EtherNet/IP - CIP over Industrial Ethernet

EtherNet/IP means EtherNet Industrial Protocol. The slash matters: it is not a generic name for Ethernet or IP. EtherNet/IP is an ODVA protocol that carries CIP (Common Industrial Protocol) over standard Ethernet and IP networks.

It is common in Rockwell/Allen-Bradley automation ecosystems and widely used for PLCs, remote I/O, drives, robots, sensors, and safety devices.

Learning Objectives

By the end of this lesson, you should be able to:

Explain EtherNet/IP as CIP over Ethernet/IP.
Distinguish explicit messaging from implicit I/O messaging.
Identify scanner and adapter roles.
Understand assemblies, objects, and connections.
Compare EtherNet/IP with PROFINET and Modbus TCP.
Recognize commissioning and troubleshooting checks.

Protocol Stack

flowchart TD classDef app fill:#dbeafe,stroke:#2563eb,color:#1e3a5f classDef trans fill:#dcfce7,stroke:#16a34a,color:#14532d classDef ip fill:#fef9c3,stroke:#ca8a04,color:#713f12 classDef eth fill:#fee2e2,stroke:#dc2626,color:#7f1d1d CIP["CIP\nObjects, services, assemblies"]:::app ENIP["EtherNet/IP encapsulation"]:::app TCP["TCP 44818\nExplicit messaging"]:::trans UDP["UDP 2222\nImplicit I/O"]:::trans IP["IP addressing"]:::ip ETH["Ethernet physical and data link"]:::eth CIP --> ENIP ENIP --> TCP --> IP --> ETH ENIP --> UDP --> IP

EtherNet/IP uses standard Ethernet, but the application layer behavior is industrial: cyclic I/O, identity objects, diagnostics, and device profiles.

Scanner and Adapter

Role	Meaning
Scanner	PLC or controller that initiates I/O connections
Adapter	Remote I/O, drive, sensor, or device providing data
Originator	Device that opens a CIP connection
Target	Device that accepts the connection

flowchart LR classDef s fill:#dbeafe,stroke:#2563eb,color:#1e3a5f classDef a fill:#dcfce7,stroke:#16a34a,color:#14532d classDef n fill:#fef9c3,stroke:#ca8a04,color:#713f12 PLC["Scanner\nPLC"]:::s SW["Industrial switch"]:::n IO["Adapter\nRemote I/O"]:::a VFD["Adapter\nVFD"]:::a ROB["Adapter\nRobot"]:::a PLC <--> SW SW <--> IO SW <--> VFD SW <--> ROB

Explicit vs Implicit Messaging

This is the most important EtherNet/IP distinction.

Messaging type	Transport	Purpose	Example
Explicit	TCP port 44818	Request/response, configuration, diagnostics	Read identity object
Implicit I/O	UDP port 2222	Cyclic real-time I/O	Drive status every 10 ms

sequenceDiagram participant PLC as Scanner PLC participant VFD as Drive Adapter PLC->>VFD: Explicit message - read identity VFD-->>PLC: Vendor, product, revision loop Requested Packet Interval PLC->>VFD: Output assembly - control word, speed reference VFD-->>PLC: Input assembly - status word, actual speed end

The Requested Packet Interval (RPI) defines how often cyclic I/O packets are produced.

CIP Objects and Assemblies

CIP models devices as objects. Each object has classes, instances, attributes, and services.

CIP concept	Meaning
Class	Object type, such as Identity or Assembly
Instance	Specific object instance
Attribute	Data field inside the object
Service	Operation such as get or set attribute
Assembly	Grouped I/O data exchanged cyclically

Example drive assembly:

Direction	Bytes	Data
Output assembly	4 bytes	control word, speed reference
Input assembly	4 bytes	status word, actual speed

Worked Example - RPI and Network Load

A remote I/O adapter sends 32 input bytes and receives 16 output bytes every 10 ms.

Packets per second = 1 / 0.010 = 100 packets/s each direction
Input payload rate = 32 bytes x 100 = 3200 bytes/s
Output payload rate = 16 bytes x 100 = 1600 bytes/s

The raw payload is small. The engineering question is whether the PLC, switch, and device can handle the total packet rate for all adapters, especially with multicast traffic.

EtherNet/IP vs PROFINET

Feature	EtherNet/IP	PROFINET
Organization	ODVA	PI
Core model	CIP objects and assemblies	Slots, subslots, records
Cyclic I/O	Implicit UDP messaging	PROFINET RT/IRT
Configuration file	EDS	GSDML
Common ecosystem	Rockwell/Allen-Bradley	Siemens
Time-critical motion	CIP Motion / CIP Sync	PROFINET IRT

Both are Industrial Ethernet protocols, not replacements for TCP/IP itself.

Practical Checks

Install the correct EDS file or vendor profile.
Confirm IP address, subnet mask, and gateway.
Check assembly instance numbers and data sizes.
Set RPI according to process need, not as fast as possible.
Watch multicast behavior; use IGMP snooping where required.
Verify switch quality, grounding, cable category, and industrial connectors.
Check connection timeout settings and device firmware compatibility.

Common Mistakes

Saying "Ethernet IP" when the intended protocol is EtherNet/IP.
Confusing explicit TCP messages with cyclic implicit I/O.
Choosing overly aggressive RPI values.
Ignoring multicast and switch configuration.
Using the wrong EDS file or assembly size.
Assuming office network troubleshooting is enough for real-time I/O behavior.

Summary

EtherNet/IP carries CIP objects and I/O assemblies over standard Ethernet/IP networks. Explicit messaging handles configuration and diagnostics; implicit messaging handles cyclic real-time I/O. The main practical checks are EDS/profile match, assembly size, RPI, IP addressing, switch behavior, and device compatibility.

Mind Map

mindmap root((EtherNet/IP)) Core concept CIP over Ethernet Objects and assemblies Scanner to adapter Applications Remote I/O Drives Robots Safety devices Calculations RPI sets packet rate packets/s = 1/RPI payload = bytes*rate Design rules Correct EDS Match assembly size Manage multicast Practical checks IP settings RPI load Connection timeout Common mistakes Wrong assembly RPI too fast No IGMP planning