Concept
MNB Configuration 5: Single End-point Bias (MNB Device Provided Source)
This configuration applies to the situation where AS-P or AS-B is not available in the system but an MNB device with bias capability (MNB-300 or MNB-1000) is part of the network and can be used to supply the required network bias. In this configuration, no bias resistors are required on the Automation Server. The 510 ohm bias of an MNB device can be used in place of external resistors on the Automation Server. This configuration allows the termination resistance of 120 ohm to be applied due to the +5 V bias supply of the MNB device.
The 120 ohm termination resistors are placed on the two ends of the network, regardless of the physical location of the MNB device supplying the network bias. The low resistance bias configurations add common mode load on the RS-485 network. The common mode load (Unit Load) of the bias network must be added with the accumulated unit load of the collection of MNB devices to identify the total unit load on the bus.
This configuration differs from Configuration 4 only in the 5 V level being supplied from an MNB-300 or MNB-1000 device instead of AS-P or AS-B. For more information, see MNB Configuration 4: Single End-point Bias (SmartStruxure Server Device Provided Source) . The MNB-300 or MNB-1000 device providing the bias can be located anywhere on the bus. The typical end of bus scenario is assumed here which presents the same reduced cable lengths as for Configuration 4. The 120 ohm termination selection on the MNB device can only be used if the device is positioned on the end of the bus.
Connect a 120 ohm termination resistor across the + and - data lines at the head end of the bus (typically at the Automation Server). Connect another 120 ohm termination resistor across the + and - data lines on the last node at the far end of the bus.
Connect the shield drain wire to earth ground terminal rail in the panel with the Automation Server. This is the only direct ground connection of the shield for the complete segment. The shield drain wire from the cable segments are connected to the RS-485 SHLD terminal at each MNB BACnet device.
Use only the terminal block designated for MS/TP communications for Shield connection.
The example diagram below shows the RS-485 Com B connections on the Automation Server. The guidelines apply the same to Com A.
If an MNB-300 or MNB-1000 device is positioned at the end of the bus, the termination option jumper on the device may be enabled, instead of attaching a 120 ohm resistor on the terminal block. This also applies to the head end of the bus if a MNB-300 or MNB-1000 device is positioned there instead of a typical Automation Server.
If an MNB-300 or MNB-1000 is used to provide the single point bias, the standard plug-on shunt connectors are used to achieve the bias. The plug-on resistor (MNB-BIAS510-50) is only used in Configuration 1.
The recommended maximum cable length and associated wire sizes are listed in the following table. These wire size and lengths are selected to maintain the minimum idle state voltage at the far terminated end of the network (away from the head end of the bus where the bias is attached).
|
Maximum Length |
Wire Size |
|
150 m (500 ft) |
24 AWG (0.20 mm²) |
|
240 m (800 ft) |
22 AWG (0.33 mm²) |
MNB BACnet Devices
MNB Configurations
MNB Configuration 3: Mid-point Bias
MNB Configuration 4: Single End-point Bias (SmartStruxure Server Device Provided Source)