QUESTION:  
On a PROFIBUS master system, on which the connection among stations is implemented via Power Rail Booster (PRB), can I use S7 communication, MPI, FDL or FMS?

ANSWER:  
The use of above mentioned protocols for communication within a PROFIBUS network with PRB is possible. The PRB supports and transfers all kind of telegrams which comply with the PROFIBUS standard. However, some further explanations are needed, since all above mentioned protocols require the use of active stations, which may be configured as master or as slave. 

If all active stations are on the stationary side of the contact rail network, or if the connection between PRBs is implemented with cable (crane facilities, EMV loaded environment), then there are no restrictions. All PROFIBUS protocols can be used.

If there are active stations on both sides of the contact rail network, then, in case of a physical interruption of the connection, there may be disturbances of the communication (interruption of the rail through a lift of the contacts, too high transfer resistance of switch gears of the Power Rail Line, etc.).

This shall be explained with the example of a stationary control as DP master and several vehicles as active DP slave. The controls are connected via PRB in a contact rail network. If the communication between the master and the slave does stop physically due to a lift of the rail contacts, then the slave slips out of the logical token ring. The master only communicates with reachable slaves and exchanges usage data and tokens. The slave without connection to the contact rail waits a specific period of time, and then generates (according PROFIBUS standard) a token of its own, since it no longer observes any activity on the PROFIBUS. Hence, two bus systems are created, that run asynchronously to each other, and have one token (permission to send) each. If the slave, disconnected from the connection, receives contact to the master system again (rail contact snaps back into the rail again), then the active stations recognize a double token on the bus. All stations withdraw from the bus. The communication is disturbed. The PROFIBUS will be initialized automatically and all stations will be accepted to the token ring. The communication is secured again.

Summary: 
Therefore it is possible to use communication mechanisms of the S7 communication, MPI, FDL, or FMS, via a Power Rail Booster transmission line. However, contact interuptions, which cannot be excluded, lead to a creation of two tokens with subsequent, brief interruption of the communication. This needs to be taken into account when configuring the plant.