wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

40-10 Industrial Communication Systems
which communication can be implemented flexibly. It only has to be ensured that all IRT frames can be sent
within the “orange interval.” The synchronized “send cycle” causes all nodes participating in IRT communication
to start transmitting I/O data at the start of the “orange interval.” As a result, all possible wait times
are reduced to a minimum (same as in unsynchronized communication), whereas the I/O traffic is secured
against other data traffic. The data frames are transmitted to the end node via the respective destination port
solely on the basis of their MAC address (and corresponding Frame_ID). This enables a very flexible method
of communication that is not subject to any special rules. Changes in the plant topology have no effect here.
However, based on the concept, the enhanced flexibility and resulting ease of adaptability of a system is made
possible at the expense of incomplete optimization of the bandwidth utilization because a small reserve is
provided for in the “orange interval.” This ensures that all frames have been sent. Frames within the “orange
interval” can be transmitted with or without VLAN tag.
40.3.2 Communication Based on RT_CLASS_3 (Red Interval)
In this case, the communication in the “red interval” is based on a schedule configured in advance (during
engineering phase), i.e., in addition to the information for the end nodes, the network components
located in between require information defining the forwarding of frames. Frames are forwarded based
exclusively on the planning algorithm defined in IEC 61158. As a result of planning, an Ethernet controller
(or more precisely, the integrated switch in an Ethernet controller) knows exactly which frame
arrives at which port and when it must be forwarded to where. This enables a very high utilization
of the bandwidth available. If a system requires RT_CLASS_3 communication, the bus cycle must be
divided into a “red interval” and a UDP/IP part (green interval) during engineering. Here, the timing
and length of each frame to be sent is specified on a port-by-port basis. The plant topology, the respective
frame length, and the cable lengths between the individual nodes are critical factors in the timing for
the purpose of its optimal utilization. If the system is changed, the planning algorithm must therefore
be repeated. Data transmission in the IRT portion is always scheduled. The “schedule” is geared only
to the sequence of arriving frames, which is determined by their Frame_ID and the frame length. The
time-controlled processing of jobs within the “red interval” aids in eliminating the final sources of inaccuracies.
Since RT_CLASS_3 communication is oriented only on the basis of timing, the throughput
times of data frames through a switch are significantly shorter. Likewise, the performance in branched
networks can be increased by optimized use planning of the same communication path. The topology
information is sent to the respective IO controller during system power-up. RT_CLASS_3 frames are
always sent without VLAN tag since the chronological position is always known.
40.3.3 Cycle Duration and Constrains
The maximum frame length in Ethernet/PROFINET yields a minimum duration of the “green interval” of
125.μs (4 × 31.25.μs). However, certain rules must be defined in order to carry out efficient data communication
involving different RT classes, ensuring that the timing and the isochronous operation are adhered to in
every configuration. The general rule is that at least two TCP/IP frames with maximum length can be sent
per millisecond. This corresponds to a transmission time of approximately 250.μs. For bus cycles