18 Network Guide
Data Bus Physical Specifications and Cable Require-
ments
Cables composed of stranded conductors are preferred over solid conductors as stranded
conductor cable better resist breakage during pulling operations. Distech Controls strongly
recommends that the following data bus segment cable specifications be respected.
Shielded cable offers better overall electrical noise immunity than non-shielded cable.
Unshielded cable or cable of a different gauge may provide acceptable performance for
shorter data bus segments in environments with low ambient noise.
Table 5: BACnet MS/TP Data Bus Segment Physical Specifications and Cable Requirements
Parameter Details
Media Twisted pair, 24 AWG (see also Metric Conversions for Wire
Gauge on page 133).
Shielding Foil or braided shield.
Shield grounding The shield on each segment is connected to the electrical system
ground at one point only; see Data Bus Shield Grounding Require-
ments on page 24.
Characteristic impedance 100-130 Ohms. The ideal is 100-120 Ohms.
Distributed capacitance
between conductors
Less than 100 pF per meter (30 pF per foot). The ideal is less than
60 pF per meter (18pF per foot).
Distributed capacitance
between conductors and
shield
Less than 200 pF per meter (60 pF per foot).
Maximum length per segment 1220 meters (4000 feet).
Data Rate 9600, 19 200, 38 400, and 76 800 baud.
Polarity Polarity sensitive
Multi-drop Daisy-chain (no T-connections)
EOL terminations 120 ohms at each end of each segment
Data bus bias resistors 510 ohms per wire (max. of two sets per segment)
Table 6: Distech Controls Recommended Cable Types for BACnet MS/TP Data Buses
Cable Type Part Number O.D. (Ø)
300 meters (1000 feet), 24 AWG Stranded,
Twisted Pair Shielded Cable - FT6, Rated
for Plenum Applications
CB-BACN6BL1000 3.75mm (0.148 in.)
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