BECKHOFF BC3100 User Manual

Page 25

PROFIBUS-DP basics

BC3100

RS-485

Data transfer technology

to the Profibus standard

RS485 fundamental characteristics:

Network

topology

Linear bus, active bus terminator at both ends,
branch lines are possible

Medium

Shielded twisted cable, shielding may be
dispensed with in suitable environments
(electromagnetic compatibility)

Number of stations

32 stations in each segment without repeaters,
extendible to 127 with repeaters

Max. bus length

without repeaters

9.6, 19.2, 93.75 kBaud = 1200 m
187.5 kBaud = 1000 m
500 kBaud = 400 m
1500 kBaud = 200 m
3, 6 12 Mbaud = 100 m

with repeaters

Using repeaters (line amplifiers) increases the
max. bus length to the order of 10 km. The
number of possible repeaters is at least 3 and
may be up to 10, depending on the
manufacturer

Transfer rates

9.6, 19.2, 93.75, 187.5, 500, 1500 Kbit/s, up to
12 Mbit/s in discrete steps

Plug

9-Pin D-Sub plug

Pin assignments of the
D-Sub socket

5: GND
3: RxD/TxD- P

8: RxD/TxD- N

Cables for
PROFIBUS DP and
PROFIBUS FMS

In systems which contain more than two stations, all the subscribers are
connected in parallel. The bus cable must always be terminated at the
ends of the lines, to prevent reflections and the transmission problems they
cause.

In order to loop the cable through without any gaps it is necessary to affix
two cables within one plug. Siemens’ SINEC L2 bus connections are very
suitable for this. These SINEC plugs are constructed to accommodate two
bus cables with the corresponding wire terminals and shielding. At the end
of the line you can use a small switch in the plug to activate the terminating
resistor. Please observe the manufacturer’s assembly instructions.

You should also note that the terminating resistor requires a 5 V power
supply for optimal operation. This means that if the plug is removed from
the bus coupler, or the power supply of the bus coupler fails, the level at