A General Guide for RS-485 Communication

Problem (Issue) Statement

A General Guide For RS-485 Communication.

Description:

A general guide for RS-485 communication networks.

The RS-485 standard allows for very robust communications over distances of up to 4000 feet on networks wired with relatively inexpensive cable types.

Related Software:

NIL

Related Hardware:

NIL

System Application:

NIL

Symptom and Finding:

Improper cable termination and ignoring RS-485 topology guidelines are classic culprits behind signal noise and communication errors. 

Cause:

A general guide for RS-485 networks.

Solution:

• Network Topology.

• Cable Distance.

• Cable Type.

• Network Topology:

Step 1: To avoid signal reflections which can cause communication errors, the Daisy Chain topology is highly recommended. The Daisy Chain topology is designed as one continuous line with multiple drops instead of similar to a star.

Step 2: The picture below shows several typical wiring configurations observed from site, with Daisy Chain's topology being the only acceptable configuration.

• Cable Distance:

Step 1: RS-485 standards allow communications of up to a maximum of 1km cable length for a network segment. This length is the total cable length between the first controller in the network and the last controller in the network. It is not the length between one controller to the next successive controller. Refer to the following picture for further information

• Cable Type:

Step 1: RS-485 specification recommends shielded twisted pair cable to reduce two major sources of problems for external noise: radiated EMI and received EMI.

Step 2: The recommended cable specifications are as follows:

Example of the shielded twisted pair cable:

Common Issue

• Hardware devices connected using star topology, which resulted in hardware status down. 

• Project deployment using incorrect cable type, noise induced by the environment can also cause hardware status to be down.

Date Documentation:

5/11/2018 (Rev 1.0)