AST-CAN485 Hookup Guide

Introduction to RS485

RS485 is a standard used extensively in serial communications systems. Only the electrical interface is defined by RS485. No specific communication protocol is specified by the standard; instead it forms the physical layer for many different protocols. For example a serial port may run over a RS485 physical link.

RS485 uses differential signaling over a twisted pair making it resilient to noise. Multiple nodes are also supported, the number of which is usually determined by the protocol in use. Distances up to 1200m and transmission speeds up to 10Mb are possible. However, there is a trade-off between distance and speed. For example, a 50m cable can run at 2Mbps.

RS485 is one of the commonly used physical layers. Applications include industrial systems, computing, automotive, and building management. Modbus and Profibus are two common industrial protocols that make use of RS485.

Signal Description

The interface consists of two lines with signals A and B. When the bus is idle, both lines float. When in operation one node acts as a controller and takes control of the bus, driving the two lines to the appropriate voltages. The other nodes act as peripherals and receive the data being transmitted. The lines are driven to opposing polarities; if A is positive, B is negative. By inverting the signals, logic levels of 0 or 1 are possible.

Network Structure

Multiple nodes may be connected in parallel. The lines must be terminated on each end with a termination resistor (typically 100Ω to 120Ω).