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Siemens
Desigo TRA Engineering, mounting and installation
CM111043en_09
Building Technologies
MS/TP networks
2016-02-22
10.2 Technical data BACnet MS/TP
Communications on BACnet MS/TP networks take place over EIA-485 physical
media.
Desigo TRA devices use the 3-wire interface without noise suppression.
Therefore, the cables must provide noise protection. This is guaranteed by the
reference wire and the shielding.
The interface is protected against short circuit, wrong polarity and faulty wiring
with AC 24 V. However, communications do not work with wrong polarity.
Transmission medium
(bus cable)
1.5-Pair (1 TP & 1 conductor)
with overall Shield and drain wire
Pair
1)
Gauge
Min. 0.2 mm
2
(24 AWG) stranded or solid
2)
Capacitance
conductor to
conductor
conductor to shield
Max. 41 pF/m / 12.5 pF/ft
Max. 79 pF/m / 24 pF/ft
Twists
Min. 13/m / 4/ ft
Reference wire ( )
Gauge
Min. 0.2 mm
2
/ 24 AWG stranded, or solid
Twists with pair
Min. 13/m / 4/ ft
Shield
100% overall foil with drain wire
3)
Voltage rating
300 V
4)
Temperature rating
75°C / 167 °F or higher
Standards
EU CE Mark
2011/65/EU (ROHS II)
2002/96/EC (WEE)
2003/11/EC (BFR)
MII Order #39 (China RoHS)
1)
Stranded conductors are easier to connect in narrow places.
Less risk of breaking in case of vibrations
2)
Higher capacitance must be compensated by
– smaller network length or
– smaller number of devices or
– smaller Baud rate.
3)
Infrastructure network cabling is normally separated from mains cables, 30 V
would be sufficient. But in cabinets, very small distance to main cables,
therefore 300 V or even 600 V is required.
National standards may also require 300 V.
4)
As the room automation stations require less than 50 °C, cabling in the
cabinet can have the same rating. But outside the cabinet, avoid hot places
like lighting troughs.
3-wire RS-485 network
interface
Cable specifications
