BECKHOFF BC3100 - Manual (Page 24)

PROFIBUS-DP basics
24 BC3100
The transfer medium: plugs and cables
Physics of the transmission The physical data transfer is defined in the PROFIBUS standard. See
PROFIBUS layer 1 (physical layer).
The sphere of operation of a fieldbus system is substantially determined by
the selected transfer medium and the physical bus interface. Besides the
requirements of data transfer security, the necessary expenditure for
procuring and installing the bus cable is of crucial significance. The
PROFIBUS
standard therefore provides for various forms of
communications technology while maintaining its standard bus protocol.
Cable transfer: this version, which confirms to the US standard EIA
RS-485, has been specified as the basic version for applications in the field
of production technology, building management technology and drive
technology. It uses a single twisted-pair copper cable. Shielding may be
unnecessary, depending on the planned application (take electromagnetic
compatibility aspects into consideration).
Channel-related
disturbances Two cable types with different maximum cable lengths are available. See
table entitled "RS485". The pin assignments on the connector and the
wiring are shown in the figure. Please pay attention to the special
requirements for the data cable at board rates in excess of 1.5 MBaud. The
right cable is a basic requirement for disturbance-free operation of the bus
system. When using the "normal" 1.5 Mbaud cable, astonishing
phenomena may occur as the result of reflections and excessive
attenuation. This may consist of the following: any one station is without a
connection and it resumes the connection when the neighbouring station is
extracted. Or, data transfer errors may occur when a certain bit pattern is
transferred. This means that the Profibus operates without disturbance but
without functioning of the system and randomly reports bus errors after
start up. The error behaviour described is eliminated by reducing the baud
rate ( < 93.75 kBaud).
If reducing the baud rate does not remedy the problem, this is frequently
due to a wiring error. Either the two data lines have been swapped on one
or several connectors or the terminators are not on or are activated in the
wrong place.
Fiber-optic conductor: the specification for a data transfer technology
based on fiber-optic conductors was elaborated in PNO for applications in
environments that are subject to extreme interference and to increase the
range at high data transfer rates. The specification is currently available as
a draft PNO guideline. The PROFIBUS COUPLER requires an external
module for conversion from RS485 to fiber-optic. The setup is clearly more
complex because of the need for an optical converter to convert from
RS485 to the "fiber-optic sub ring".