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RS485 Bus

Last Updated: Sep 22, 2016 11:31AM CEST

Definition

RS485 is a balanced, multi-point communications standard. It enables the sending and receiving of binary information between devices connected to the bus.
What is meant by “balanced” is that the signal is sent as both positive and negative by the wire in order to increase the reliability of the system.

RS485 defines how to physically send the information. It allows any type of information to go through the bus (for example Modbus data). 
 

Uses and Characteristics

  • Resistant and stable, so it’s ideal for industrial environments, where reliability is an important factor.
  • Up to 1200m (ideal).
  • Termination resistors recommended​ which eliminates the majority of the signal reflection.
 

Topology
 

The recommended topology is a serial bus without any branching:
 
Example of adaptacion from existing topologies to a serial bus:
 

Cable type

We recommend to use a shielded twisted pair (general shield) with minimal section 0,20 mm 2 (AWG 24) of 120 ohms impedance and type L IYCY-CY.

Identification of terminals accoriding to RS485 standard

The protocol uses 3 wires: the signal (represented by a + sign, the inverse of the signal (represented by a - sign) and the reference voltage, or ground (represented by 0). Some manufacturers call the wires A, B and 0, but whether A will is positive or negative varies by manufacturer, so it is recommended to rely on the polarity instead of the manufacturer.
Recommendations:
- in a 3 wire network connect the 3rd terminal (C) in the terminal (OV / NC)
- in a 2 wire network use the 3rd terminal (C) to provide the shield continuity.
 

Line fail-safe biasing

The RS485 standard imposes one differential level of 200 mV minimum to detect the signal. If the RS485 line is not biased, this level will not be reached (without communication on the line) and successful communication will not be guaranteed. For this, we apply a bias to only one place on the bus and it is best applied at the master. On certain models of interface unit, it is possible to activate this biasing otherwise, it would be necessary to add an external supply which guarantees a level of 250-280 mV on the whole bus when there is no active communication.

In order to prove this, it is best to apply the biasing at one end of the bus (on the interface side) and verify the voltage level on the other end of the bus ; this ensures adequate biasing throughout the bus.

 

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