlled by the PLC.

These devices perform physical actions based on PLC logic decisions.

Examples include:

• Motors
• Contactors
• Solenoid valves
• Alarm buzzers
• Indicator lamps
• Cooling fans
• Pumps
• Heaters

Outputs allow the PLC to “control” the industrial process.

Communication Symbols

Modern PLC systems rely heavily on networking and data exchange.

Communication symbols identify connections between:

• PLC controllers
• HMIs
• SCADA systems
• Remote I/O stations
• Industrial networks

Examples include:

• Ethernet symbols
• Modbus symbols
• Profibus symbols
• DeviceNet symbols
• Remote communication modules

Power Supply Symbols

PLC systems require stable electrical power.

Power-related symbols represent:

• AC power supplies
• DC power supplies
• Grounding systems
• Circuit breakers
• Fuses
• Terminal blocks

Power symbols are critical for safe and reliable system operation.

Ladder Logic Symbols

PLC programming uses specialized logic symbols.

Common ladder logic symbols include:

• Normally open contacts
• Normally closed contacts
• Output coils
• Timers
• Counters
• Internal relays
• Set/reset functions

These symbols form the building blocks of PLC control programs.

Understanding PLC Symbol Labels and Tagging Conventions

Symbols rarely appear alone in engineering drawings. They are typically accompanied by identification labels and device tags.

A tagging system helps engineers quickly determine:

• Device type
• Device number
• Equipment location
• Functional purpose

Common examples include:

Different companies may use different naming conventions based on IEC, ANSI, ISA, or internal engineering standards.

Learning how to interpret both symbols and tags is essential for accurately reading PLC documentation and troubleshooting industrial automation systems.

3. PLC Input Symbols Chart

In every automation system, PLC input symbols represent the devices that send information into the programmable logic controller. Inputs are essential because they allow the PLC to monitor machine status, process conditions, operator commands, and environmental changes.

A PLC cannot make control decisions without receiving data from field devices. These devices convert physical conditions—such as pressure, temperature, position, flow, or operator actions—into electrical signals that the PLC can interpret.

A PLC Input Symbols Chart helps engineers and technicians quickly identify the symbols associated with various input devices used in industrial automation.

Digital Input Symbols

Digital inputs, also called discrete inputs, operate using binary logic.

They only have two states:

• ON / OFF
• True / False
• Open / Closed
• 1 / 0

Digital inputs are commonly used for status monitoring and machine control.

Typical digital input symbols represent:

• Push buttons
• Limit switches
• Emergency stop buttons
• Door switches
• Float switches
• Proximity sensors

For example, when an operator presses a push button, the device sends an ON signal to the PLC input channel.

Digital input symbols are among the most frequently used elements in PLC diagrams and ladder logic.

Analog Input Symbols

Unlike digital inputs, analog inputs measure continuously changing process variables.

Analog signals can vary over a range, such as:

• 0–10 V
• 4–20 mA
• 1–5 V

These signals allow the PLC to monitor dynamic operating conditions.

Common analog input devices include:

• Pressure transmitters
• Temperature transmitters
• Flow transmitters
• Level sensors
• Gas analyzers
• pH analyzers

Analog input symbols are important in process industries such as oil and gas, chemical plants, semiconductor manufacturing, and water treatment systems.

Push Button Symbols

Push buttons are one of the simplest PLC input devices.

They allow operators to manually interact with machinery.

Common push button types include:

• Start push button
• Stop push button
• Reset button
• Jog button
• Emergency stop button

Typical PLC push button symbols may represent:

• Normally Open (NO) push buttons
• Normally Closed (NC) push buttons
• Illuminated push buttons

Push buttons are commonly found on:

• Machine control panels
• Conveyor systems
• Packaging equipment
• Pump stations
• Industrial process skids

Selector Switch Symbols

Selector switches provide manual selection between operating modes.

They allow operators to choose different system functions.

Examples include:

• Auto / Manual selection
• Local / Remote control
• Forward / Reverse motor operation
• High / Low speed selection

PLC selector switch symbols are widely used in electrical panel drawings and control schematics.

