PROFIBUS Manual

To guarantee that individual stations can, in all cases, transmit after a maximum time, a time period must be specified by the end of which the token must have completed one rotation of the logical ring. There is little sense assigning fixed transmission times to individual active stations, because time will be wasted on stations with no need to transmit, while time will be too short for stations whose need to transmit is urgent. Since transmission requirements change in the course of bus operation, dynamic distribution of transmission time is required. This is achieved by the active station, once it has received the token, starting up a timer to measure the token rotation time. When it next receives the token (i.e. after one rotation) the timer will be read, the time measured will be compared with the defined maximum target rotation time, and the timer started up again.  The active station can only send telegrams if the time measured is less than the defined maximum target rotation time.

 

In this form, the procedure has the disadvantage of being unable to guarantee transmission times without further action. One avoids this disadvantage by assigning 'high' or 'low' priority to individual message cycles. The service with high priority will be processed first.

 

The minimum desired token time, which must be set in any installation, is calculated as follows:

 

min TTR = n ( TTC + TMC ) + k * TMC

n          Number of master stations

 

The first expression contains one high priority message cycle per master station and token rotation. This guarantees the maximum reaction time for high priority messages without retries for all load conditions. The second term contains the estimated number of low priority telegrams per token rotation.  

 

To take account of possible retries, the minimum value chosen should be approx. 10 to 20% greater. Modern configuration tools for PROFIBUS independently calculate token rotation time from the number of projected stations in a network.

 

Please note: If there is to be any subsequent inclusion during runtime of an additional master (e.g. for engineering work) within a PROFIBUS network, the projected token rotation time must be raised. Some manufacturers suggest a value of 20 000 tBit in this case.

 

Users have a fundamental choice between two priorities (high or low) for FDL user interface services. This choice is passed on with the service request in the telegram's frame control byte (see next chapter for FC byte).

 

To guarantee the transmission of important data, a high priority message cycle may be processed after every receipt of the token, even if no more token holding time is actually available. However, the token must then be forwarded promptly to the next station.

 

If token real rotation time (TRR) is less than token target rotation time (TTR) there is still time for message transmission.  Additional high priority services, followed by low priority ones, can be processed. Among low priority services, the following sequence applies:

1.Processing the poll list (cyclic services)

2.Processing low priority, acyclic services

3.Station registration (establishing the live-list)

4.Gap updating (maximum one address from gap list)