Norgren VS18 Series Valve Advantage - Manual (Page 54)

VS18/VS26 MAINTENANCE & INSTRUCTION BOOKLET
Wiring termination
The active bus termination using a resistor combination avoids signal reflections during data
transfer and ensures a defined zero-signal voltage on the data lines when none of the stations
on the bus are active. Active termination must be provided at the beginning and end of each
RS485 bus segment.
5V (1)
Data Line - B(4)
Data Line - A(2)
GND(3)
390
Ω
220
Ω
390
Ω
Line termination of wire A in acc. With EN 50 170
If the bus termination is missing, this can cause errors during data transfer. Problems can also
arise if too many bus terminators are fitted since each bus terminator also represents an
electrical load and reduces the signal levels and thus the signal-to-noise ratio. Too many or
missing bus terminators can also cause intermittent data transfer errors, particularly if the bus
segment is operated close to the specified limits for maximum numbers of stations, maximum
bus segment length and maximum data transfer rate.
The power required by the active bus termination is usually obtained through the bus connector
from stations connected to the bus. Alternative measures must be taken if there is no guarantee
that the power required by the bus termination is permanently provided while the bus is
operating. For example, in a particular installation the station providing the power to the bus
termination may need to be repeatedly switched off or removed from the bus for operational
reasons. In such cases, the bus termination should use an external power supply or a repeater
should be used for bus termination instead.
VS2672926-KG00E_05/07 Page 54/97