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What cable to use for a Modbus network?
Modbus RTU – how to interconnect Sentera devices
Sentera products share information via Modbus communication. Modbus is a serial communication protocol that uses RS485 technology.
Based on that technology, Sentera developed the concept Power over Modbus or PoM to interconnect Sentera products. The PoM concept means that Modbus data communication and 24 Volt supply voltage can be distributed via one single UTP cable (Unshielded Twisted Pair cable):
Sentera advises to use Unshielded Twisted Pair (UTP) cable or Shielded Twisted Pair (STP) cable with following impedance characteristics: 120 Ω ±10%
For longer networks, it might be necessary to activate Network Bus Terminator resistances (NBT) in two connected devices to avoid disturbances and data losses. We advise to activate NBT's for network lengths above 50 meters. NBT in Sentera devices can be activated via the Modbus Holding registers (download the product Modbus register map from our website for more information).
The PoM concept reduces the risk of data losses due to disturbances. When Sentera products are interconnected via PoM technology, the combined branch lenghts become less critical compared to a standard Modbus network. More and longer branches can be added to the network before disturbances and data losses will occur.
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