How PROFINET Works ?

 

How PROFINET Works ?

The Engineering Guide to PROFINET: Architecture, Operation, and Industrial Ethernet Excellence

In the world of industrial automation, communication is the central nervous system of the factory. Among the various protocols that have emerged to bridge the gap between the office floor and the factory floor, PROFINET (Process Field Net) stands as the global leader. Developed by PROFIBUS & PROFINET International (PI), it is an open Industrial Ethernet standard that leverages the speed of IT networking with the rugged determinism required for high-speed manufacturing.

This article explores the mechanics of PROFINET, explaining how it achieves real-time performance, how it manages data, and why it has become the backbone of Industry 4.0.

 

What is PROFINET? The Convergence of IT and OT

PROFINET is not simply "PROFIBUS over Ethernet." While it shares the same user profiles and much of the philosophy of its predecessor, PROFINET is built natively on IEEE 802.3 (Ethernet).

The brilliance of PROFINET lies in its ability to handle "Office" traffic (HTTP, SNMP, DHCP) and "Industrial" traffic (I/O data, motion control, safety) on the same cable. It achieves this by bypassing certain layers of the standard TCP/IP stack for time-critical data—a process known as Channel Hopping or Direct L2 Communication.

The Functional Roles: Controllers, Devices, and Supervisors

To understand how PROFINET works, we must define the three primary "actors" in a PROFINET network:

A. The IO Controller (The "Brain")

Typically a Programmable Logic Controller (PLC) or a high-end Industrial PC. The Controller is the master of the configuration. It holds the hardware configuration for all connected devices and is responsible for establishing connections and exchanging cyclic I/O data.

B. The IO Device (The "Hands")

These are field devices such as Remote I/O modules, VFDs (Variable Frequency Drives), Servo Drives, and Sensors. A Device waits for a Controller to establish a connection. Once connected, it sends its inputs and receives its outputs at a predefined "Update Rate."

C. The IO Supervisor

Typically a laptop running engineering software (like TIA Portal) or an HMI. Supervisors are used for commissioning, diagnostics, and monitoring, but they do not control the cyclic I/O logic.

Communication Classes: Solving the Real-Time Challenge

Standard Ethernet is non-deterministic; collisions and "best-effort" delivery can cause unpredictable delays. PROFINET solves this by categorizing data into three communication classes:

Class 1: Non-Real-Time (NRT) / TCP/IP

For data that is not time-sensitive (e.g., configuring a device, downloading a program, or viewing a web page hosted on the PLC). This data uses the standard TCP/IP or UDP/IP stack (Layers 3 and 4). It is flexible but slow, with latencies in the 100ms range.

Class 2: PROFINET Real-Time (RT)

Used for the vast majority of industrial I/O. RT communication bypasses the TCP/IP layers and communicates directly at the Ethernet Layer (Layer 2). By removing the overhead of IP headers and routing, PROFINET RT can achieve update rates of 1ms to 10ms, which is sufficient for most factory automation tasks.

Class 3: PROFINET Isochronous Real-Time (IRT)

For high-speed motion control (e.g., 32 axes of servos synchronized to within 1 microsecond). IRT uses a hardware-based "Time Slot" method. The network cycle is split into a "Reserved" phase for IRT data and an "Open" phase for standard traffic. This ensures that the IRT data is never delayed by other network traffic.

 

The GSDML File: The Device's Passport

Every PROFINET device is accompanied by a GSDML (General Station Description Markup Language) file. This XML-based file tells the PLC:

• What sub-modules the device has.
• What parameters can be configured.
• The structure of the input and output data.
• The supported update rates.

Without a GSDML file, the Controller cannot communicate with the Device. It is the "driver" that allows the PLC to understand the device's "language."

Device Identification: Name vs. IP

In a standard home network, devices are found via IP addresses. In PROFINET, things are different.

1. MAC Address: Fixed by the manufacturer.
2. Device Name: Assigned by the engineer (e.g., Conveyor_Motor_1).
3. IP Address: Assigned by the PLC based on the Device Name.

