Image Processing Reference
In-Depth Information
timing behavior, special care has to be taken in the RTE equipment. Sufficient computing
resources are needed to handle the TCP/IP protocol in real-time or the protocol stack
must be simplified and reduced to get these reaction times on simple, cheap resources.
•
Third and most demanding class is defined by the requirements of motion control: To
synchronize several axes over a network, a time precision well below ms is needed. Cur-
rent approaches to reach this goal rely on modifications of both protocol medium access
and hardware structure of the controllers.
During the last few years, a number of industrial solutions appeared that tackled the real-time
requirements, mostly on the basis of switched Ethernet. Still, as with fieldbus systems, they were
tailored to specific needs. Not even the use of standard Ethernet is really a common denomi-
nator, and above the data link layer, the approaches are completely different. Some use standard
TCP/UDP/IP mechanisms for transmitting data, maybe enhanced by additional software layers to
support both real-time and non-real-time communication, and some use dedicated communication
stacks that bypass the entire IP suite. Figure . sketches the various appearances of the protocol
stack. Manifold differences are also possible on the physical layer. Some approaches foresee redun-
dant media (VNET/IP, TCnet), PROFINET-I/O uses dedicated built-in switches to reduce the data
transmission jitter [], and EtherCAT [] as well as SERCOS III [] need dedicated controllers.
Ethernet Powerlink uses the old shared Ethernet and places a master-slave scheduling system on top
of it []. Common to many proposed networks is that they employ clock synchronization to support
real-time applications. To this end, the recent standard IEEE [], which originally emerged in
theinstrumentationarea,wasoiciallyadoptedalsobyIEC.hespeciicrequirementsintheautoma-
tion domain have led to several suggestions for improvement of the standard [,], and work on
the next revision was already started.
Profiles
Function blocks
OPC
Clock sync.
CIP
Mod-
bus
RTPS
(IDA)
VNET/
IP
EPA
PROFINET
CBA
TCnet
Ether-
CAT
EPL
I/O
Encaps.
RDP
DCOM
Empty layers
UDP
TCP
IP
Scheduling extensions
Ethernet DLL/MAC
Red.
Red., Switches, Sync.
Ethernet PHY
CIP:
Control and information protocol
Encaps. Encapsulation layer
EPL:
RTPS:
Real-time publisher-subscriber
Ethernet powerlink
EPA:
Ethernet for plant automation
Red.
Redundant medium
RDP:
Realtime-reliable datagram protocol
Non-standard extensions
CBA:
Component-based automation
FIGURE .
Protocol architecture of selected RTE solutions proposed for IEC -.
