QUESTION:
Where and how are the STEP7 bus time calculations corrected?

ANSWER:  
When you configure the PROFIBUS DP/PA link connections in your hardware configuration, it may be necessary to correct the bus time calculations afterwards. This affects connection modules IM157 (MLFB: 6ES7 157-0AA81-0XA0, 6ES7 157-0AA80-0XA0 and 6ES7 157-0AA00-0XA0). Configurations with more than five DP/PA links should also be checked for this.   

Select the connection module with your cursor in STEP7 hardware configuration, then click the right mouse button and select "Properties" to open the following dialog box.  

 
Fig. 1: Dialog box for Properties of DP slave  

In the "DP Slave Properties" box click "PROFIBUS". This takes you to the screen called "PROFIBUS Interface IM157".  
Fig. 2: Dialog box for Properties of IM 157  

Click "Properties" to open the dialog box "PROFIBUS Properties". Here select the "Network Settings" tab and set the profile to "User Defined".  
Fig. 3: PROFIBUS Properties dialog box  

Then click "Bus Parameters" and in the "Bus Parameters" box increase the value for the TTR (Target Rotation Time).

 
Fig. 4: Setting the bus parameters

We recommend calculating the Ttr (Target Rotation Time) using the following formula:

Ttrnew = Ttrold + (n * 11 t_bit + 680 t_bit) * 2 * k 

n := number of all input and output bytes on the PROFIBUS PA segment concerned.

k := number of DP/PA links on the PROFIBUS segment. In H systems k corresponds to the number of slaves on a segment, even if the slave has two connections.

Please also note that increasing the Ttr (Target Rotation Time) affects the DP transfer time and the DP error recognition time of the SIMATIC H station.

If you change the Ttr, then you also have to change the response monitoring accordingly.

Response monitoring_new = Response monitoring_old * Ttr_new / Ttr_old

Keywords:
PROFIBUS parameters, Bus parameters, Bus setting