These symbols help technicians understand machine operating modes during commissioning and troubleshooting.

Proximity Sensor Symbols

Proximity sensors detect objects without physical contact.

They are extremely common in industrial automation because they offer:

• High reliability
• Fast response times
• Reduced mechanical wear

Types of proximity sensors include:

• Inductive proximity sensors
• Capacitive proximity sensors
• Magnetic proximity sensors

Typical applications include:

• Position detection
• Part counting
• Conveyor monitoring
• Robotic movement control
• Machine safety systems

PLC diagrams use standardized symbols to distinguish proximity sensors from traditional mechanical switches.

Limit Switch Symbols

Limit switches are mechanical devices used to detect physical movement or equipment position.

They are widely used in automated machinery.

Applications include:

• End-of-travel detection
• Cylinder position monitoring
• Conveyor stop positions
• Valve position indication

Common PLC limit switch symbols may represent:

• Roller lever limit switches
• Plunger limit switches
• Rotary limit switches

Limit switches are important safety and positioning devices in many PLC-controlled systems.

Temperature Sensor Symbols

Temperature sensors provide critical process data to the PLC.

Industries rely on temperature monitoring for:

• Equipment protection
• Product quality control
• Process stability
• Safety systems

Common temperature sensor types include:

• Thermocouples
• RTDs (Resistance Temperature Detectors)
• Infrared temperature sensors

Temperature sensor symbols often appear in:

• Furnace control systems
• HVAC automation
• Chemical processing
• Semiconductor manufacturing
• Food production lines

These sensors usually connect to analog PLC input modules.

Pressure Sensor Symbols

Pressure sensors monitor gas and liquid pressure within industrial systems.

Typical PLC pressure sensor applications include:

• Hydraulic systems
• Pneumatic systems
• Steam systems
• Gas distribution systems
• Process control systems

Pressure sensor symbols commonly represent:

• Pressure switches
• Pressure transmitters
• Differential pressure transmitters

Pressure monitoring is essential for maintaining process efficiency and equipment safety.

Flow Sensor Symbols

Flow sensors measure the movement of liquids or gases.

PLC systems use flow monitoring to control:

• Chemical dosing
• Cooling water circulation
• Gas delivery systems
• Fuel systems
• Water treatment operations

Common flow measurement devices include:

• Flow switches
• Turbine flow meters
• Magnetic flow meters
• Mass flow controllers
• Coriolis flow meters

Flow sensor symbols are commonly found in PLC diagrams for process industries.

Common PLC Input Symbols Chart Table

Understanding the PLC Input Symbols Chart is fundamental for interpreting automation drawings, troubleshooting equipment, and developing reliable PLC control systems.

4. PLC Output Symbols Chart

While input devices allow the PLC to monitor conditions, PLC output symbols represent the devices that perform physical actions.

Outputs receive commands from the PLC and translate logic decisions into real-world operations.

A PLC Output Symbols Chart helps engineers identify the symbols associated with devices controlled by PLC output channels.

PLC outputs typically control:

• Motors
• Valves
• Alarms
• Lighting systems
• Pumps
• Heaters
• Industrial machinery

Digital Output Symbols

Digital outputs operate using ON/OFF control logic.

They are used whenever a device only requires two operating states.

Common PLC digital output devices include:

• Relay coils
• Solenoid valves
• Contactors
• Pilot lights
• Alarm horns

Digital outputs are common in machine automation and manufacturing systems.

Analog Output Symbols

Analog outputs generate continuously variable signals.

They allow the PLC to control equipment proportionally rather than simply switching devices ON or OFF.

Analog outputs commonly use:

• 4–20 mA signals
• 0–10 V signals

Typical applications include:

• Control valve positioning
• Variable speed drives
• Temperature control systems
• Pressure regulation systems

Analog output symbols frequently appear in advanced process control systems.

Relay Output Symbols

Relay outputs provide electrical switching capability.

PLC relay output symbols represent electromechanical switching devices used to control larger electrical loads.

Applications include:

• Motor starters
• Lighting circuits
• Pump controls
• Alarm systems

Relay outputs offer isolation between PLC electronics and external equipment.

