PROFIBUS Manual

The estimation formula can be further simplified for PROFIBUS PA. Data is transferred at a fixed bit rate of 31.25 kBit/s and we can assume that for every process value (PV) 5 bytes are transferred. The time required to transfer such a process value therefore lies approximately around 1.1 ms ( = 5x8 bits).

 

Depending on the coupler used, other bus parameters must be adjusted, slightly altering the calculation.  

 

For a bit rate = 45.45 kBit/s the following applies:

TBCycle ≈ 21.3 + (14.5 * slaves) + (1.1 * PV) ms

 

For a bit rate = 93.75 kBit/s the following applies:

TBCycle ≈ 29.5 + (11.5 * slaves) + (1.1 * PV) ms

 

These estimation formulae are relatively accurate in practical measurements, as long as no further master is added and no acyclic data is transferred. An acyclic service will, as a rule, require between 40 and 60 ms.

 

When using a transparent coupler or a DP/PA link, it is necessary to add together the listed PA bus cycle times and DP bus cycle time to determine the overall reaction time. Often the DP bus cycle time will then appear negligibly small compared to the PA bus cycle time.