Bus Termination - Omron PROFIBUS DP GATEWAY Operation Manual

Setting up a PROFIBUS Network
Stub Lines
2-5-2

Bus Termination

Termination Resistors
VP
390 Ohm
B-Line
220 Ohm Network
A-Line
390 Ohm
DGND
Inductors
110 mH
B-Line
Network
A-Line
110 mH
Note If the network must be extended beyond the range of the advised cable
length, the use of a fibre optic segment to cross the larger distance should be
considered.
Passive Stub lines (branches from the main line) should be avoided for data
transmission speeds of more than 500 kbit/s. Except at end devices with ter-
mination, it is recommended to always use plug connectors that permit two
data cables to be connected directly to the plug. This method allows the bus
connector to be plugged and unplugged at all times without interrupting data
communication between other devices.
In order to minimize cable reflections and ensure a defined signal level on the
data lines, the data transfer cable must be terminated at both ends with a ter-
minating resistor combination. The bus termination diagram is shown on the
left.
The bus terminator connects the two data lines via a 220 ohm resistor which,
in turn, is connected to VP 5Vdc and DGND via two 390 ohm resistors. Pow-
ering the terminator resistor via VP 5V and DGND ensures a defined idle
state potential on the data lines.
To ensure the correct functioning up to the highest baud rate, the bus cable
must be terminated at both its ends.
A missing bus termination can cause errors during data transfer. Problems
can also arise if too many bus terminators are fitted, since each bus termina-
tor represents an electrical load and reduces the signal levels and thus the
signal-to-noise ratio. Too many or missing bus terminators can also cause
intermittent data transfer errors, particularly if the bus segment is operated
close to the specified limits for maximum numbers of devices, maximum bus
segment length and maximum data transfer rate.
In addition to the bus termination, additional precautions must be taken to
ensure proper operation at high baud rates, i.e. baud rates of 500 kbit/s and
higher. Due to the capacitive load of the device and the resulting cable reflec-
tions, bus connectors must be provided with built-in series inductors, of 110
mH each, as shown in the figure on the left.
Installing the inductors applies to all devices on the network, and not only to
the devices at both ends of the bus cable.
Section 2-5
33