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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.

Connections, Ownership, and Timeouts

EtherNet/IP cyclic I/O is connection-oriented at the CIP layer. A scanner opens a connection to an adapter with the expected assembly instances, data sizes, RPI, and connection type. If those do not match the adapter configuration, the connection fails or enters a faulted state.

Item What to verify
Input assembly Instance number and byte count from adapter to scanner
Output assembly Instance number and byte count from scanner to adapter
Configuration assembly Required setup bytes, if the device uses them
RPI Packet interval supported by both sides
Connection type Unicast or multicast behavior
Timeout multiplier Fault reaction after missed packets

A drive can respond to explicit identity reads and still fail cyclic I/O if the configured assembly or RPI is wrong. Treat explicit connectivity as a starting check, not proof that the real-time connection is healthy.

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.

Multicast and Switch Planning

Many EtherNet/IP systems can use unicast I/O, but multicast still appears in existing designs and some device profiles. Without IGMP snooping and a querier where required, multicast traffic may be flooded to ports that do not need it.

Approximate packets/s per adapter = 2 / RPI
For 40 adapters at 10 ms: 40 x 2 / 0.010 = 8000 packets/s

The payload rate may look small, but packet rate, multicast flooding, controller connection limits, and switch buffering decide whether the cell remains stable during faults and recovery.

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.
  • Confirm controller connection limits before adding many adapters.
  • Use device web pages or diagnostics to compare configured and actual assemblies.

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.
  • Ignoring unicast versus multicast settings when replacing adapters or switches.

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.

Further Reading

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 adapter packets/s = 2/RPI payload = bytes*rate Design rules Correct EDS Match assembly size Manage multicast Check connection limits Practical checks IP settings RPI load Connection timeout Common mistakes Wrong assembly RPI too fast No IGMP planning