Westermo ODW-641 - Manual (Page 21)

21 6650-2291
Calculating system processing delay
Data exchange between a PROFIBUS DP master and slave via ODW-641 fibre optic link,
will be delayed due to the length of the optical fibre and the signal processing within the
ODW-641. The signal processing delay is dependent on the data rate, and the fibre delay
is dependent on the total length of the optical fibre. The additional time resulting from
the optical fibre and ODW-641 is the Overall system delay.
Delay @ ≤ 1.5 Mbit/s Delay @ 3 to 12 Mbit/s
Optical fibre length delay (typical) 5 μs/km 5 μs/km
Signal processing, electrical to fibre (max) 1 tbit + 1 μs 9 tbit + 1 μs
Signal processing, fibre to electrical (max) 0.3 μs 0.3 μs
Signal processing, fibre to fibre (max) 1.3 μs 1.3 μs
Note! tbit = 1 / Baud rate (Baud rate in bit/s)
Example
One PROFIBUS DP master and slave using a data rate of 12 Mbit/s. and a total fibre length of 40 km.
A data frame sent from the master to a slave.
1. Fibre: The total optical fibre length delay.
40 x 5 μs = 200 μs
2. Converter electrical to fibre: Signal processing delay
(ODW-641 connected to PROFIBUS DP master).
9 tbit + 1 μs = 9 x 0.083 μs + 1 μs ≈ 1.1 μs
3. Converter fibre to electrical: Signal processing delay
(ODW-641 connected to addressed slave).
0.3 μs
4. The system delay is calculated by summing the delays in item 1 to 3 above:
200 μs + 1.1 μs + 0.3 μs = 201.4 μs