Viconics VT7600F Integration Guide BACnet - Manual (Page 22)

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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.