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Profibus Communication Rate and Bandwidth

Profibus Communication Rate and Bandwidth

 

The update rate of the Profibus communications is set in the Profibus master controller.  The GSD file of the drive allows a minimum time of 2ms.

The Profibus master will send the complete telegram every cycle: 4 words (parameter channel) plus 6 words (process data channel).  It does not necessarily change the data every cycle.  So, the cycle includes 10 words Output and 10 words Input, totaling 160 bits per transmission cycle.

 

Calculate Bandwidth (data transmission capacity):

At 12,000k baud and an update of 2ms, Profibus can transmit 12,000,000 bps / 0.002 s = 24,000 bits.

20 words = 160 bits per cycle

At 160 bits, this is capable of 24,000 bits / 160 bits = 150 drives on the bus.

 

Calculate Required Baudrate:

For 13 drives, the minimum required baudrate is: 160 bits x 13 drives / 0.002s = 1.04 Mbps (Mbaud).

 

Selecting Baudrate:

The baudrate should be specified only as high as the application requires.  The higher the baudrate, the higher the chance to see transmitting problems caused by the wiring and connectors.  Most applications actually run with 1.5 Mbaud.  With 12MBaud you really need to be very careful about your cable installation.

 

Cable Wear:

Did you know that a Profibus system wears out?  It may happen that a system runs, but after a year of operation it suddenly shows transmission problems.  The reason is the insulation of the cable degrades and suddenly changes its behavior.  A cable will degrade faster with higher baudrates.