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What is a Modbus Network Bus Terminator or NBT?


Disruptions in a communication network can be prevented by using Network Bus Terminators or NBTs. Modbus Network Bus Terminators reduce voltage reflections in the lines of a Modbus communication network and restore stable communication.
  
What is Modbus communication?
Modbus is a serial communication protocol developed in 1979 for industrial applications. In simple terms, it is a method used for transmitting information over serial lines (RS485 technology) between electronic devices. The Modbus communication protocol can be seen as the language, RS485 technology can be seen as the technology that is used to communicate. 
 
Each Modbus communication network contains one Master device and one or more slave devices. The device requesting the information is called the Modbus Master and the device(s) supplying information are Modbus Slaves. On a standard Modbus network, there is one Master and up to 247 Slaves, each with a unique Slave Address from 1 to 247. The Master can also write information to the Slaves.

   
Wiring guidelines for a Modbus network
Modbus communication is designed for industrial applications and is relatively resistant to interference. It is much more stable and robust than a classic 0-10 Volt signal. However, there are still limitations. There are limitations to the total cable length on a Modbus network. When the total cable length becomes too high, the Modbus communication will be disturbed. It is recommended to keep the total cable length shorter than 1.000 m. A Modbus network must have a line topology. Therefore, the branches should be kept as short as possible.
 

In order to avoid communication disturbances, it is important that all Modbus cables are physically separated from power cables, electric motors and other sources that can cause EMC pollution. It is recommended to use separate cable guidance for data communication lines and power lines. Metal cable guidance systems, connected to PE will provide the best results. Apart from keeping the wires protected from EMC pollution, it is also essential to separate or maintain enough distance between more sensitive devices as sensors / distribution boxes / HVAC controllers and power devices like frequency inverters, contactors, relays, etc.
 
Network Bus Terminators or NBTDrawing with lines
If the cable length becomes too long and partially or completely disrupts the Modbus communication, NBTs can be used. If the data loss occurs only sporadically, it can be very difficult to detect the problem. To ensure proper RS485 communication, terminating resistors are used at both ends of a bus to reduce voltage reflections. There must always be two terminations or Network Bus Terminators (NBT) on the network, one at each bus end. Every Sentera devices features an NBT that can be activated via the parameter settings. By default, NBT is deactivated. We advise to use Sentera 3SModbus software to activate the NBTs.


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