Bus length and speed
When a PROFIBUS network is installed, boundary conditions of the RS485 transfer technology must be observed. The electrical network uses a shielded, twisted pair cable. All subscribers are connected in a line-shaped bus. The transmission rate can be adjusted in steps from 9.6 kbit/s to 1.5 Mbit/s. For extremely time-critical PROFIBUS-DP applications, additional transmission rate of 3, 6 and 12 Mbit/s are possible. The maximum segment length depends on the transmission rate.
RS485 limits and network design
As described in the PROFIBUS RS485 specification, each bus segment can have a maximum of 32 active devices. In order to be able to connect a larger number of PROFIBUS DP devices, it is necessary to segment the bus. The segments are then interconnected with repeaters which amplify and refresh the data signals. Repeaters can also be used for galvanic isolation of bus segments or bus sections. Each repeater allows the PROFIBUS system to be extended by an additional bus segment with the maximum admissible cable length and the maximum number of fieldbus
devices. Repeaters increase the signal propagation times. This should be taken into account during planning.
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