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What is the maximum cable length for a Modbus RTU network?

For Modbus communication, the total cable length per segment must not exceed 1.000 m. (Total cable length = sum of the main network line and all branches). Hereby it is important to mention that a standard Modbus network should follow a line topology. This means that it is important to minimise the length of the branches. If branches are present, they should be kept as short as possible. The combined length of all branches should not exceed 20 m. 
 Modbus topology
When the total cable length becomes too high, the Modbus RTU communication might get disturbed. To compensate these communication losses, a Modbus repeater (type DPOM-24-20) can be used to fix the problem.
Sentera developed the concept 'Power over Modbus' or PoM. This means that Modbus data communication and 24 Volt supply voltage are distributed via one single UTP cable. This technology reduces the risk of data losses due to disturbances. When Sentera products are interconnected via PoM technology, the combined branch lenghts become less critical. More and longer branches can be added to the Modbus network before disturbances and data losses will occur.  
However, the higher the total cable length, the higher the resistance of the cable. When the PoM network becomes too long, the supply voltage will drop below 24 Volts due to the high cable resistance. To ensure stable operation, it is recommended to use a Modbus repeater after 250 m of PoM cable. If the total power consumption of all connected devices is too high, a repeater will be needed after a shorter PoM cable length to provide sufficient supply voltage in that segment of the PoM network.
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