29
Setting up a PROFIBUS network
Section 2-5
Note
The PROFIBUS standard EN 50170 also specifies a Type B cable with
slightly different cable characteristics. This Type B cable is no longer recom-
mended for use.
Maximum PROFIBUS
Cable Length
The transmission speed defines the maximum advised cable distance or
cable segment in metres before the use of a repeater is recommended. The
cable lengths specified in the following table are based on PROFIBUS type A
cable.
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.
Stub Lines
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.
2-5-2
Bus Termination
Termination Resistors
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
Characteristic
Value
Impedance
135 - 165 ohms
Capacitance per unit length
< 30 pF/m
Loop resistance
110 ohms/km
Core diameter
0.64 mm
Core cross section
0.34 mm
2
Baud rate
(kbit/s)
Distance/segment
(m)
Baud rate
(kbit/s)
Distance/segment
(m)
9.6
1200
500
400
19.2
1200
1500
200
45.45
1200
3000
100
93.75
1200
6000
100
187.5
1000
12000
100
390 Ohm
390 Ohm
220 Ohm
VP
B-Line
A-Line
DGND
Network
