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Rockwell Automation Publication 842E-UM001C-EN-P - September 2016
Chapter 3
EtherNet/IP Overview
The encoder is an I/O adapter in the EtherNet/IP. It receives and sends explicit
and implicit messages either cyclic or on request (polled).
TCP/IP and UDP/IP
EtherNet/IP uses TCP/IP or UDP/IP for communication. (TCP is
transmission control protocol and UDP is user datagram protocol.)
Implicit messaging is used for real-time communication between a
programmable logic controller (PLC) and the encoder in EtherNet/IP. With
implicit messaging a connection is established between exactly two devices
within the CIP protocol. Implicit messaging uses UDP/IP via port 2222.
Explicit Messaging is used in EtherNet/IP for communication that does not
need to take place in real time. Explicit Messaging uses TCP/IP; it is used, for
example, to transfer parameters from the PLC to the encoder.
MAC ID
Devices that originate or use data on the network have factory-assigned media
access control (MAC) addresses for unique identification. The MAC ID
(MAC ID) consists of 6 bytes. The first three bytes identify the manufacturer.
The last three bytes are unique to the device. An example of a MAC ID is
00:00:BC:C9: D7:14.
Communication Frame
EtherNet/IP is based on the standard Ethernet frame. The frame contains the
Ethernet header, the Ethernet data, and the Ethernet trailer. The MAC IDs of
the receiver (destination address) and of the source (source address) are
contained in the Ethernet header.
Figure 2
Transmission sequence
Data field
Trailer
Header
Destination
Address
Source
Address
46...1500 Byte
