Protocols & Communication

Problem

When is it reccomended that the bias resistors be used when communicating with a SCADAPack via RS485

Environment

RS485, 485, SCADAPack, SCADAPack32, serial commnunications,

Fix

The most recent position on termination resistors is "don't use them". Some manuals are better at describing this than others.

The two important things that must be considered are establishing a ground or return along with biasing and terminations.

RS-485 is usually referred as a "two wire". This ignores the most important connection or "third wire". RS-485 is a differential signaling system. Signals at every node must be established relative to the same common point. This shared common, the "third wire" is generally a connection to earth. 4000 series products have their RS-485 transceivers referenced to the negative side of the power supply. The negative side of this supply should be connected to earth directly or through a resistor of up to 100 ohms. If the RS-485 network is going to include a SCADAPack then this connection has already been made through the chassis.

The inputs on RS-485 receivers must be always asserted to a valid state. If not, these receivers will pick-up noise that may be interpreted as data. All SCADAPacks and 4000 series products have bias resistors or biased receivers that ensure that the RS-485 lines are in a valid state when the lines are not be driven. Do not use termination resistors. Terminations will defeat the effectiveness of the biasing by lowering the signal levels and waste power. Terminations provide no benefit at bit rates of 115k or lower.