Mounting and wiring
CX8030 / CX8031
27
Version: 1.6
5.2.3
PROFIBUS
5.2.3.1
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).
Table 3: 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.
