Introduction

This guide explains the RS-485/Modbus module features and functions. This product was designed for Waspmote v12 and continues with no changes for Waspmote v15.

The standard

RS-485 is the most versatile communication standard. The RS-485 standard defines the electrical characteristics of drivers and receivers for use in digital systems. The standard is published by the Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA). RS-485 signals are used in a wide range of computer and automation systems and are used in programmable logic controllers and on factory floors. Since it is differential, it resists electromagnetic interference from motors and welding equipment. It may be used to control video surveillance systems or to interconnect security control panels and devices such as access control card readers. It does not specify or recommend any communications protocol. In the next table the electrical characteristics of the standard are defined.

RS-485

‚Äč

Standard

EIA RS-485

Physical media

Twisted pair

Network topology

Point-to-point, multi-dropped, multi-point

Maximum devices

32 drivers or receivers

Mode of operation

Differential signaling

Voltage levels

-7 V to +12 V

Mark (1)

Positive voltages (B-A > +200 mV)

Space (0)

Negative voltages (B-A \< -200 mV)

Available signals

Tx+/Rx+, Tx-/Rx- (Half Duplex), Tx+,Tx-,Rx+,Rx- (Full Duplex)

The number of maximum devices could increase improving some features. The Waspmote RS-485 module uses half duplex communication and the SP3485 integrated circuit wich provides low consumption and good communication speed.

Figure: The RS-485 module on Waspmote

This list includes some of the most common uses of the standard:

  • Industrial equipment

  • Machine to Machine (M2M) communications

  • Industrial Control Systems, including the most common versions of Modbus and Profibus

  • Programmable logic controllers

  • RS-485 is also used in building automation

  • Interconnect security control panels and devices

Functional features

The differential transmission is the base of the functioning. The same information is sent trough the two wires, but with a phase difference of 180 degrees. Any interference introduced in the signal will affect equally both wires. By reversing the signals interferences are eliminated each other. Another noise immunity is the use of twisted pairs. Twisted pairs in RS-485 communication however adds immunity which is a much better way to fight noise. The resulting noise current is many factors lower than with an ordinary straight cable.

Figure: Differential signaling operation

Connectivity

Network topology is probably the reason why RS-485 is now the favorite interface in data acquisition and control applications. RS-485 is the only one of the interfaces capable of Internet working multiple transmitters and receivers in the same network. It is possible to connect 32 devices to the network. Currently available high-resistance RS-485 inputs allow this number to be expanded to 256. With the introduction of \"automatic\" repeaters and high-impedance drivers / receivers this \"limitation\" can be extended to hundreds (or even thousands) of nodes on a network. In RS-485, the communication is half duplex. It means bidirectional but not simultaneously communication. Only one device may be transmitting at any given time and all other are receiving.

Figure: Multi-drop Network connection