30
Setting up a PROFIBUS network
Section 2-5
close to the specified limits for maximum numbers of devices, maximum bus
segment length and maximum data transfer rate.
Inductors
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.
2-5-3
PROFIBUS Cable Connector
Bus Cable Connector
The plug connector to be used on the PROFIBUS-DP Gateway Unit is a 9-pin
male sub-D type, preferably encased in metal and having a facility to connect
the shield of the cable to the case or to pin 1. The cable should be connected
to the receive / transmit lines, pin 3 (B-line) and pin 8 (A-line).
The use of special PROFIBUS-DP cable connectors, which are available from
several manufacturers, is highly recommended. Various models are widely
available, with or without the bus termination and inductors built-in. If provided
in the connector, the Bus termination can often be enabled or disabled
through a switch on the connector.
The special PROFIBUS-DP cable connectors often provide a convenient way
of connecting the cables. The figure on the left, provides an example of such
a bus cable connector.
A standard 9-pin sub-D plug can only be used if the PROFIBUS-DP Gateway
Unit is not at the start or the end of a bus segment, or on a stub line at a baud
rate of 500 kbit/s or less.
The two PROFIBUS data lines are designated A and B. There are no regula-
tions on which cable core color should be connected to which of the two data
terminals on each PROFIBUS device; the sole requirement is to ensure that
the same core color is connected to the same terminal (A or B) for all devices
throughout the entire system (across all devices and bus segments). The
PROFIBUS Organization recommends the following rule for data line color
codes: PROFIBUS cables in general will use the colors red and green for the
data lines, with the following assignment:
Data cable wire A - green
Data cable wire B - red
This rule applies to both the incoming and the outgoing data lines.
B-Line
A-Line
110 mH
110 mH
Network
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