BECKHOFF BC3150 - Manual (Page 90)

Safety Instructions
88 Fieldbus Components
Cables, Plugs and Switches
The Medium: Plugs and Cables
The physics of
the
transmission
Cable-related
malfunctions Physical aspects of the data transmission are defined in the Profibus standard. See
Profibus layer 1 (physical layer).
The types of area where a fieldbus system can be used is largely determined by the
choice of the transmission medium and the physical bus interface. In addition to the
requirements for transmission security, the expense and work involved in acquiring and
installing the bus cable is of crucial significance. The Profibus standard therefore allows for
a variety of implementations of the transmission technology while retaining a uniform bus
protocol.
Cable-based transmission: This version, which accords with the American EIA RS-485
standard, was specified as a basic version for applications in production engineering,
building management and drive technology. A twisted copper cable with one pair of
conductors is used. Depending on the intended application area (EMC aspects should be
considered) the screening may be omitted.
Two types of conductor are available, with differing maximum conductor lengths; see the
„RS-485“ table. The pin assignment at the plug and the wiring are illustrated in the
diagram. Note the special requirements on the data cable for baud rates greater than 1.5
Mbaud. The correct cable is a basic requirement for correct operation of the bus system. If
a „simple“ 1.5 Mbaud cable is used, reflections and excessive attenuation can lead to
some surprising phenomena. This could, for example be that some station is not
connected, but when the neighbouring station is unplugged the connection appears again.
Or there may be transmission errors when a specific bit pattern is transmitted. The result
of this can be that when the equipment is not operating, Profibus works without faults, but
that there are apparently random bus errors after start-up. Reducing the baud rate (<
93.75 kbaud) corrects this faulty behaviour.
If reducing the baud rate does not correct the error, then in many cases this can indicate a
wiring fault. The two data lines maybe crossed over at one or more connectors, the
termination resistors may not be switched on, or they may be active at the wrong
locations.
Note
Installation is made a great deal more straightforward if pre-assembled cables from
Beckhoff are used. Wiring errors are avoided, and commissioning is more rapidly
completed. The range includes fieldbus cables, power supply cables, sensor cables and
accessories such as terminating resistors and T-pieces. Connectors and cables for field
assembly are nevertheless also available.
Profibus
Connection
of the fieldbus
box modules The M12 socket is inverse coded, and has five pins. Pin 1 is 5 V DC and 3 is GND for the
active termination resistor. These must never be misused for other functions, as this can
lead to destruction of the device. Pin 2 and pin 4 are the Profibus signals. These must
never be swapped over, as this will prevent communication. Pin 5 is the shield, and this is
capacitatively coupled to the Fieldbus Box chassis.
Profibus socket pin assignment
Profibus conductor colours
Profibus conductors M12 D-Sub
B red Pin 4 Pin 3
A green Pin 2 Pin 8