BECKHOFF BC3150 - Manual (Page 25)

Safety Instructions
Fieldbus Components 23
PROFIBUS Cabling
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).
RS485 - Fundamental properties
RS-485 transmission according to the Profibus standard
Network topology Linear bus, active bus terminator at both ends, stubs are possible.
Medium Screened twisted cable, screening may be omitted, depending upon the
environmental conditions (EMC).
Number of stations 32 stations in each segment with no repeater. Can be extended to 127 stations
with repeater
Max. bus length without
repeater 100 m at 12 MBit/s
200 m at 1500 KBit/s, up to 1.2 km at 93.75 KBit/s
Max. bus length with
repeater Line amplifiers, or repeaters, can increase the bus length up to 10 km. The
number of repeaters possible is at least 3, and, depending on the manufacturer,
may be up to 10.
Transmission speed
(adjustable in steps) 9.6 kBit/s; 19.2 kBit/s; 93.75 kBit/s; 187.5 kBit/s; 500 kBit/s; 1500 kBit/s; 12 MBit/s
Plug connector 9-pin D-Sub connector for IP20
M12 round connector for IP65/67
Cabling for Profibus DP and Profibus FMS
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. It is possible, for instance, for a connected Profibus station not
to achieve a connection, but for it to be included again when the neighboring station is disconnected. 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 behavior.
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, or the termination resistors may not be active, 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 BECKHOFF 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.