August 11, 2026 · Comparison

CIP Explicit vs Implicit Messaging: When to Use Each

By Dzulfikar Ats Tsauri, founder of Voltrus / JoyoDigitama. He builds the protocol tools covered in these posts. Updated October 28, 2026.

Once you get past the EtherNet/IP encapsulation layer, every CIP message is one of two types: explicit or implicit. They use different transports, different ports, and exist for different reasons. Confuse them and you will be debugging a controller that "cannot see its I/O" while tag reads work fine, or vice versa.

This guide compares the two and tells you when each is appropriate. For the surrounding protocol structure, see the CIP protocol guide.

The Core Distinction

Explicit messaging spells out exactly what it wants on every message: the service code, object path, and attribute. It is request-response and flexible, but carries addressing overhead on every frame. It runs over TCP 44818.

Implicit messaging sets up a connection once (via ForwardOpen), then exchanges data at a fixed rate with no per-message addressing: the meaning of each byte is "implied" by the connection configuration. It is fast and deterministic, ideal for cyclic I/O. It runs over UDP 2222.

Feature Comparison

When to Use Explicit Messaging

• Reading and writing tags on a Logix controller: service 0x4C (Read Tag), 0x4D (Write Tag). See reading tags without RSLinx.
• Browsing the tag database and reading device identity or diagnostics.
• SCADA and monitoring tools that poll on demand rather than at a fixed cycle.
• Configuration and setup operations on any CIP object.

Explicit messaging is what almost every non-Rockwell tool (SCADA drivers, data loggers, custom apps) uses, because it does not require the connection bookkeeping of implicit messaging.

When to Use Implicit Messaging

• Real-time I/O between a Logix controller and remote I/O (POINT I/O, ArmorBlock, etc.).
• Drive control: speed reference and start/stop to a PowerFlex drive on a fixed cycle.
• Safety I/O and any data that must update at a deterministic rate.

Implicit connections are configured in Logix Designer (Studio 5000) when you add a module to the controller tree and set its RPI. The controller handles ForwardOpen and the cyclic exchange; you never write implicit-messaging code yourself in normal project work.

Why the Split Exists

It is a latency/overhead trade-off. Explicit messaging carries enough metadata to be self-describing, which is exactly what you want for occasional reads against a large, changing data set. Implicit messaging strips that metadata to recover the bandwidth and determinism that real-time I/O demands, at the cost of pre-configuration. Real Allen-Bradley systems run both at once: implicit for the I/O rack, explicit for the HMI and SCADA.

Further Reading

• EtherNet/IP Port 44818 Explained
• EtherNet/IP vs PROFINET