Concept
Viconics VT/VZ/SE 7xxx Series Devices
This application note provides recommendations and guidelines for the configuration of RS-485 communications between the automation server and Viconics room controllers. The recommendations are associated with the Viconics VT/VZ/SE 7xxx series thermostats / room controllers configured with the Viconics plug-on MS/TP network adapter. This information is intended to supplement the instructions you receive with the various RS-485 network devices. The guidelines focus on the arrangement of the electrical interface to the automation server's RS-485 port in regards to biasing, termination, cable selection, cable lengths, and cable routing. The guidelines on maximum unit load (node count) and common mode voltage tolerance are associated with AS-P and AS-B servers.
The general wiring of the BACnet MS/TP adapter is described in the following Viconics documents:
BACnet Integration Manual ITG-VT(R) 72_73-BAC-E11 (028-6009 R11, 2012-02-07)
BACnet Integration Manual ITG-VT76xx-PIR-BAC-E02 (028-6015 R2, 2011-01-23)
BACnet Integration Manual ITG-VZ7xxx-BAC-E04 (028-6011 R4, 2012-05-31)
BACnet Zoning System Application Guide VBZS_Rel2_Application_Guide-E04 (2012-01-10)
General Viconics VT/VZ7xxx Device Properties
The RS-485 transceiver encountered on the Viconics MS/TP adapters reviewed was the STM ST485C. This transceiver alone has a RS-485 unit load rating of 0.5. With consideration only for the transceiver unit load, the 0.5 unit load rating would indicate up to 64 units transceivers could be attached to a single RS-485 network wire segment. This is the unit load and node limit described in Viconics' documents. The network adapter includes an on-board weak bias resistor of 100 kohm on the A+ and B- bus signals. This added 100 kohm resistance presents an additional unit load of 0.12UL making each of the controllers have an aggregate unit load of 0.62. This is the value that should be used in totaling the network load.
For more information, see General Viconics VT/VZ7xxx Device Properties .
VT/VZ7xxx Configurations
The recommendations include two different configuration options to choose from with differences in performance and/or resources required.
For more information, see VT/VZ7xxx Configurations .
Unit Load Definition, Maximum Network Load and Affects of Excess Unit Load
According to the TIA-485A standard, a single unit load is equivalent to a 12 kohm impedance attached to the + and – data lines (connected to ground or supply). A 1/8UL transceiver would have an impedance of 96 kohm. The TIA-485A defined total network load limit of 32UL is based on a common mode load resistance of 375 ohm connecting both the + and – data lines to ground (or CMV source). The standard requires the RS-485 drivers be capable of driving a network load of 32UL along with a Common-Mode Voltage (CMV) difference of -7 V to +12 V and produce a guaranteed minimum of 1.5 V transmit signal level. Such a full UL load with severe CMV conditions exhausts the maximum drive current of 60 mA provided by all standard RS-485 drivers. The specified minimum of 375 ohm resistance for the common mode load is the resulting resistance seen when 32 transceivers with 12 kohm input impedance are placed in parallel (12,000 / 375 = 32).
For more information, see Unit Load Definition, Maximum Network Load and Affects of Excess Unit Load (Viconics VT/VZ7xxx Devices) .
Expanded Unit Load with Network of Isolated Devices Only
If the network is comprised exclusively of devices with isolated RS-485 interfaces with the only exception being the automation server, it is recommended that the maximum unit load limit can be stretched higher. It is recommended that a maximum load extension should be 16UL (that is, 50% overload) giving a total expanded unit load limit of 48UL. Using a maximum network load of 48UL and subtracting the 24UL for the bias network and automation server leaves 24UL available for the sensors/controllers. With the example device load of 0.62UL each, it is suggested that the isolated bus arrangement could support the full collection of up to 38 sensors.
For more information, see Expanded Unit Load with Network of Isolated Devices Only (Viconics VT/VZ7xxx Devices) .
Cable Routing
The RS-485 network cable should be routed in a continuous daisy chain bus configuration. There should not be any stub connections, stars or ring configurations. The bussed cable should pass through each node to be connected with no splits or branches in the cable network.
For more information, see Cable Routing .
Cable Selection
This is one of the most important selections having significant impact on the performance and reliability of the RS-485 network being installed. An incorrect cable selection can be difficult and expensive to reverse. The decision should not be made on previous examples of seeing some alternate non-compliant cable work.
For more information, see Cable Selection .
RS-485 Communications
General Viconics VT/VZ7xxx Device Properties
VT/VZ7xxx Configurations
Unit Load Definition, Maximum Network Load and Affects of Excess Unit Load (Viconics VT/VZ7xxx Devices)
Expanded Unit Load with Network of Isolated Devices Only (Viconics VT/VZ7xxx Devices)
Cable Routing
Cable Selection