PROFIBUS Manual

Estimation formula

Estimation formula

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Estimation formula

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To simplify the estimation of PROFIBUS-DP bus cycle time, findings set out in the preceding chapters can be reduced to the following formula:

 

TBCycle = (380 + slave x 300 + bytes x 11 ) x tBit + 75 μs

 

or directly with the bit rate

 

TBCycle = (380 + slave x 300 + bytes x 11 )/bit rate + 75 μs

 

where

TBCycle = estimated bus cycle time ±10%
Slaves= number of DP slave stations in the network
Bytes= number of bytes for all inputs and all outputs added together
tBit= duration of one bit, corresponds to the inverse of the PROFIBUS bit rate

 

This estimation formula results in a magnitude ±10% for bit rates > 500 kBit/s and no minimum or maximum value.

 

Simple example:

We have a DP master that must control 640 digital inputs and outputs. We consolidate this digital data in the best possible way as 80 bytes and share it among a total of 20 terminal blocks as DP slaves. We want to use a bit rate of 1.5 MBit/s.

 

Slave = 20, bytes = 80, bit rate = 1500 kBit/s  i.e. tBit = 0,667 μs

 

We obtain an estimated bus cycle time of

 

TBCycle = (380 + 20 x 300 + 80 x 11) x 0.667 + 75 =  4917 μs 5 ms

 

More comprehensive example:

We have a PLC as DP master, which has to control various field devices as DP slaves with a cycle time lower than one ms. What should be the minimum bit rate?

 

Address

Number of input bytes

Number of output bytes

2

3

3

3

5

0

4

10

20

5

0

5

Total

18

28

 

The minimum bit rate can be determined through

 

Bit rate > (380 +slaves x 300 + bytes x 11) / (TBCycle - 75 μs )

 

With slaves =  4, bytes = 46 and a bus cycle time of 1 ms we obtain a minimum bit rate of 2.2 MBit/s. We therefore select the next highest adjustable bit rate of 3 MBit/s. Recalculation with the estimation formula results in a bus cycle time of 0.77 ms, which meets our specification.