Concept
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.
The transceiver used in the b3 BACnet devices includes an integrated failsafe receiver function. This means strong network biasing resistors are not required on the RS-485 bus to ensure the maintenance idle state (failsafe) voltage. This integrated failsafe feature avoids the additional unit load caused by a strong bias resistor network discussed in other application notes.
The RS-485 circuitry on the b3 BACnet devices is not an isolated interface. The RS-485 network depends on the local device ground potentials being within the -7V to +12V common mode voltage (CMV) range of the RS-485 transceivers. The RS-485 interface provided by the b3 BACnet devices has the interface attributes according to the table below.
|
Transceiver |
Transceiver Unit Load |
Transceiver Failsafe |
Adapter Circuit Bias |
Circuit Bias Load |
Total Unit Load |
Isolated 485 Bus |
|
Multiple Parts |
0.125 |
Yes |
47 kohm |
0.255 |
0.38 |
No |
The typical MS/TP data rates of 38.4 kbps or 76.8 kbps used with the b3 BACnet devices make the installation of End Of Line (EOL) resistor termination necessary. Because the b3 BACnet devices do not require network bias, there is no negative effect from termination. The 120 ohm termination is recommended regardless of the data rate used.
When it is necessary to combine one or more b3 BACnet family devices on the same MS/TP bus segment with MS/TP devices that use isolated RS-485 interfaces (that require network bias), you should evaluate the impact such combination can have on the recommended node counts for the two products. Isolated RS-485 interface products have a tolerance for Common Mode Voltage that allows them to have an expanded unit load budget within our recommendations (48UL instead of 32UL). When the isolated interface product needs bias, the expanded budget is more than consumed by the load. The addition of any non-isolated devices to the bus (with exception of the automation server) nullifies the recommended budget expansion returning the recommended limit to 32UL. Review the load status of the isolated products to determine if they can tolerate the standard 32UL budget.
For more information, see Generic RS-485 Network Devices .
For more information, see MNB BACnet Devices .
RS-485 Communications
b3 BACnet Devices
Generic RS-485 Network Devices
MNB BACnet Devices