Viconics VT8600 Series BACnet Integration Guide - Manual (Page 24)

VT8600 Series BACnet ® Integration Manual 24
Viconics Technologies Inc. | 9245 Langelier Blvd. | St.-Leonard | Quebec | Canada | H1P 3K9 | Tel: (514) 321-5660 | Fax: (514) 321-4150
028-0437-00 www.viconics.com | [email protected] March 2015
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WIRING GUIDE
Cable Type
Viconics recommends the use of balanced 22-24 AWG twisted pair with a characteristic impedance of 100-
130 ohms and capacitance of 30 pF/ft or lower. A braided shield is also recommended.
Impedance
A value based on the inherent conductance, resistance, capacitance, and inductance that represent the
impedance of an infinitely long cable. The nominal impedance of the cable should be between 100 ohms and
120 ohms. Using 120 ohms results in a lighter load on the network.
Capacitance (pF/ft)
The amount of equivalent capacitive load of the cable (per foot basis). One factor limiting total cable length is
the capacitive load. Systems with long lengths benefit from using low capacitance cable (17pF/ft or lower).
NETWORK CONFIGURATION
EIA-485 networks use a daisy chain configuration. A daisy chain has only one main cable and every network
device is connected directly along its path.
Figure 3 illustrates two improper network configurations and the proper daisy chain configuration.
Maximum Number of Devices
A maximum of 64 nodes are allowed on a single daisy-chain segment. A node is defined as any device (panel, zone,
or Repeater) connected to the RS485 network. Terminators do not count as a node.
Add the following to determine the number of nodes on a network:
• One node for each device, including main panels
• One node for each repeater on the chain
Figure 3 : Three different network configurations: star, bus, and daisy chain. Only the daisy chain configuration is correct for an
EIA-485 network
Other methods of wiring an EIA-485 network may give unreliable and unpredictable results. There are no troubleshooting methods
for these types of networks. Site experimentation may be required with no guarantee of success. As a result, Viconics only sup-
ports daisy chain configurations.