Viconics VTR7300 BACnet Integration Manual - Manual (Page 24)

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Cable Type
Viconics recommends the use of balanced 22-24 AWG twisted pair with characteristic impedance of 100-
130 ohms, 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 100Ωand
120Ω. However using120Ω will result in a lighter load on the network.
Capacitance (pF/ft)
The amount of equivalent capacitive load of the cable, typically listed in a per foot basis. One of the factors
limiting total cable length is the capacitive load. Systems with long lengths benefit from using low
capacitance cable (i.e. 17pF/ft or lower).
Network Configuration
EIA-485 networks use a daisy chain configuration. A daisy chain means that there is 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.
Other methods of wiring an EIA-485 network may give unreliable and unpredictable results. There are no
troubleshooting methods for these types of networks. Therefore, a great deal of site experimentation may
have to be done, making this a difficult task with no guarantee of success. Viconics will only support daisy
chain configurations.
Figure 3: Three different network configurations: star, bus, and daisy chain. Only
the daisy chain configuration is correct for an EIA-485 network.