Bus Termination - ABB AC 800M User Manual

Bus termination

Bus termination
According to the PROFIBUS set-up instructions, the transmission line must be
terminated actively on both ends to minimize line reflections and to ensure
quiescent levels on the transmission lines. The RS485 interface operates on voltage
differences. This network must be provided with a ground-free voltage of 5 volts.
Depending on the device the required bus termination is already integrated into the
device so that it can be inserted into the circuit easily if the device is located at the
beginning or end of the line structure. According to the rules of certification, each
PROFIBUS DP device needs a minimum voltage of 5 V.
Figure 21. RS 485 Bus termination
If it is not sure from the very beginning that, for a planned installation, the required
power supply for the active bus terminator will be ensured during plant operation,
suitable measures must be taken. A typical example of this application is, if the bus
participant supplying the terminating resistor with voltage, is switched idle
repeatedly for operational reasons or separated from the bus when the plant is
running. In that case, a bus terminator with external power supply or a repeater must
be used for the respective bus termination of the bus.
Details about Repeater are described in
on page 102.
66
Section 3 Fieldbus Topologies
Active network terminator and repeater
3BDS009029R5001 B