Concept
b3 BACnet Devices
This application note provides recommendations and guidelines for the configuration of RS-485 communications between the automation server and b3 BACnet devices. This information is intended to supplement the existing configuration documents, such as the “b3 BACnet and b4920 Controller Technical Reference (30-3001-862)”. 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.
General b3 BACnet Device Properties
The b3 BACnet devices provide a RS-485 transceiver with unit load rating of 0.125 along with a weak bias resistance of 47 kohm creating a total unit load of 0.38.
For more information, see General b3 BACnet Device Properties .
b3 BACnet Device Configuration
The configuration for the b3 BACnet devices is simply the addition of a 120 ohm (1/4W 1%) termination resistor across the TX/RX+ and TX/RX- signal pair (assuming the automation server is located at one end of the bus).
For more information, see b3 BACnet Device Configuration .
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 (b3 BACnet 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 b3 BACnet Device Properties
b3 BACnet Device Configuration
Unit Load Definition, Maximum Network Load and Affects of Excess Unit Load (b3 BACnet Devices)
Cable Routing
Cable Selection