Concept
VT/VZ7xxx Configurations
The recommendations include two different configuration options to choose from with differences in performance and/or resources required.
VT/VZ7xxx Configuration 1: Single End-point Bias (Automation Server Provided Source)
This configuration is the simplest to arrange and supports bus lengths out to 150 m (500 ft) on 24 AWG (0.20 mm²). The automation server should be configured with two 510 ohm bias resistors. One resistor connecting the + data line to the Bias+ terminal on the automation server and the other resistor connecting the – data line to the RET terminal on the automation server. The bias is required to support the transceivers in the VT/VZ nodes and the termination of the bus. AS-P servers and AS-B servers provide 5 V for bias, which supports 120 ohm termination resistors at the two ends of the bus. This will achieve the minimum idle line bias voltage of +200 mV required by the transceivers. Low resistance bias configurations (with 5 V supplies) add significant common mode load on the RS-485 network. The common mode load (Unit Load) of the bias network must be summed with the accumulated unit load of the collection of VT/VZ thermostats/controllers to identify the total unit load on the bus.
For more information, see VT/VZ7xxx Configuration 1: Single End-point Bias (Automation Server Provided Source) .
VT/VZ7xxx Configuration 2: Dual End-point Bias (External Supply Source)
Dual end-point bias applies a separate 5 V DC supply at each end of the network to generate the RS-485 bias. Instead of a single pair of 510 ohm resistors, the dual end scheme uses a pair of 1000 ohm resistors at each end. The dual end-point is the best technique for avoiding the drop in the bias voltage over extended cable lengths as you move away from the bias connection location. By applying bias at the two termination points, the arrangement maintains an equal bias across the complete length of the cable.
For more information, see VT/VZ7xxx Configuration 2: Dual End-point Bias (External Supply Source) .
Power Supply Selection for VT/VZ7xxx Configuration 2
The power required from the 5 V DC supply for the bias circuit is extremely small (approximately 5 mA) so just about any small isolated 5 V DC supply will have more than necessary power rating. A low noise power supply with an output isolated from local ground is recommended to minimize the injection of differential noise onto the bus.
For more information, see Power Supply Selection for VT/VZ7xxx Configuration 2 .
Viconics VT/VZ/SE 7xxx Series Devices
VT/VZ7xxx Configuration 1: Single End-point Bias (Automation Server Provided Source)
VT/VZ7xxx Configuration 2: Dual End-point Bias (External Supply Source)
Power Supply Selection for VT/VZ7xxx Configuration 2