BECKHOFF BK3000 - Manual (Page 29)

PROFIBUS coupler BK3xx0 in the PROFIBUS DP
BK3xxx/LC3100 29
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. Whenusing 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.
BK3500 Profibus coupler
with fiber optic conductor Fiber optic conductor: the Profibus User group elaborated the specification
of a transmission technology based on fiber optic conductors for
applications in highly interference-prone environments and also to increase
the range.
Using the Beckhoff Profibus bus couplers with fiber optic interface
(BK3500) the realization of optical Profibus networks with ring technology
(optical one fiber with plastic fiber conductor) is possible. As the head of a
Profibus fiber optic ring a Coordinator (e.g. OZD Profi from Hirschmann) is
required. The maximum amount of stations in a Profibus fiber optic ring
depends on the baud rate. With 1.5 MBaud a maximum of 10 Stations in
one ring are allowed. Between two stations a minimal and maximal fiber
optic conductor length has to be observed (1 - 25 m). The baud rate can be
adjusted via DIP switches on the BK3500. Additional information can be
find in the following table.