Siemens SIMATIC NET OLM12M Description And Operating Manual page 77

6ZB5530–3AB00–0BA0/01
Note:
If the configuration software you are using to configure your PROFIBUS network does not support
the PROFIBUS parameter TTD, then increase the 2 x min. TSDR and max. TSDR by 2 x TTD (the
reaction time of the responder is increased by the transmission delay time for the outward and
inward path).
Calculating the Transmission Delay Time TTD
Find out which transmission path has the longest transmission time between the transmitter
and receiver of a frame. PROFIBUS nodes that do not communicate with each other (for
example DP slave with DP slave) can be ignored.
Indications for a long transmission time are as follows:
Long FO cables or copper cables
High cascading depth of active components such as OLM12M
1. Delay time of the FO cable and RS 485 cables
The delay time is approximately 5 µs per km of cable
Converted to bit times, this produces the following:
Transmission
Rate in Kbps
187.5
500.0
1500.0
12000.0
Table 3: Delay Times of FO and RS-485 LAN Cables
To calculate the cable delay time, multiply the maximum cable length in kilometers with the
delay time corresponding to the transmission rate from the table above.
2. Delay time of the optical link modules
The delay per module is 4 bit times. The total delay is the number of modules through which a
frame passes between the sender and receiver multiplied by 4 bit times.
3. Delay time of other active PROFIBUS network components
Please check the product documentation for the delay time of other components.
4. Transmission Delay Time (TTD)
The total transmission delay time is the sum of the values of 1., 2. and 3.
Copyright Siemens AG 1997
Delay Time in
tBIT per km
0.94
2.50
7.50
60
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SIMATIC NET OLM12M