Image Processing Reference
In-Depth Information
21.4.1.1 Modbus/TCP (Profiles 15/1 and 15/2)
Modbus/TCP, defined by Schneider Electric uses the well-known Modbus
∗
over a TCP/IP network
[], using port and defined as profile /. This is probably one of the most widely used Eth-
ernet solutions in industrial applications today and fulfills the requirements of the lowest class of
applications which we called human control.
Modbus is a request/reply protocol (send a request frame and get back a reply frame) and offers
services specified by function codes to read or write data objects. hese data objects can be discrete
inputs, coils,
†
input registers, or holding registers. In fact, this protocol is very simple and the actual
definition must be extended with service definitions for the integration in international standards.
In additional to the historical Modbus protocol, new RT extensions have been defined as profile
/. These RT extensions use the RT publisher-subscriber (RTPS) protocol []. The RTPS pro-
tocol provides two main communication models: the publish-subscribe protocol, which transfers
data from publishers to subscribers; and the composite state transfer protocol, which transfers state
information from a writer to a reader.
IntheCTSprotocol,aCTSwriterpublishesstateinformationasavariablewhichissubscribed
by the CTS readers. The user data transmitted in the RTPS protocol from the publisher to one or
several subscribers is called an issue. The attributes of the publication service object describe the
contents(thetopic),thetypeoftheissue,andthequality(e.g.,timeinterval)ofthestreamofissues
that is published on the network. A subscriber defines a minimum separation time between two
consecutive issues. It deines the maximum rate at which the subscription is prepared to receive issues.
The persistence indicates how long the issue is valid. The strength is the precedence of the issue sent
by the publication. Strength and persistence allow the receiver to arbitrate if issues are received from
several matching publications. Publication relation may be best effort (as fast as possible but not
faster as the minimum separation), or strict. In the case of the strict publisher-subscriber relation,
the timing is ensured with a heartbeat message sent from the publisher to the subscriber (exact timing
is middleware dependent) and a replied acknowledge message. he RTPS protocol is designed to run
over an unreliable transport such as UDP/IP and a message is the contents (payload) of exactly one
UDP/IP datagram.
In the standard, any concrete indication for values for the performance indicators is missing. hey
depend very strongly on the performance and implementation of the UDP/IP communication stack.
So it is not possible to define an implementation independent message delivery time, for instance.
21.4.1.2 EtherNet/IP (Profiles 2/2 and 2/2.1)
EtherNet/IP,
‡
defined by Rockwell and supported by the Open DeviceNet Vendor Association
(ODVA, see www.odva.org) and ControlNet International (see www.controlnet.org), makes use of
the common interface protocol (CIP) which is common to the following networks: EtherNet/IP,
ControlNet, and DeviceNet [].
The EtherNet/IP communication technology, standardized in IEC - as Profile / (using
type specifications in IEC ), already provides ISO/IEC - based RT communica-
tion. In full-duplex switched Ethernet, there is no possibility to get delays due to collisions. But
in the switching device, Ethernet frames may be delayed, if an output port is busy with the
∗
Industrial de facto standard since .
†
In Modbus, for the representation of binary values, the term coil is used. his is originating from the ladder-logic where
the coil of a relay is used to store binary information.
‡
EtherNet/IP
TM
is a trade name of ControlNet International, Ltd. and Open DeviceNet Vendor Association, Inc. IP stands
here for Industrial Protocol.