Solenoid Valve Symbols

Solenoid valves are extremely common PLC-controlled devices.

These valves use electrical signals to control fluid or gas flow.

Common applications include:

• Pneumatic cylinders
• Hydraulic systems
• Process fluid control
• Gas distribution systems
• Automated machinery

PLC symbols for solenoid valves may indicate:

• Normally closed valves
• Normally open valves
• Directional control valves

Solenoid valve symbols are frequently found in industrial automation drawings.

Indicator Light Symbols

Indicator lights provide visual system feedback.

These devices help operators understand machine conditions quickly.

Common indicator light functions include:

• Power ON indication
• Alarm status
• Machine running condition
• Fault warning
• Maintenance alerts

Indicator light symbols often appear in PLC panel layouts and HMI integration diagrams.

Motor Starter Symbols

Motor starters control electric motor operation.

PLC motor output symbols frequently represent:

• Contactors
• Soft starters
• Variable frequency drives (VFDs)
• Motor overload protection devices

Motor control is one of the most common PLC applications in industry.

Examples include:

• Pumps
• Fans
• Compressors
• Conveyors
• Mixers

Alarm and Buzzer Symbols

Alarms warn operators about abnormal system conditions.

PLC alarm symbols may represent:

• Audible alarms
• Warning sirens
• Flashing lights
• Fault annunciators

Alarm systems improve operational safety and reduce downtime.

Heater Control Symbols

Many industrial systems use PLC outputs for temperature regulation.

Heater output symbols may represent:

• Electric heaters
• Heating jackets
• Furnace heating elements
• Process heat tracing

Temperature-controlled processes frequently depend on these outputs.

PLC Output Symbols Chart Examples

Understanding PLC output symbols allows engineers to interpret how a PLC physically controls industrial processes.

5. PLC Communication and Network Symbols

Modern industrial automation increasingly depends on data communication.

A PLC no longer operates as an isolated controller. Instead, it exchanges information continuously with HMIs, SCADA systems, remote I/O stations, drives, analyzers, and enterprise networks.

A PLC Communication Symbols Chart organizes the symbols used to represent these communication links and networking components.

Ethernet Communication Symbols

Industrial Ethernet is one of the most widely used PLC communication methods.

Ethernet communication symbols commonly represent:

• PLC-to-PLC communication
• PLC-to-HMI communication
• SCADA integration
• Industrial switches
• Network routers

Industrial Ethernet protocols include:

• EtherNet/IP
• Profinet
• Modbus TCP
• Ethernet TCP/IP

Ethernet symbols frequently appear in modern control architecture drawings.

Fieldbus Symbols

Fieldbus systems provide digital communication between controllers and field devices.

Fieldbus communication reduces wiring complexity compared to traditional hardwired systems.

Common fieldbus protocols include:

• Foundation Fieldbus
• Profibus
• DeviceNet
• CAN Bus

Fieldbus symbols are common in large process automation facilities.

Modbus Communication Symbols

Modbus remains one of the most widely used industrial communication protocols.

Modbus symbols often appear in PLC diagrams involving:

• Energy meters
• VFDs
• Instrumentation devices
• Building automation systems
• Remote monitoring equipment

Modbus communication may operate through:

• Modbus RTU
• Modbus TCP

Profibus Symbols

Profibus is commonly used in European automation systems and Siemens PLC environments.

Profibus symbols often represent:

• Distributed I/O modules
• Smart instruments
• Motor drives
• Process automation equipment

These symbols are common in manufacturing and process industries.

HMI Connection Symbols

Human Machine Interfaces (HMIs) allow operators to interact with PLC systems.

PLC HMI communication symbols represent:

• Touchscreen panels
• Operator interfaces
• Remote display terminals
• Graphical process dashboards

HMI communication is essential for modern automation monitoring and control.

SCADA Communication Symbols

SCADA systems provide centralized supervisory monitoring.

PLC diagrams use SCADA symbols to indicate connections between controllers and supervisory systems.

SCADA systems typically handle:

• Data acquisition
• Alarm management
• Historical trending
• Process visualization
• Remote control

Remote I/O Symbols

Remote I/O allows input and output devices to be installed away from the main PLC rack.