How it works: When the PLC starts, it broadcasts a "DCP" (Discovery and Configuration Protocol) request: "Is there a device named Conveyor_Motor_1?" The device responds with its MAC address, and the PLC then sends it an IP address. This makes replacing a faulty device easier, as the PLC can often automatically assign the name to a new device based on its physical position in the network topology.

Cyclic vs. Acyclic Data Exchange

Cyclic Data (The Heartbeat)

This is the process data (Start/Stop bits, Speed setpoints, Sensor values). It is exchanged continuously. If a device fails to send its cyclic packet within a "Watchdog Time" (usually 3 cycles), the PLC will trigger a "Module Plug/Pull" fault and stop the process for safety.

Acyclic Data (On-Demand)

This is used for "Events."

• Alarms: A drive over-temperature alarm.
• Diagnostics: Reading the firmware version or serial number.
• Record Data: Changing a parameter value while the machine is running. This data is only sent when needed and does not impact the speed of the cyclic I/O.

Network Topologies: Flexibility by Design

Unlike old fieldbuses that required a "Daisy Chain," PROFINET supports:

• Line: Devices connected one after another (Integrated switches).
• Star: Devices connected to a central industrial switch.
• Tree: A combination of stars.
• Ring (MRP): Using Media Redundancy Protocol. If one cable breaks, the network detects the break and re-routes data in less than 200ms, preventing a machine shutdown.

Why PROFINET is the Standard for Industry 4.0

PROFINET is more than just I/O; it includes specialized "Profiles":

1. PROFIsafe: Allows safety data (E-Stops, Light Curtains) to travel over the same Ethernet cable as standard data, eliminating miles of expensive safety wiring.
2. PROFIenergy: Provides a standard way to put devices into "sleep mode" during breaks, significantly reducing factory energy consumption.
3. PROFIdrive: A standardized way for controllers to talk to drives, making it easier to swap drive brands without rewriting the entire PLC code.

 

PROFINET works by taking the standard Ethernet infrastructure we use in our offices and "hardening" it for the factory. By utilizing different communication classes (NRT, RT, IRT), it ensures that a simple web request won't delay a critical E-Stop signal or a high-speed servo move.

Its reliance on Device Names, GSDML files, and Layer 2 communication makes it a fast, robust, and user-friendly protocol. As we move deeper into the era of Big Data and the Industrial Internet of Things (IIoT), PROFINET’s ability to move massive amounts of data while maintaining microsecond precision ensures it will remain the dominant force in global automation.

 

 

 

 

 

 

 

 

 

Why Overload Relays Always Use 95–96 and 97–98 contacts

Why Overload Relays Always Use 95–96 and 97–98 contacts

Introduction

If you’ve ever opened a motor control panel, you’ve likely seen overload relays marked with 95–96 and 97–98. These numbers appear across Siemens, Schneider, ABB, Allen-Bradley, and countless other brands. At first glance, it might seem arbitrary. But in reality, this numbering is part of a global language of automation, defined by IEC 60947 and NEMA standards.

This article explores why these numbers are standardized, how they function, and why they matter for technicians, engineers, and students.

What is an Overload Relay?

An overload relay is a protective device used in motor circuits. Its job is to protect motors from excessive current caused by overloads or phase failures. Unlike fuses or circuit breakers, overload relays are thermal or electronic devices that trip when current exceeds a preset value for a certain time.

Key functions:

•        Detect overload conditions

•        Interrupt the control circuit to stop the motor

•        Provide auxiliary signaling for alarms or indicators

Why 95–96?

•        95 = Common terminal of the NC (Normally Closed) overload relay contact

•        96 = NC terminal

Together, 95–96 form the trip contact. When an overload occurs, this contact opens, breaking the control circuit and stopping the motor.

This is critical because:

•        It ensures the motor is de-energized during overload.

•        It provides a predictable wiring point for technicians.

•        It aligns with IEC/NEMA conventions, making diagrams universal.

Why 97–98?

