Concept
General MNB BACnet Device Properties
MNB BACnet devices use an RS-485 transceiver that provides integrated galvanic isolation (NVE IL3286E). This transceiver has an RS-485 unit load rating of 0.125 along with a weak bias resistance of 220 kohm creating a total unit load of 0.18. This is a little better (lower) than the 0.25UL listed in “TAC I/A Series MicroNet BACnet Wiring and Networking Practices Guide (F-27360-10)”.
The transceiver used in the MNB devices does not include an integrated failsafe receiver function. This means network biasing resistors are required on the RS-485 bus to insure the idle state of the bus is kept at a minimum of +200 mV (target bias value +225 mV or higher for a noise buffer). The biasing of the network has been a jumper selectable feature provided by several of the head-end I/A series devices or routers in the family. The MNB-300, MNB-1000, and I/A Series Network Controllers included the jumper enabled bias. If one or more of these devices are present on the bus, one of these could continue to provide the network bias function.
The MNB device's bias capability mentioned above provides the jumper selected ability to connect 510 ohm pull-up resistor on the MSTP+ data line and 510 ohm pull-down resistor on the MSTP- line.
Configurations 1 and 2 provide dual end-point bias supporting the maximum 1200 m (4000 ft) network length. Configurations 4 and 5 provide the common single end-point bias which is simpler to implement and supports a shorter network length, and frequently covers the distances required in the HVAC applications. For more information, see MNB Configurations .
The RS-485 circuitry on the MNB devices provides an isolated interface. The circuit common/reference used by the RS-485 transceiver is isolated from the local ground potential. This isolated common created by the isolated DC power supply on the adapter is presented on the terminal labeled SHLD on the three position network terminal block.
In addition to the MSTP+ and MSTP- twisted pair signal conductors, the shield drain wire in the RS-485 cable is also bussed in a daisy chain arrangement from each device to the next. The shield conductor provides an isolated reference/common for all MNB devices to use with their isolated RS-485 interfaces. The RS-485 interface provided by the MNB devices has the following interface attributes, as shown in the table below.
|
Transceiver |
Transceiver Unit Load |
Transceiver Failsafe |
Adapter Circuit Bias |
Circuit Bias Load |
Total Unit Load |
Isolated RS-485 Bus |
|
NVE IL3286E |
0.125 |
No |
220 kohm |
0.055 |
0.18 |
Yes |
The typical MS/TP data rates of 38.4 kbps or 76.8 kbps used with the MNB devices make the installation of EOL resistor termination necessary.
RS-485 Communications
MNB BACnet Devices
MNB Configurations