Benefits include:

• Reduced wiring costs
• Flexible equipment layouts
• Easier system expansion

Remote I/O symbols commonly appear in large industrial facilities.

PLC Network Architecture Symbol Examples

Understanding PLC communication and network symbols is increasingly important as automation systems become more connected, data-driven, and digitally integrated.

6. PLC Power Supply and Hardware Symbols

A PLC system is more than just ladder logic and field devices. It also depends on hardware infrastructure and electrical power to operate reliably. PLC power supply and hardware symbols represent the physical components that make up the controller architecture.

These symbols appear frequently in:

• PLC panel drawings
• Electrical schematics
• Control cabinet layouts
• Wiring diagrams
• System architecture drawings

A PLC Power Supply and Hardware Symbols Chart helps engineers identify the hardware building blocks of industrial automation systems.

PLC CPU Symbols

The PLC CPU (Central Processing Unit) is the brain of the automation system.

The CPU executes the control program, processes input signals, performs logic calculations, and sends commands to outputs.

PLC CPU symbols generally represent:

• Compact PLC controllers
• Modular PLC CPUs
• Safety PLC controllers
• Redundant CPU systems

The CPU symbol often appears at the center of a PLC architecture drawing because nearly every device communicates with it.

Typical PLC CPU functions include:

• Program execution
• Data processing
• Memory management
• Communication handling
• System diagnostics

Different manufacturers may use slightly different CPU symbol styles depending on their drawing standards.

PLC Rack and Module Symbols

Large PLC systems commonly use modular hardware designs.

Instead of a single compact controller, engineers build systems using racks containing multiple modules.

Common PLC rack symbols represent:

• PLC chassis
• Backplanes
• I/O racks
• Expansion racks

Module symbols typically include:

• Digital input modules
• Digital output modules
• Analog input modules
• Analog output modules
• Communication modules
• Specialty control modules

Modular designs provide flexibility, easier expansion, and simplified maintenance.

These symbols are commonly found in:

• Process control plants
• Manufacturing lines
• Large automation systems
• Distributed control architectures

Power Supply Symbols

Every PLC requires stable electrical power.

Power supply symbols identify the electrical source that feeds the controller and supporting hardware.

Common PLC power supply symbols include:

• AC power supplies
• DC power supplies
• 24 VDC supplies
• Redundant power systems
• UPS backup power supplies

Many industrial PLC systems operate using 24 VDC power, especially for sensors, relays, and communication modules.

Power supply symbols are extremely important because unstable power can cause:

• PLC shutdowns
• Program interruptions
• Communication failures
• Equipment malfunction
• Production downtime

Grounding Symbols

Proper grounding is critical in industrial automation.

Grounding symbols represent electrical connections designed to improve safety and reduce electrical noise.

Common grounding symbol categories include:

• Protective earth grounding
• Signal grounding
• Equipment grounding
• Shield grounding

Grounding becomes especially important in systems containing:

• Analog signals
• High-frequency drives
• Communication networks
• Sensitive instrumentation

Incorrect grounding can create problems such as:

• Signal instability
• Communication errors
• Sensor noise
• Electrical safety hazards

Terminal Block Symbols

Terminal blocks simplify wiring connections inside PLC control panels.

These devices organize electrical conductors and provide secure connection points between equipment.

Terminal block symbols commonly represent:

• Standard terminal blocks
• Fuse terminal blocks
• Disconnect terminal blocks
• Ground terminals

Terminal block symbols help technicians:

• Trace field wiring
• Perform maintenance
• Replace components
• Verify wiring accuracy

They are essential elements in electrical panel drawings.

Fuse and Circuit Breaker Symbols

Protection devices are vital for electrical safety.

PLC hardware drawings frequently use symbols for:

• Fuses
• Miniature circuit breakers (MCBs)
• Molded case circuit breakers
• Overcurrent protection devices

These components protect PLC systems against:

• Short circuits
• Overloads
• Electrical faults
• Equipment damage

Protection symbols are standard elements in control panel electrical documentation.

