Ladder Diagram

A ladder diagram is a graphical representation of logical control within PLC systems based on classic relay logic. The diagram is the visual programming form of Ladder Logic and is used worldwide in Industrial Automation, machine building, SCADA environments and Process Automation.

The name “ladder diagram” refers to the visual structure of the schematic:

• Two vertical power rails
• Horizontal lines (“rungs”)
• Contacts and coils

This makes the diagram resemble a ladder.

Ladder diagrams are used for:

• Machine control
• Start/stop logic
• Motor control
• Safety functions
• Alarm handling
• Interlocks
• Process sequences

In OT environments, the ladder diagram remains one of the most widely used programming forms thanks to its high readability for electrical and maintenance engineering teams.

⚙️ Basic structure of a ladder diagram

A ladder diagram consists of:

Basic example:

|----[ Start ]----[/ Stop ]----( Motor )----|

Meaning:

• Start activates the logic
• Stop breaks the logic
• Motor is activated

The PLC interprets this as a logical circuit.

🔌 Contact types

Normally Open (NO)

A Normally Open contact closes when the linked variable becomes TRUE.

Symbol:

[ ]

Example:

|----[ Sensor ]----( Output )----|

Normally Closed (NC)

A Normally Closed contact opens when the linked variable becomes TRUE.

Symbol:

[/]

Example:

|----[/ Alarm ]----( Motor )----|

⚡ Output coils

Coils represent outputs or internal bits.

Examples:

• Relays
• Lamps
• Motor starters
• Internal memory bits
• Alarms

Symbol:

( )

Example:

|----[ Start ]--------( Conveyor )----|

🧠 Logical functions

Ladder diagrams support logical operators.

AND logic

Contacts in series:

|----[ A ]----[ B ]----( Output )----|

Output is active only when:

A = TRUE and B = TRUE

OR logic

Contacts in parallel:

|----[ A ]----||             |----( Output )----||----[ B ]----|

Output is active when:

A = TRUE or B = TRUE

NOT logic

Using an NC contact:

|----[/ Alarm ]----( Motor )----|

Motor runs only when no alarm is active.

⏱️ Timers in ladder diagrams

Timers are crucial in industrial processes.

Main timer functions:

Example:

|----[ Sensor ]----[TON T1 10s]----|

Function:

• Sensor activates
• Timer starts
• After 10 seconds the output becomes active

Applications:

• Motor delays
• Alarm delays
• Process sequences
• Soft-start functions

🔢 Counters

Counters count events or products.

Types:

Example applications:

• Production counts
• Batch recording
• Cycle counts
• Maintenance intervals

🔄 PLC scan cycle

PLCs execute ladder diagrams cyclically.

The standard scan cycle:

Read inputs    ↓Execute logic    ↓Write outputs    ↓Repeat

Important properties:

This predictability is essential in real-time OT processes.

🏭 Ladder diagrams in industrial applications

Machine building

Commonly used functions:

• Conveyor control
• Robot interlocks
• Start/stop circuits
• Sensor logic

Process industry

Applications:

• Pump control
• Valve actuation
• Batch processes
• Alarm management

Power supply

Used for:

• Switchgear
• Interlocks
• Emergency procedures
• Generator control

Building automation

In Building Automation for:

• HVAC
• Lighting
• Access control
• Energy management

🛡️ Safety logic

Ladder diagrams are widely used in safety functions.

Examples:

• Emergency stop logic
• Safety doors
• Light curtains
• Two-hand control

Safety-related applications often run on a Safety PLC.

Important standards:

⚙️ Interlocks and permissives

A ladder diagram often contains interlocks.

Example:

Motor may start only if:- Safety door closed- No fault active- Pressure sufficient- Emergency stop reset

Interlocks protect:

• Operators
• Machines
• Processes
• Product quality

🔄 Retentive functions

Many PLCs support retentive variables.

These retain values after:

• Power loss
• PLC reboot
• Power interruptions

Important for:

• Production counters
• Batch numbers
• Process status

📡 Integration with SCADA and HMI

Ladder diagrams send variables to:

• SCADA
• HMI
• Historian
• MES

Examples:

Communication often uses:

• Modbus TCP
• OPC UA
• Ethernet IP
• ProfiNET

🧪 Diagnostics and troubleshooting

A major benefit of ladder diagrams is visual fault diagnosis.

Maintenance technicians can see in real time:

• Active contacts
• Energised coils
• Timer status
• Counter values
• Interlock conditions

Common problems:

⚠️ Common design errors

Over-complex rungs

Large parallel structures reduce readability.

Poor naming

Unclear tags make troubleshooting difficult.

Excessive use of internal bits

Too many helper variables increase complexity.

No modular structure

Large monolithic programs are hard to maintain.

Best practices:

• Standardised templates
• Function blocks
• Clear tagging
• Modular architecture

🔐 Cybersecurity risks

PLC logic is an important OT attack target.

Possible attacks:

• Manipulation of outputs
• Tampering with interlocks
• Process sabotage
• Disabling safety

Known threats:

• Malware
• Insider threats
• Unauthorised engineering
• Malicious firmware

Stuxnet showed how PLC logic can be manipulated without operators noticing.

🧱 Security measures

Important OT security measures:

PLC programs often fall under formal Change Management.

🌐 Ladder diagrams in Industry 4.0

Although ladder diagrams are a classic technology, they remain relevant within Industry 4.0.

Modern developments:

• Virtual PLCs
• Edge-based runtime
• Integration with Industrial AI
• Cloud connectivity
• Digital twins

Many modern PLC platforms support hybrid programming models with:

• Ladder diagrams
• Structured Text
• Function Block Diagram
• SFC

📈 Benefits of ladder diagrams

Key benefits:

• High readability
• Visual debugging
• Widely supported
• Familiar to maintenance teams
• Deterministic behaviour
• Easy troubleshooting

⚡ Limitations

Key limitations:

• Less suited to complex algorithms
• Limited abstraction
• Difficult to scale in very large systems
• Less suitable for data processing
• Large programs become hard to read

For these reasons, complex applications are often combined with:

• Structured Text
• Function Block Diagram
• SFC