•        97 = Common terminal of the NO (Normally Open) auxiliary contact

•        98 = NO terminal

This contact closes during an overload, typically used for:

•        Fault indication lamps

•        Alarms or buzzers

•        Sending signals to SCADA/HMI systems

Other Standard Numbers

IEC and NEMA standards also define numbering for contactors:

•        13–14 → NO contacts in contactors

•        21–22 → NC contacts in contactors

This consistency ensures that whether you’re wiring a Schneider relay in India or an Allen-Bradley relay in the US, the diagrams look familiar.

 

Benefits of Standard Numbering

Transparency

Any technician can read the diagram without brand-specific training.

Safety

Reduces wiring mistakes and hazards. Miswiring in motor control circuits can cause costly downtime or dangerous accidents.

Global Practice

Aligns with IEC 60947 and NEMA standards, ensuring interoperability across industries and countries.

Real-World Example

Imagine a multinational plant with equipment from Siemens (Germany), Schneider (France), and Rockwell (USA). Without standardized numbering, each relay would have different terminal assignments. Troubleshooting would be chaotic. Thanks to 95–96 and 97–98, technicians can quickly identify trip and auxiliary contacts regardless of brand.

Educational Perspective

For students learning PLCs and motor control, understanding these numbers is essential. It’s not just about memorization — it’s about recognizing that automation systems rely on global conventions.

Teaching tip:

•        Draw a motor starter circuit with overload relay.

•        Label 95–96 in series with the coil (trip circuit).

•        Label 97–98 connected to a pilot lamp (fault indication).

This simple exercise reinforces the concept of standardization in practice.

Safety & Compliance

IEC 60947 and NEMA standards mandate these numbering conventions. Compliance ensures:

•        Reduced risk of misinterpretation

•        Easier audits and inspections

•        Safer installations across industries

Future Outlook

As industries move toward smart relays and IoT-enabled devices, the numbering convention remains unchanged. Even with advanced diagnostics and communication protocols, 95–96 and 97–98 continue to serve as the backbone of relay wiring.

Conclusion

Overload relays don’t use 95–96 and 97–98 by chance. These numbers are part of a global language of automation, defined by IEC and NEMA standards. They ensure transparency, safety, and interoperability across brands and borders.

So next time you wire an overload relay, remember: 95–96 isn’t random — it’s a universal code that keeps the world of automation connected.

Key Takeaways

95–96 = NC trip contact (opens on overload to stop motor)

97–98 = NO auxiliary contact (closes on overload for fault indication)

Standardization prevents chaos in wiring diagrams

IEC 60947 & NEMA define these conventions globally

Benefits: transparency, safety, global practice

 

Pressure Relief Valve (PRV) vs. Pressure Reducing Valve (PRV): Functions, Operation & Industrial Importance

Pressure Relief Valve (PRV) vs. Pressure Reducing Valve (PRV): Functions, Operation & Industrial Importance

Pressure management is one of the most critical aspects of any hydraulic, pneumatic, steam, or process-fluid system. Whether the application involves boilers, pumps, pipelines, or industrial automation equipment, controlling pressure ensures not just operational efficiency—but more importantly, safety.

Among the most frequently discussed and often misunderstood pressure-control devices are the Pressure Relief Valve (PRV) and the Pressure Reducing Valve (PRV). Though their abbreviations appear similar, their functions, mechanisms, and applications differ dramatically. One protects a system from catastrophic failure, while the other ensures stable downstream pressure for proper functioning of equipment.

This article provides a comprehensive deep dive into the working principles, mechanical construction, flow behaviour, failure modes, and industrial relevance of both valves—supported by the illustration you generated.

 

1. Introduction: Why Pressure Control Matters

Fluid systems, whether hydraulic oil circuits, steam distribution networks, compressed air systems, or water supply pipelines, inherently face pressure variations due to:

• Pump start/stop cycles
• Load fluctuations
• Temperature changes
• Blockages or line restrictions
• Rapid demand changes

If these pressure variations are left unmanaged, the system can suffer from:

• Burst pipes
• Seal failures
• Equipment damage
• Inefficient operation
• Complete system downtime
• Dangerous explosions (especially in boilers and high-pressure steam applications)

This is why two foundational devices—Pressure Relief Valves and Pressure Reducing Valves—are used to maintain safe and stable operating conditions.