Expansion Module Symbols

As automation systems grow, engineers often add expansion hardware.

Expansion module symbols represent additional devices connected to the primary PLC.

Examples include:

• Additional I/O modules
• Communication expansion cards
• High-speed counter modules
• Motion control modules
• Temperature control modules

Expansion symbols are commonly used in scalable PLC designs where future system growth is expected.

PLC Hardware Symbols Chart

Understanding PLC power supply and hardware symbols is essential for designing, wiring, and maintaining reliable industrial control systems.

7. PLC Ladder Logic Symbols Chart

PLC ladder logic symbols are among the most important elements in industrial automation programming.

Ladder logic is a graphical programming language used to develop PLC control programs. It resembles traditional electrical relay diagrams, making it familiar to electricians, engineers, and maintenance technicians.

A PLC Ladder Logic Symbols Chart organizes the symbols used within ladder logic programs.

These symbols define how the PLC processes inputs, executes logic, and controls outputs.

Normally Open (NO) Contact Symbols

The Normally Open (NO) contact is one of the most fundamental ladder logic symbols.

An NO contact only allows logical continuity when the referenced condition becomes TRUE.

Typical uses include:

• Start push buttons
• Sensor activation
• Process conditions
• Enable functions

When the associated input activates, the NO contact “closes” logically.

NO contacts are heavily used in:

• Motor controls
• Conveyor systems
• Pump logic
• Safety interlocks

They are foundational building blocks of PLC programs.

Normally Closed (NC) Contact Symbols

The Normally Closed (NC) contact behaves opposite to an NO contact.

An NC contact allows logical continuity until its referenced condition becomes TRUE.

Typical applications include:

• Stop push buttons
• Emergency shutdown logic
• Alarm logic
• Safety conditions

NC symbols are widely used because industrial control systems often use fail-safe logic.

In fail-safe design:

• Broken wires
• Device failures
• Power loss

can immediately generate protective shutdown actions.

Coil Symbols

Coils represent output actions inside ladder logic.

When logical conditions become TRUE, the PLC energizes the associated coil.

Common coil applications include:

• Output activation
• Internal memory bits
• Motor control commands
• Alarm activation
• Status signals

Types of PLC coil symbols may include:

• Standard output coils
• Internal relay coils
• Set coils
• Reset coils

Coils form the execution point of ladder logic rungs.

Timer Symbols

Timers add time-based functionality to PLC programs.

Timer symbols are critical when processes require controlled delays or sequencing.

Common timer types include:

• TON (Timer On Delay)
• TOF (Timer Off Delay)
• Retentive timers

Typical timer applications include:

• Motor startup delays
• Pump sequencing
• Alarm delays
• Batch processing
• Conveyor timing

Timer symbols are essential in nearly every industrial PLC application.

Counter Symbols

Counters track events or occurrences.

PLC counter symbols are commonly used when systems need to count:

• Product quantities
• Machine cycles
• Sensor activations
• Production batches

Typical counter types include:

• Count Up (CTU)
• Count Down (CTD)
• Bidirectional counters

Applications include:

• Packaging systems
• Assembly lines
• Production monitoring
• Machine maintenance tracking

Counters improve automation intelligence and operational monitoring.

Comparator Symbols

Comparator instructions compare values inside PLC logic.

These symbols help the PLC evaluate conditions based on data relationships.

Common comparison functions include:

• Equal To
• Greater Than
• Less Than
• Greater Than or Equal
• Less Than or Equal

Comparator symbols are especially common in:

• Analog process control
• Temperature regulation
• Pressure control
• Production monitoring

They allow more advanced decision-making inside automation programs.

Internal Relay Symbols

Internal relays are software-based logic elements.

Unlike physical relays, internal relays exist only inside PLC memory.

Internal relay symbols help programmers:

• Simplify logic
• Organize program structure
• Reuse logic conditions
• Reduce program complexity

These symbols are commonly used in large ladder logic applications.

Set and Reset Symbols

Set and reset instructions create latched logic behavior.

Set symbols activate a condition and hold it TRUE.

Reset symbols deactivate the stored condition.

