EtherCAT Protocol Implementation Issues
on an Embedded Linux Platform
Sorin Potra
LVD-Napomar
Bd-ul Muncii, nr. 14, Cluj-Napoca,
România
Sorin.Potra@lvdnapomar.ro Gheorghe Sebestyen
Technical University of Cluj-Napoca
G. Bariţiu, Nr. 26-28, Cluj-Napoca,
România
Gheorghe.Sebestyen@cs.utcluj.ro
Abstract – The paper presents the most important
implementation issues of an industrial communication protocol, the EtherCat protocol, on an embedded Linux platform. The authors underscore critical aspects (e.g. reliability, timeliness, predictability) concerning the use of an Ethernet-like protocol in an industrial environment.
I. INTRODUCTION
Today, the use of industrial communication networks is
mandatory in most automation systems [8]. Industrial
protocols are specially designed to fulfill the specific
communication requirements of control applications, such as:
real-time data delivery, high reliability and dependability,
priority-based messaging, etc. [6][7].
An industrial network assures the communication
environment between different automation devices, from the
simplest ones, like intelligent sensors and actuators, until the
more complex, process computers. Figure 1 shows a
hierarchical automation system where industrial networks are
the links between the system's components (sensors – S,
actuators – A, Programmable Logic controllers – PLC, and
industrial PCs).
The implementation of an industrial protocol involves a
number of issues [6]:
- the protocol must be developed on systems (e.g.
intelligent sensors or actuators) with limited hardware
and software resources; such limitations are: small
memory capacity, limited processor speed, few or no
operating system support
- time restrictions and message delivery deadlines must
be guaranteed
Controlled process Ind. PC Ind. PC
PLC PLC PLC Industrial
networks
S S SS AA
Figure 1. A network-based automation system
- the roundtrip time of a request-answer message pair is
much smaller (usually microseconds) than in the case
of usual networks;
- the protocol drivers must have a highly predictable
behavior; for instance, uncontrolled delays caused by
message queues are not allowed
- automatic fault recovery mechanisms must be
included in the protocol drivers
The following chapters present the way in which these
issues were solved during the implementation of an industrial
protocol, namely the EtherCAT.
II. BRIF DESCRIPTION OF THE ETHERCAT PROTOCOL
In the last decade there were a number of attempts to adapt
general-purpose computer protocols (e.g. Ethernet, TCP/IP)
[4][5] for industrial purposes. EtherCAT (Ethernet Control
Automation Technology) is a relatively new industrial
protocol built on the Ethernet specifications; it incorporates
some new features that make it adequate for control
applications. This protocol solves the compatibility gap
between an industrial protocol and a computer network
protocol.
The EtherCAT combines the efficient and relatively high-
speed message transmission (specific for Ethernet networks),
with the predictability imposed by a master/slave medium
access control policy. This access policy works in the
following way: there is a single master node on a network
segment that has the right to initiate data transfers; this node
sends an Ethernet frame to the slave nodes; a slave node
extracts the data from the frame addressed to it, puts some
new data in the frame and than sends the frame to the next
slave; the frame arrives back to the master node confirming
the correctness of the transmission. All the message
reception, data processing and frame retransmission
operations are made "on the fly" by the slave nodes, without
any extra delays. Special hardware components, embedded in
the slave's Ethernet interface, are responsible for these
operations. This solution assures a minimum roundtrip
(reaction time), better than in the case of other industrial
protocols (e.g. CAN, Profibus, etc.).
Figure 2 shows a typical EtherCAT segment, with one
master node (a data acquisition, control and supervision
device) and a number of slave nodes (intelligent sensors and
actuators, PLCs, etc.).
1-4244-0361-8/06/$20.00 ©2006 IEEE
Figure 2. A Master/Slave EtherCAT network segment
In order to farther improve the data transmission efficiency
the protocol allows data addressing at bit level. Logical
addresses are allocated to every slave node inside of a single
Ethernet frame. When a frame arrives to a slave, the node
will take the data addressed to it (control information to the
process) and in the same time it will put its own data (process
parameter values) on the frame. So, data is transferred