Page 49 of 82
Date: 27.01.06
Bus terminating resistor
In order to avoid signal reflections on the bus, each bus segment must be
terminated at its physical start and end point with a terminating resistor. A
terminating resistor is connected between bus wires A and B.
This provides a defined open-circuit potential when no data transmission is taking
place on the bus.
Bus terminating resistor in the IP20 version
A terminating resistor is integrated in most 9-pin sub-min D connectors. If the
module is connected at the end of the bus line, a terminating resistor must be
connected by setting the switch to ON.
Bus terminating resistor in the IP65 version
A built-in terminating resistor is located near the Profibus terminal block. If the
module is connected at the end of the bus line, a terminating resistor must be
connected by setting both contacts of the DIP switch to ON.
7.6
Shielding
Shielded cables are recommended where there is increased risk of interference
e.g. in an industrial environment. The supply cables and signal leads should also
be shielded in this case. In general, the shield of each bus circuit should be
connected to the protective ground (not DataGround).
7.7
Equipotential bonding
The potential difference between the data reference potentials of each device
connected to the bus must not 7 volts. If this cannot be guaranteed,
equipotential bonding must be provided. For the bus devices this usually means
that the negative terminal of the power supply needs to be looped through from
device to device as the equipotential bonding conductor. Since the Sensor
Profibus Module has a power supply that is d.c. isolated from the bus, the module
does not need to be included in the equipotential bonding system.
7.8
Setting the baud rate
No setting is required for the Sensor Profibus Module because the baud rate is
detected automatically. Stubs create numerous problems and should therefore be
avoided completely (especially for a high bus speed). See EN50170 standard.
