Adding field devices to the RS485 network Section 2 Transmission Technology
30 3BDS009029R5001 B
Adding field devices to the RS485 network
A shielded twisted-pair copper cable is the hardware required for RS485
transmission. It connects all devices to the bus structure (line). The bus structure
allows for non-reactive coupling and decoupling of stations or step wise
commissioning of a system. Later extensions have no impact on stations that are
already working on the bus.
When connecting the field devices it must be ensured that the data lines are not
reversed. Always use a shielded data line to ensure high interference immunity of
the system against electromagnetic emissions. The shield should be grounded on
both sides and large-area shield clamps should be used for grounding to ensure good
conductivity. Furthermore the data lines should be laid separately and away from all
power cables. Never use spur lines for transmission rates >
1,5 Mbit/s (see also Line
Topology with spurs on page 60).
A 9-pin D-Sub connector is primarily used to connect PROFIBUS devices on the
bus, which complies with DIN 19245/ EN 50170. Normally connectors are used,
which supports direct connection of the incoming and outgoing data cable.
This eliminates the need for spur lines and the bus connector can be connected and
disconnected to the field device at any time without interrupting data
communication. The type of connector suitable for RS485 transmission technology
depends on the degree of protection.
To avoid signal reflections on the bus line, the line structure must be terminated by a
bus termination resistor network at either end. The bus is terminated at each end of a
segment with an active bus termination. Both bus terminators have a permanent
Figure 3. PROFIBUS connector
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