Real-Time Ethernet for Automation Applications - Networked Embedded Systems - page 538

TABLE . Protocol Types for Different RTE Profiles
Defined in IEC 
IEC  Profile Brand Name Protocol Type

Family  PROFIBUS/PROFINET x

Family  TCnet xB

Family  EtherCAT xA

Family  EPL xAB

Family  EPA xBC

Family  SERCOS xCD

for IP is Ethertype x. These RTE protocols do use, beside the standard IP protocol stack,

their own protocol stack identified with their own protocol type. Table . lists the different values
assigned to this Ethertype for these protocols.

=

21.4.2.1 Ethernet Powerlink (Profile 13/1)

Ethernet Powerlink (EPL) was defined by Bernecker and Rainer, and is now supported by
the Ethernet Powerlink Standardisation Group (see www.ethernet-powerlink.org).
Itisbasedontheprincipleofusingamaster-slaveschedulingsystemonasharedEthernetseg-
ment called Slot Communication Network Management (SCNM). he master, called managing node
(MN), ensures RT access to the cyclic data and lets NRT TCP/IP frame pass through only in time slots
reserved for this purpose. All other nodes are called controlled nodes (CN) and are only allowed to
sendonrequestbytheMN.heMNsendsamulticastStart-of-Cycle(SoC)frametosignalthebegin-
ningofacycle.hesendandreceivetimeofthisframeisthebasisforthecommontimingofallthe
nodes. It is important to keep the start time of an EPL cycle as exact (jitter-free) as possible. he fol-
lowing time periods exist within one cycle: start period, isochronous ∗ period, asynchronous † period,
and an additional idle period. he length of individual periods can vary within the preset period of
an EPL cycle. In the isochronous period of the cycle, a Poll-Request (PReq) frame is sent unicast to
every configured and active node. he accessed node responds with a multicast Poll-Response (PRes)
frame.Intheasynchronousperiodofthecycle,accesstotheEPLnetworksegmentmaybegrantedto
one CN or to the MN for the transfer of a single asynchronous message only. he preferred protocol
for asynchronous messages is UDP/IP. he Start-of-Asynchronous (SoA) frame is the first frame in
the asynchronous period and is a signal for all CNs that all isochronous data has been exchanged
during the isochronous period (compare also Figure .). .).Thus the transmission of isochronous and
asynchronous data will never interfere and precise communication timing is guaranteed.
An EPL network is a “protected Ethernet” deined with one controller acting as the MN and several
field devices implemented as CNs. In order to protect the SCNM access mechanism of the MN, non-
EPLnodesarenotpermittedwithintheprotectedEthernetitself,astheywouldcorrupttheSCNM
access mechanism.
Messages exchanged between MN of diferent protected Ethernet segments are synchronized based
on distributed clock. With the IEEE  [] protocol in every MN, a clock is synchronized and the
messages between the different networks are sent based on the synchronized time in the MNs. he
MN includes the routing functionality, including the IP address translation from the network to the
outside world. With this synchronization mechanism, RTE communication is also possible among
different networks. Performance indicators for a small- and a large-size automation system within a
protected Ethernet are listed in Table ..

∗ From Latin for iso

time based, so communication at the same time interval.
† Asynchronous is without any synchronization to a reference.

= thesameandchronous =