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Bus Parameters Appendix A
The parameters to calculate are:
• Target Rotation Time
• Min Slave Interval
• Watchdog timeout
• Data Control Time
These four values depend on the number of slave stations allocated to the PROFIBUS-DP Master Unit, the
number of I/O bytes each of the slave stations will exchange with the Master Unit, and how many other Master
Units are on the PROFIBUS network at the same time.
Furthermore, the Unit will require additional time to process the PROFIBUS I/O data and exchange this with
the PLC CPU. Based on this, the following empirical formula is used:
Min. Slave Interval = A
1
+ (B
1
* slave stations) + (C
1
* bytes to transfer) +
(bits per µs)*((A
2
* slave stations) + B
2
* modules) + (C
2
* Words to exchange) [1]
Target Rotation Time T
TR
=
å
(Min. Slave Interval) [2]
T
WD
= T
TR
+ 20000 [3]
Data Control Time = (T
WD
* 7) * 100 / Baud rate [4]
Based on measurements, the following values have been determined for the 6 factors in equation [1]:
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