Distech Controls ECLYPSE APEX, Руководство пользователя

Страница: 119 / 171

119 ECLYPSE APEX
Data Bus Physical Specifications and Cable
Requirements
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.
Parameter Details
Media Twisted pair, 24 AWG
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
Requirements .
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 (18 pF 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
When VCC=5V: 510 ohms per wire (max. of two sets per segment)
When VCC=3.3V (The APEX is 3.3V internal): 330 ohms per wire
Table 13: BACnet MS/TP Data Bus Segment Physical Specifications and Cable Requirements
Shielded cable offers better overall electrical noise immunity than non-shielded cable. Unshielded ca-
ble or cable of a different gauge may provide acceptable performance for shorter data bus segments
in environments with low ambient noise.
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.)
Table 14: Distech Controls Recommended Cable Types for BACnet MS/TP Data Buses
Distech Controls BACnet cable offers the best performance over the full range of baud rates, cable
lengths, and number of connected devices. This is primarily due to lower conductor-to-conductor ca-
pacitance of this cable.
Data Bus Topology and EOL Terminations
Function of EOL Terminations
The first and last device on the data bus must have End-of-Line (EOL) termination resistors connected
across the two data lines/wires of the twisted pair. These resistors serve the following purposes:
£
EOL terminations dampen reflections on the data bus that result from fast-switching (high-speed
rising and falling data edges) that otherwise would cause multiple data edges to be seen on the
data bus with the ensuing data corruption that may result. The higher the baud rate a data bus is
operating at, the more important that EOL terminations be properly implemented. Electrically, EOL
terminations dampen reflections by matching the impedance to that of a typical twisted pair cable.
BACnet MS/TP Communication Data Bus Fundamentals