I/O Terminal /
MOUNTING AND WIRING
22
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
Maximum 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
Maximum 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
3.6.2 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 are
used! Wiring errors are avoided, and commissioning is more rapidly completed. The 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.
