This section considers the network loads generated by the PROFINET devices. It starts with a description of the structure of the PROFINET transmission cycle, fol-lowed by an explanation of the impact that the update time has on the PROFINET network load.
Planning of update times
Figure 5-1 shows the general structure of the PROFINET transmission cycle.
Figure 5-1: Structure of PROFINET transmission cycle
This phase corresponds to the minimum transmission clock of a transmission cycle.
It is determined by the PROFINET device that requires the fastest update time. In the example, the minimum transmission clock has been determined at 1 ms since PROFINET device 1 (blue) requires this update time.
You should consider the PROFINET network load especially if the application requires very short response times.
The update time only refers to the communication clock on the PROFINET network and does not include the additional internal processing time of the network nodes (e.g.
PROFINET device and controller etc.) Transmission cycle
Phase 2 Phase 3 Phase 4
"n+1"
Phase 1
"n-1"
Performance Consideration
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ple, this is PROFINET device 3 (red) requiring an update time of 4 ms. PROFINET device 2 (green) requires an update time of 2 ms.
The engineering tool provides the configuration of the clock parameters for control of the PROFINET transmission cycle. This engineering tool uses preset data to calculate a number of clock factors which are then transmitted to the PROFINET device.
The minmimum (fastest) transmission cycle period is given by:
TPhase = SendClock Factor * Base clock
Where the base clock is 31.25 µs. After deciding upon the fastest requirement for the transmission cycle clock period, the required SendClock factor can be deter-mined. Each individual PROIFINET device will have its own requirement for trans-mission clock cycle which is always an integer multiple of the minimum transtrans-mission cycle period.
TCycle = ReductionRatio * SendClock-Faktor * Base clock
The ReductionRatio acts as multiplier of the minimum transmission clock. In the above example the minimum transmission clock of 1 ms is achieved by using a SendClock factor of 32. While PROFINET device 1 (blue) requires a ReductionRa-tio of 1 using a transmission clock of 1 ms, PROFINET device 2 (green) requires a ReductionRatio of 2 and PROFINET device 3 (red) a ReductionRatio of 4.
Determine the update times for your PROFINET devices.
Make sure to adjust the update times to the relevant task.
You should always configure the update times according to the process requirements even if the bus system allows for much shorter update times. This reduces the PROFINET network load and the load of the PROFINET controller to the required extent.
Performance Consideration
Impact of update times on the PROFINET network load
The specific configuration of update times has a major impact on the PROFINET network load. Figure 5-2 shows that all data streams meet at certain locations. The access to the controller is therefore typically the link with the highest network load.
Figure 5-2: Weaknesses caused by high network load
Table 5-1 illustrates the impact of update times on the PROFINET network load which has been determined by means of the network load calculation tool.
To determine the network load, use the network load calcula-tion tool which is available at
www.profinet.com
for download. You will find an overview of the user interface as well as a short user manual in Annex 9.9 of this Design Guideline.
In many cases, engineering tools also provide a function for the calculation of the network load in networks.
Performance Consideration
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Update time 50 devices 100 devices 150 devices
8 ms 6 % 12 % 18 %
4 ms 12 % 24 % 36 %
2 ms 24 % 48 % 73 %
Table 5-1: PROFINET network load in percent
For this, the update time of all PROFINET devices have been changed and defined as identical each for input and output. It can be seen clearly that the network load can assume a high volume in case of a certain configuration.
The percentage network load refers to a network speed of 100 MBit, for the link which sums up all the network traffic (Connection to the PROFINET IO controller).
The PROFINET network load rises almost linearly with the number of PROFINET devices and with the transmission clock.
The network load generated by PROFINET should always be below 50%.
Transmit and receive direction are independent of each other.
In practice it is sufficient to analyze the direction which carries the larger data volume.
Based on the communication relations you should check which data streams meet at which locations.
Avoid high network loads and optimize the update times of the PROFINET devices.
Mutual disturbances of the device communication may occur in case of high network loads. However this does not concern the communication according to CC-C.
Performance Consideration
Involvement of transmission times
For extensive automation plants it makes sense also to consider the transmission times. The transmission link will introduce a small delay in the transmission in each direction which depends upon the medium length. Depending on the length of the medium used this delay can often be neglected compared to the processing time in a switch.
The switches as well as the integrated switches in PROFINET devices have the major impact on the transmission time. The switches are basically differentiated in two categories.
“Store and Forward” “Cut Through”
Buffering of the entire data packet, check, reading of address and sending to addressee in case the data packet is correct. The routing delay depends on the telegram length.
Reading of address and direct for-warding of message to the relevant address without checking the data packet. The forwarding delay is not depending on the telegram length.
Table 5-2: Comparison of “Store and Forward” and “Cut Through”
Switches operating according to the “Store and Forward” principle usually have longer processing times than those operating according to the “Cut Through“ princi-ple.
In long line topologies it may make sense to use a switch with cut through. Figure 5-3 shows that the processing time of a line topology increases with the length of the line.
1 2 3 1 1
Performance Consideration
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The line topology can produce a significant delay for telegrams to and from the last device. To minimize the switch delay time, a star topology can be used. The proc-essing time is thus identical for all nodes.
The left part of the figure shows a line topology based on integrated switches. A data packet from the controller to the last PROFINET device runs through a switch several times, thus being delayed. The right part of the figure shows a star topology where each packet is directly routed to its target. The delay therefore is minimal.
The decisive factor is the ratio of update time and processing time. The processing time must be shorter than the update time. If this is not the case, you should either modify the topology or use Cut Through switches. Figure 5-4 shows the possibility to adjust the network topology. Here, segmenting of the line changes the line depth and thus the processing time.
Figure 5-4: Adjustment of line depth
You will find details about processing times in the manufac-turer documentation. You should check whether these meet your specifications.
The processing times of a switch as well as the data packet sizes transmitted along the line determine the maximum line depth. Follow the manufacturer instructions to determine the maximum line depth.
1 1
1 2 3
Performance Consideration