Is it possible to place the I/O units more than 300 meters from the CPU ? To understand better my question please read the details below.

Yes, for better result use Optic fiber

PROFIBUS DP (Process Field bus with Decentralized Peripherals) is designed to provide high data transmission rates (up to12 Mbit/s) and short response times (up to1 ms ) and is particularly suitable for direct control of:

·         Intelligent field devices (e.g. drives, motor starters, analyzers, process controllers or panels)

·         Distributed I/O devices: remote I/Os such as ET200M, ET200iSP or ET200   

It is possible to reach distances up to 1 km (at12 Mbit/s) or up to10 km (at187.5 kbit/s) by using segments and by boosting the signal with up to 9 repeaters.

A segment can have up to 32 participants (master/slaves), and the total network up to 126 participants. The start and end of each segment must be terminated by an active bus resistor, which is typically pre-integrated in the device (e.g. repeater) or is available as an active RS485 termination element.

We have executed a work to monitor the motor current in SCADA system wih Simocode DP with the following configuration

SCADA station –SCADA based on Simatic Wincc

Master- S7-3002DP

Slaves-54 Number of Simocode for monitoring motor current

Total distance from the54th slave to Master is220M

We have used total three numbers of repeaters. (Placed after18th slave).As per standard repeater to be placed after32th slave.

Please note that the maximum node is limited to126.

For details please refer the following link

www.siemens.com/profibus

Regards, 

there are may ways as mentioned below.

1-use the profibus repeators for extention (i think you can go up to9 in serial but not proven ).

2-the otherway is profibus to fiber optic  to optocoupler .but i am not sure about the realtime communication please reffer to system manual .......regards