Typical applications include:

• Machine start/stop memory
• Alarm acknowledgment
• Batch process control
• Mode selection logic

Latch logic is extremely common in industrial PLC programming.

PLC Ladder Logic Symbols Chart Examples

Mastering the PLC Ladder Logic Symbols Chart is essential for PLC programming, troubleshooting, and automation system design.

8. PLC Symbols Standards and Drawing Conventions

Standardization plays a major role in industrial engineering documentation.

Without standardized symbol conventions, interpreting PLC drawings across companies, countries, and industries would become extremely difficult.

PLC symbol standards establish common graphical rules that improve consistency, readability, and communication.

A PLC Symbols Standards and Drawing Conventions guide helps engineers understand how symbols are organized and applied.

Common PLC Symbol Standards

Several standards influence PLC symbol design and documentation practices.

These standards help ensure engineering drawings remain understandable across different organizations and industries.

Common standards include:

• IEC standards
• ANSI standards
• NFPA conventions
• ISA standards
• Manufacturer drawing guidelines

Although minor differences may exist, most standards pursue the same goal: consistent engineering communication.

IEC Standards for PLC Symbols

The IEC (International Electrotechnical Commission) develops internationally recognized electrical and automation standards.

IEC-based PLC drawings commonly use standardized graphical symbols for:

• Switches
• Sensors
• Relays
• Contactors
• PLC modules
• Communication devices

IEC conventions are widely adopted in:

• Europe
• Asia
• Global industrial projects
• International engineering firms

Many multinational automation projects rely heavily on IEC documentation practices.

NFPA Electrical Drawing Conventions

The National Fire Protection Association (NFPA) strongly influences electrical documentation practices in North America.

NFPA conventions frequently appear in:

• Industrial machinery
• Manufacturing equipment
• Electrical panel documentation
• Safety-related control systems

NFPA drawing styles may differ slightly from IEC practices, particularly regarding:

• Device labeling
• Wire numbering
• Panel documentation structure

Engineers working globally often need familiarity with both systems.

ANSI Electrical Symbol Practices

The American National Standards Institute (ANSI) provides additional symbol and drawing guidance.

ANSI standards influence:

• Electrical diagrams
• Industrial controls
• Wiring schematics
• Instrumentation drawings

ANSI symbols are commonly used throughout the United States industrial market.

ISA Instrumentation Symbol Influence

The International Society of Automation (ISA) significantly influences process automation documentation.

ISA standards are especially important for industries such as:

• Oil and gas
• Petrochemical plants
• Chemical processing
• Water treatment
• Power generation

ISA conventions strongly affect symbols related to:

• Instrumentation
• Transmitters
• Controllers
• Process loops
• Control valves

Many PLC diagrams in process industries combine ISA symbols with PLC hardware symbols.

Manufacturer-Specific Symbol Variations

PLC manufacturers sometimes introduce their own documentation styles.

Common examples include:

• Siemens PLC symbols
• Allen-Bradley PLC symbols
• Mitsubishi PLC symbols
• Omron PLC symbols
• Schneider Electric PLC symbols

Although core concepts remain similar, graphical appearances and naming methods can vary.

Engineers must understand both industry standards and manufacturer-specific conventions.

Symbol Naming and Numbering Conventions

Most PLC drawings include structured identification systems.

Common labeling conventions may include:

• Device tags
• Wire numbers
• Terminal numbers
• Module addresses
• Panel references

Example device tags:

• PB-101 → Push Button
• LS-202 → Limit Switch
• M-301 → Motor
• SV-401 → Solenoid Valve

Consistent naming improves:

• Troubleshooting efficiency
• Documentation clarity
• Installation accuracy
• Maintenance productivity

Understanding PLC symbol standards and drawing conventions is critical for reading industrial control documentation accurately across different industries, countries, and automation platforms.

9. How to Read and Use a PLC Symbols Chart

Understanding a PLC Symbols Chart is not just about memorizing symbols. Engineers and technicians must know how to interpret the symbols in context, follow signal paths, and understand how devices interact inside a control system.

