4
Technical data
4.4
Internal switch latency
18
Lenze · Inverter Drive 8400 motec (PROFINET® option) · DMS 5.0 EN · 02/2019 · TD23
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4.4
Internal switch latency
The integrated 2-port switch causes runtime delays which can be calculated as follows:
Example :
20 process data words => 40 bytes
• ((36 permanent bytes + 40 bytes) x 8 x 10 nsec) + 4
μ
sec
• (76 bytes x 8 x 10 nsec) + 4
μ
sec
• 6.08
μ
sec + 4
μ
sec =
10.08
μ
sec
In accordance with the PROFINET specification, the shortest PROFINET I/O frame must have a data
length of 72 bytes. If the 36 permanent bytes are subtracted from the 72 bytes, 36 bytes are
available for process data. If now less than 36 bytes of process data are used, the PROFINET I/O
frame is filled with "zero bytes" until it can be transmitted. As a consequence for the calculation
formula, the shortest PROFINET I/O frame with 18 process data words (36 bytes) has always the
same length and thus the runtime delay is the same, too.
Runtime delay = ((36 permanent bytes + process data in bytes) x 8 x 10 nsec) + 4
μ
sec
Note!
The use of external switches can also lead to runtime delays. Depending on the system
constellation, it may be useful to create a star topology or a line/mix topology.
