Siemens SIMATIC NET PROFIBUS - Manual (Page 386)

Infrared Link Modul (ILM) 6ZB530–3AC30–0BA1
44 Copyright

by Siemens
10.2 Errors Due to Incorrect Network Configuration
In large PROFIBUS networks with numerous modules and long cable lengths, the delay caused by network
components and cables (transmission delay) must be taken into account when setting the monitoring times. If
you do not take these delay times into account, problems will occur during operation. In such situations, the
active partner does not receive a response to a request within the slot time of PROFIBUS because this has
been configured too short.
In large networks, or networks with looped-in active components, the transmission delay time(TTD) must be
calculated to allow correct configuration.
The transmission delay time is the maximum time that can elapse during the transmission of a frame between
the sender and receiver on the transmission medium.
Note
If the configuration software you have used to configure your PROFIBUS network does not support the PRO-
FIBUS parameter TTD, increase the two times min. TSDR and max. TSDR in each case by 2 x TTD (the reac-
tion time of the responder is increased by the transmission delay time for the outward and return path).
10.2.1 Calculating the Propagation Time on Electric Cables and Fiber-Optic Cables
The propagation times on electrical cables or fiber-optic cables are physically related to the speed of light and
certain material characteristics and are therefore almost constant (approximately 5
µ
s/km).
First calculate the transmission link with the longest propagation time between the sender and receiver of a
frame. PROFIBUS nodes that communicate with each other (for example DP slave with DP slave) do not need
to be taken into account.
Indicators for long propagation times are as follows:
â Long fiber-optic or copper cables
â
High cascading depth of active components (PROFIBUS OLMs, PROFIBUS ILMs, PROFIBUS repeaters)
The delay time is approximately 5
µ
s per km cable length.