 

2. Pressure Relief Valve (PRV): Safety Through Over-Pressure Protection

2.1 What is a Pressure Relief Valve?

A Pressure Relief Valve (also called Safety Valve, Safety Relief Valve, or PSV) is a normally closed protective device designed to open automatically when system pressure exceeds a set limit, allowing excess fluid to vent to atmosphere or return to a tank.

It is a safety component, often mandated by industrial standards, certifications, and legal requirements.

2.2 How It Works

As shown in the illustration, the PRV consists of:

• A spring-loaded poppet or disc
• A sealed valve body
• An inlet connected to the high-pressure line
• An outlet connected to a tank or atmosphere

Under normal operating pressure:

• The spring force keeps the valve fully closed.
• Fluid cannot pass through the valve.

When pressure rises above the preset relief pressure:

• The upward force of the fluid overcomes spring tension.
• The valve lifts, opening a path for the excess fluid to escape.
• Pressure in the system immediately drops.

Once pressure returns to normal, the spring pushes the valve closed again.

2.3 Key Characteristics

Attribute	Description
Function	Safety, protection from overpressure
Normal State	Closed
Action Mode	Opens fully when pressure exceeds setpoint
Fail Mode	Fails open (preferred for safety)
Flow Direction	To tank, drain, or atmosphere
Control Type	ON–OFF, not modulating

 

 

2.4 Applications

PRVs are indispensable in:

• Hydraulic power units
• Boiler systems
• Compressed air receivers
• Steam lines
• Water pipelines
• Chemical processing tanks
• Pressure vessels

Any system capable of generating excessive pressure must include a PRV to avoid damage or explosion.

2.5 Why PRVs Are Critical

A PRV is the last line of defense.
While sensors, transmitters, and PLC-based logic can help regulate pressure, none are foolproof. Electronic systems can fail; a mechanical PRV is required to prevent catastrophic failure.

 

3. Pressure Reducing Valve (PRV): Stable Output Pressure Through Regulation

3.1 What is a Pressure Reducing Valve?

A Pressure Reducing Valve (also abbreviated PRV but better called Pressure Regulating Valve or Pressure Reducing Regulator) is a normally open, modulating valve that reduces high inlet pressure to a consistent, lower outlet pressure.

Its purpose is control, not safety.

3.2 How It Works

From the illustration, the reducing valve operates using:

• A spring and diaphragm/piston assembly
• An inlet for high-pressure fluid
• An adjustable screw or set-spring to set downstream pressure
• An outlet providing reduced pressure

When downstream demand increases:

• Pressure falls → the diaphragm moves → the valve opens more, allowing more fluid.

When downstream pressure rises:

• Pressure pushes the diaphragm → compresses the spring → the valve throttles or closes slightly.

This constant adjustment ensures the downstream pressure remains within a narrow, stable range.

3.3 Key Characteristics

Attribute	Description
Function	Control, maintaining constant downstream pressure
Normal State	Open
Action Mode	Modulating (throttling)
Fail Mode	Fails closed (to avoid overpressure on downstream side)
Flow Direction	Inline (inlet → outlet)
Control Type	Continuous, proportional

 

 

3.4 Applications

Pressure Reducing Valves are widely used in:

• Pneumatic control circuits
• Water distribution systems
• Steam applications
• Hydraulic circuits requiring stable pilot pressure
• Domestic plumbing
• Industrial gas distribution
• HVAC systems

They enable the system to deliver consistent pressure even when the inlet pressure fluctuates.

