This Technical Note describes the use of termination resistors and biasing resistors for RS-485 networks. This note describes the use of these resistors on Control Microsystems products but the techniques can be applied to all RS-485 networks.
Some RS-485 receivers require biasing and must have sufficient voltage on their inputs to generate a stable output. The differential voltage must be greater than 200mV. Voltages less than this may not be detected properly. The bias resistors on the inputs of these RS-485 receivers ensure the receiver output is at a known state when there are no other devices driving the RS-485 lines. The RX+ line is pulled high with 5k ohms. The RX- line is pulled low with 5k ohms. Other RS-485 receivers are failsafe and will generate an output consistent with the RX+ line pulled high and the RX- line is pulled low.
Refer to the following table to see if your SCADAPack has termination resistor or failsafe biasing.
|Product||Biasing method||Termination on board|
|SCADAPack with 5204 controller||Failsafe||Jumper selected.|
|SCADAPack 32||Bias resistors||Jumper selected.|
|SCADAPack LP||Bias resistors||None.|
|SCADAPack 100||Bias resistors||None.|
|4000 series transmitters and flow computers||Bias resistors||None.|
Termination resistors are required to match the impedance of the transmitter to the line. Termination resistors are required only in long cable runs and baud rates of 115.2k or more. Termination resistors are rarely required and can cause more problems then they solve when they reduce the effectiveness of the biasing resistors. Termination resistors are 120 ohms at each end of the network for a total termination loading of 120 ohms. If termination resistors are used, the following should be considered.
- Terminate twice and only at the extreme ends of the network. If a star network must be used then terminate at two stations that are the furthest apart.
- Calculate the effects on biasing after the installation of the termination resistors. Consider the effects of the biasing resistors of all of the devices on the network. Contact the manufacture of these devices to see if biasing resistors were used.
- The differential voltage when the network is idle should be greater than 200mV on all failsafe receivers. This can be measured or calculated.
- A network with two stations, biasing resistors and terminations may have insufficient biasing and may require either external biasing resistors. Consider removing the termination resistors.