User´s Guide Power Rail Booster
Version 12/2005
Copyright
©
Siemens AG 2005 All rights reserved.
6ES7 972-4AA02-0XA0
Page 24 of 49
5.3.2 Adapting the„Retry Limit“ Parameter
It is advisable to set the "Retry Limit" parameter in the hardware configuration to a
value of 4 to make the DP Master System less sensitive to short line interruptions,
due, for example, to bouncing of sliding contacts when transitioning between rail
sections.
Fig. 14: Configuring the network parameters in Step 7´s Hardware Configuration
5.4 Configuring the Contact Rails and Sliding Contacts
5.4.1 Introduction
The PRB enables PROFIBUS transmission over contact rails. Over than the limit
values for capacitance (Chapter
5.4.2) and contact resistance (Chapter
5.4.3), the
PRB makes no special demands on the contact rail systems. Double collectors must
be used in order to ensure continuous contact. The maintenance instructions
provided by the contact rail supplier must be observed to ensure long-term
compliance with the limit values.
5.4.2 Contact Rail Capacitance
The higher the capacitance of the contact rails, the lower the distributable rail length
per Power Booster segment. The “PRB Checker” configuration tool’s internal
calculations are based on rail capacitance of 45nF/km. Contact rails with a higher
capacitance per unit length should not be used.
5.4.3 Contact Resistance between Contact Rails and Sliding Contacts
In order to ensure reliable data transmission, the contact resistance between the
connecting terminals of the PRB involved in communication must be maximum of
100 ohms. In this connection, you must take into account all the transition resistors of
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