The ability to read PLC symbols correctly is essential for:

• System design
• Panel wiring
• PLC programming
• Equipment troubleshooting
• Maintenance and commissioning

A structured approach can greatly improve diagram interpretation.

Step-by-Step Method for Reading PLC Symbols

Reading PLC drawings becomes easier when following a logical sequence.

Step 1 – Identify the PLC Hardware

Begin by locating the PLC controller components.

Look for symbols representing:

• PLC CPU
• Input modules
• Output modules
• Communication modules
• Power supplies

Understanding the hardware layout helps establish the control system structure.

Step 2 – Locate Inputs and Outputs

Next, identify field devices connected to the PLC.

Typical inputs include:

• Push buttons
• Sensors
• Limit switches
• Pressure transmitters
• Temperature sensors

Typical outputs include:

• Motors
• Solenoid valves
• Indicator lights
• Alarms
• Pumps

Recognizing signal direction is critical.

Inputs → Information into PLC

Outputs → Commands out of PLC

Step 3 – Analyze Ladder Logic Functions

Review the ladder logic symbols associated with the control sequence.

Identify:

• Normally open contacts
• Normally closed contacts
• Coils
• Timers
• Counters
• Comparators

This step helps explain how the PLC processes information and makes decisions.

Step 4 – Follow Signal Flow

Control systems follow a logical signal path.

Example:

Push Button → PLC Input → Ladder Logic → Output Coil → Motor Starter

Following signal flow simplifies troubleshooting and process understanding.

Step 5 – Check Device Labels and References

Most PLC drawings use identification tags.

Examples:

• PB-101 → Push Button
• LS-202 → Limit Switch
• M-301 → Motor
• SV-401 → Solenoid Valve

Reference labels help locate devices across multiple engineering drawings.

Identifying Inputs, Outputs, and Control Logic

A major skill when reading PLC symbols is distinguishing between device categories.

Identifying Inputs

Inputs generally monitor machine conditions or operator commands.

Common PLC input symbols include:

• Sensors
• Switches
• Buttons
• Process transmitters

Inputs provide the PLC with process information.

Identifying Outputs

Outputs perform physical actions.

Common output symbols include:

• Contactors
• Motors
• Alarms
• Valves
• Heaters

Outputs convert PLC logic into real-world operations.

Identifying Logic Elements

Logic symbols exist within the PLC program itself.

Common examples include:

• Contacts
• Timers
• Counters
• Set/reset instructions
• Internal relays

These symbols determine system behavior.

Reading PLC Panel Drawings

PLC panel drawings provide a physical view of control system hardware.

Typical panel symbols represent:

• PLC racks
• Terminal blocks
• Power supplies
• Circuit breakers
• Relays
• Wiring terminals

When reading PLC panel drawings:

1. Start with incoming power.
2. Locate protection devices.
3. Identify the PLC controller.
4. Trace field wiring connections.
5. Verify module assignments.

Panel drawings are especially important during:

• Installation
• Commissioning
• Maintenance
• Field troubleshooting

Conclusion

A PLC Symbols Chart is an essential reference tool for engineers, electricians, technicians, and automation professionals working with industrial control systems.

PLC symbols create a standardized visual language that simplifies communication, documentation, troubleshooting, and system design.

Throughout this guide, we explored major PLC symbol categories, including:

• PLC input symbols
• PLC output symbols
• Communication and network symbols
• Power supply and hardware symbols
• Ladder logic symbols
• Standards and drawing conventions

Understanding these symbols enables professionals to:

• Read electrical and control drawings accurately
• Design automation systems efficiently
• Troubleshoot PLC systems faster
• Improve maintenance effectiveness
• Reduce installation and programming errors

Because PLC technology is used across manufacturing, oil and gas, HVAC, semiconductor, water treatment, and countless other industries, mastering PLC symbols is a valuable technical skill.

Whether you are a beginner learning ladder logic or an experienced automation engineer working on large industrial projects, using a reliable PLC Symbols Chart can improve accuracy, productivity, and system reliability.

As industrial automation continues advancing toward smarter and more connected systems, understanding PLC symbols will remain a critical foundation for successful engineering and operational performance.