RS485 not responding correctly

When using RS485 bus interface a few things should be taken into account when setting up the network:

  1. Bus structure: The network topology should follow a bus structure as closely as possible. That is, the connection between each node and the bus itself should be as short as possible. Basically, it should be short compared to the length of the bus.
  2. Bus termination: Especially for longer busses and/or multiple nodes connected to the bus and/or high communication speeds, the bus should be properly terminated at both ends. Most of our modules do not integrate any termination resistor (please check with the datasheet). Therefore, 120 Ohm termination resistors at both ends of the bus have to be added externally.
  3. No floating Bus lines: Avoid floating bus lines while neither the host/master nor one of the slaves along the bus line is transmitting data (all bus nodes switched to receive mode). Floating bus lines may lead to communication errors. In order to ensure valid signals on the bus it is recommended to use a resistor network connecting both bus lines as well defined logic levels. In contrast to the termination resistors this network is normally required just once per bus. Certain RS485 interface converters available for PCs already include these additional resistors (e.g. USB-2-485).

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