Implicit vs Explicit Messaging in CIP
EtherNet/IP moves data two very different ways, and confusing them is behind a lot of design missteps. Cyclic I/O and one-shot reads are not the same mechanism, do not use the same transport, and are not tuned the same way. This page compares implicit and explicit CIP messaging side by side - what each is for, which transport it rides, and when to reach for one over the other.
Implicit vs Explicit Messaging in one line: Implicit messaging is cyclic I/O: pre-negotiated data blocks exchanged automatically every RPI over UDP, with no request or response per packet. Explicit messaging is request-response: a message names a specific object path and service, gets an answer, and typically rides TCP. Implicit moves real-time I/O; explicit reads or writes anything else on demand.
The Difference in One Table
The two mechanisms differ on almost every axis, and lining them up makes the choice obvious for any given need.
| Attribute | Implicit (I/O) messaging | Explicit messaging |
|---|---|---|
| Purpose | Real-time cyclic I/O | On-demand read or write of any object |
| Pattern | Producer/consumer, no per-packet request | Request and response |
| Transport | UDP (port 2222) | TCP (port 44818) |
| CIP connection class | Class 0 or Class 1 | Class 3, or unconnected UCMM |
| Timing | Fixed by the RPI | Whenever the application sends it |
| Data | Pre-agreed assembly blocks | Any class/instance/attribute path |
| Overhead per exchange | Minimal, negotiated once | Full path and service each time |
The headline is that implicit messaging trades flexibility for speed - it moves a fixed block extremely efficiently but only that block - while explicit messaging trades speed for reach, letting you touch any attribute at the cost of per-message overhead.
Both are described in the EtherNet/IP overview, but the distinction is worth isolating because engineers routinely try to use the wrong one - polling a value with explicit messages that should have been in the I/O assembly, or expecting cyclic behavior from a MSG instruction that fires only when the rung tells it to.
When Implicit Messaging Is Right
Implicit messaging is for data that must move continuously and predictably: the I/O image between a controller and its remote racks, drives, and sensors. Because the connection is negotiated once and then runs automatically, there is no per-packet request overhead, and the data arrives on a fixed cadence set by the requested packet interval. This is what makes EtherNet/IP viable for time-critical control - the process variables and output commands that the control loop depends on ride the implicit connection.
The cost of that efficiency is rigidity. An implicit connection carries exactly the assembly it was opened with, no more. If you need a diagnostic counter or a rarely changed configuration value, putting it in the cyclic I/O wastes bandwidth on data that almost never changes. The discipline is to reserve implicit I/O for the fast-changing, control-relevant data and leave everything else to explicit reads. How to size the cadence itself is covered in choosing an RPI for EtherNet/IP I/O.
When Explicit Messaging Is Right
Explicit messaging is for everything that does not need to move every cycle: reading a device's diagnostic object once a minute, writing a configuration parameter during commissioning, pulling an event log, or fetching an identity attribute. Each explicit message names its target - class, instance, attribute - and its service, so it can reach any object in the device, including ones no assembly exposes. In a controller this is the MSG instruction, fired on a condition or a timer rather than continuously.
The trade-off runs the other way. Because each explicit message carries a full path and expects a response, sending them at high rates loads the device and the network far more than the equivalent cyclic I/O would. Explicit messages are also asynchronous to the scan - they complete when the response arrives, not on a fixed cadence - so control logic must not assume a fresh value is present every scan. Used for its intended purpose, on-demand access to occasional data, explicit messaging is exactly right; abused as a polling loop for fast data, it is the wrong tool.
Pitfalls That Mix the Two Up
The most common mistake is polling a fast process value with a rung of MSG instructions because the value did not make it into the I/O assembly. This works at low rates and then collapses under load, because explicit traffic does not scale like cyclic I/O. The fix is to get the value into the assembly if it must be real-time, or accept that explicit access means occasional, not continuous.
The mirror-image mistake is expecting a MSG instruction to behave cyclically. A MSG executes when its rung is true and its previous instance has finished, not on a fixed interval, so building timing-sensitive logic on its completion is fragile. When a value genuinely needs deterministic timing, it belongs in an implicit connection with an RPI, not in an explicit message. A monitoring platform such as Merobix sees only the named tags the controller ends up with, so getting this split right at the controller is what makes the SCADA data both fresh where it must be and lightweight where it can be.
Frequently Asked Questions
What is the difference between implicit and explicit messaging?
Implicit messaging is cyclic I/O: a pre-negotiated data block exchanged automatically every RPI over UDP, with no request per packet. Explicit messaging is request-response: a message names a specific object path and service and gets an answer, usually over TCP. Implicit carries real-time I/O; explicit reads or writes any object on demand.
Does implicit messaging use TCP or UDP?
Implicit I/O messaging uses UDP, on port 2222, because cyclic real-time data benefits from low overhead and does not need TCP's retransmission - a late I/O packet is worse than a missing one. Explicit messaging uses TCP on port 44818, where reliable delivery of an occasional request-response matters more than minimal latency.
When should I use explicit messaging instead of I/O?
Use explicit messaging for data that does not need to move every cycle: reading diagnostics or event logs, writing configuration during commissioning, or fetching an identity attribute. It reaches any object in the device but carries per-message overhead, so it is wrong for fast, continuous values - those belong in the cyclic implicit I/O connection instead.
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