 

 

 

 

 

 

4. PRV vs Pressure Reducing Valve: A Clear Technical Comparison

Parameter	Pressure Relief Valve (Relief PRV)	Pressure Reducing Valve (Regulating PRV)
Purpose	Safety	Control
Normal Position	Closed	Open
Operates When	Pressure exceeds set limit	Pressure varies downstream
Flow Direction	To tank, drain, or atmosphere	Inline, from inlet to controlled outlet
Operation Type	Snap open (ON/OFF)	Modulating (variable opening)
Fail Mode	Fails open (safe)	Fails closed (protects downstream equipment)
Key Application	Prevents burst/explosion	Ensures constant controlled pressure
Adjustment	Set at one value	Adjusts continuously during operation

This fundamental contrast clarifies that they cannot replace each other. A pressure reducing valve cannot protect against dangerous overpressure. A relief valve cannot stabilize downstream pressure.

 

5. Internal Mechanics and Flow Behavior

5.1 Relief Valve Flow Behavior

When pressure exceeds the setpoint:

• The valve snaps open instantly.
• A large volume of fluid is discharged.
• The system pressure drops rapidly.
• When pressure normalizes, it returns to the closed position.

This rapid response is crucial to safety.

5.2 Reducing Valve Flow Behavior

Reducing valves work in a continual equilibrium between:

• Spring force
• Downstream pressure
• Diaphragm/piston movement

The valve opening continuously adjusts to deliver consistent downstream pressure irrespective of:

• Fluctuating upstream pressure
• Changing demand on the downstream side

This "continuous throttling" is characteristic of regulating devices.

 

6. Failure Modes and Safety Considerations

6.1 Relief Valve (Fails Open)

If the relief valve fails, the safest condition is for it to remain:

• Open

This ensures pressure does not rise uncontrollably. Though it may cause loss of fluid or system shutdown, it avoids catastrophic failure.

6.2 Reducing Valve (Fails Closed)

If the reducing valve fails, the safe condition is for it to remain:

• Closed

This prevents excess pressure from reaching downstream instruments, actuators, and equipment.

 

7. Industry Standards and Codes Related to Valves

Relief Valve Standards

• ASME Boiler and Pressure Vessel Code
• API Standards (API 520, API 521, API 526)
• OSHA pressure safety guidelines

Reducing Valve Standards

• ISO 5208
• ASSE 1003 (water pressure reducing valves)
• Industrial equipment manufacturer specifications

These standards consistently reinforce that relief valves are mandatory safety components.

 

8. Real-World Example Scenarios

8.1 Hydraulic Press System

• Pressure Relief Valve prevents dangerously high pressure if the pump or hydraulic cylinder malfunctions.
• Pressure Reducing Valve is used to supply a lower pressure to a pilot-operated valve or actuator.

8.2 Water Supply System

• Reducing Valve ensures every outlet receives a consistent flow pressure.
• Relief Valve protects the system from sudden peaks, such as pump overrun or line blockage.

8.3 Steam Boiler System

• Relief Valve prevents explosion.
• Reducing Valve supplies lower pressure steam to equipment requiring controlled conditions.

 

9. Choosing the Right Valve

A common engineering mistake is selecting a pressure reducing valve thinking it can also relieve pressure—this is incorrect. When selecting valves:

Choose a Pressure Relief Valve when:

• Overpressure may cause damage or pose danger
• Legal or safety compliance is required
• Protecting pumps, pipelines, pressure vessels

Choose a Pressure Reducing Valve when:

• Constant downstream pressure is required
• Flow fluctuates based on demand
• You need stable process control

Many systems actually require both valves to ensure safe and smooth operation.

 

10. Conclusion

Pressure Relief Valves and Pressure Reducing Valves play fundamentally different roles in fluid systems. While one ensures protection, the other ensures precision control. The relief valve prevents disasters by opening during overpressure, while the reducing valve maintains constant downstream conditions essential for equipment performance.

Understanding their differences is not just beneficial—it is essential for engineers, technicians, and professionals working in hydraulics, pneumatics, water management, steam systems, and industrial automation. The illustration clearly highlights how each valve functions mechanically and how its role fits into an overall system.

A Pressure Relief Valve saves the system;
a Pressure Reducing Valve stabilizes the system.

Both are vital, but neither substitutes the other.