Lenze E84DxxxxxxxxxR - Manual (Page 15)

EDS84DPNET EN 4.0 - 11/2010
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15
Communication manual 8400 protec PROFINET
Technical data
Internal switch latency
3.4 Internal switch latency
The integrated 2 port switch causes runtime delays which can be calculated as follows:
Example:
20 process data words + 4 PROFIsafe words => 48 bytes
 ((36 permanent bytes + 48 bytes) x 8 x 10 nsec) + 4 μsec
 (84 bytes x 8 x 10 nsec) + 4 μsec
 6.72 μsec + 4 μsec = 10.72 μsec
According to the PROFINET specification, the shortest PROFINET IO telegram 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 IO telegram is filled with "zero bytes" until it can be transmitted. As a
consequence for the calculation formula, the shortest PROFINET IO telegram 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 μsec

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.
 Network topology
( 16)