Concept
MNB Configurations
The recommendations include five different configuration options to choose from with differences in performance and/or resources required.
MNB Configuration 1: Dual End-point Bias (MNB Source)
Dual end-point bias applies a separate 5 V DC supply at each end of the network to generate the RS-485 bias. Instead of a single pair of 510 ohm resistors, the dual end scheme uses a pair of 1000 ohm resistors at each end. The dual end-point is the best technique for avoiding the drop in the bias voltage over extended cable lengths as you move away from the bias connection location. By applying the bias at the same location as the termination, the arrangement maintains an equal bias across the complete length of the cable. This technique avoids the positional sensitivity of Configuration 3 in locating the middle point of the network and provides a superior bias reliability. For more information, see MNB Configuration 3: Mid-point Bias . This Configuration 1 differs from Configuration 2 only in the use of MNB-300 or MNB-1000 devices to provide the bias supply source. For more information, see MNB Configuration 2: Dual End-point Bias (External Supply Source) .
For more information, see MNB Configuration 1: Dual End-point Bias (MNB Source) .
MNB Configuration 2: Dual End-point Bias (External Supply Source)
When MNB-300 or MNB-1000 devices are not available within 60 m (200 ft) of the end of the cable, the dual end-point bias configuration can still be deployed as seen here in Configuration 2 using a separate 5 V DC supply at each end of the network to generate the RS-485 bias. The same as Configuration 1, this arrangement provides the best performance on systems where bias is needed. For more information, see MNB Configuration 1: Dual End-point Bias (MNB Source) . The dual end-point avoids the drop in the bias voltage seen with the single end-point bias arrangements. By applying the bias at the same location as the termination, the arrangement maintains an equal bias across the complete length of the cable. This technique does not encounter the positional sensitivity of Configuration 3 in locating the middle point of the network. For more information, see MNB Configuration 3: Mid-point Bias .
For more information, see MNB Configuration 2: Dual End-point Bias (External Supply Source) .
MNB Configuration 3: Mid-point Bias
Mid-point bias refers to the technique of using a separate 5 V DC power supply and locating it at the middle of the total network cable length. This technique takes advantage of the maximum DC differential resistance at the center to maximize the bias voltage applied through the typical pair of 510 ohm resistors from the 5 V bias supply. The higher bias voltage level now applied in the center can extend up to 600 m (2000 ft) in both directions from the center using 24 AWG (0.20 mm²) size cable (or larger).
For more information, see MNB Configuration 3: Mid-point Bias .
Power Supply Selection for MNB Configuration 1, 2 and 3
The power required from the 5 V DC supply for the bias circuit is extremely small (approximately 5 mA) so just about any small isolated 5 V DC supply will have more than necessary power rating. A low noise power supply with an output isolated from local ground is recommended to minimize the injection of differential noise onto the bus.
For more information, see Power Supply Selection for MNB Configuration 1, 2 and 3 .
MNB Configuration 4: Single End-point Bias (Automation Server Provided Source)
This configuration applies to the situation where existing MNB devices with bias capability (MNB-300 or MNB-1000) are not available on the network segment, and it is preferred not to add external supplies.
For more information, see MNB Configuration 4: Single End-point Bias (Automation Server Provided Source) .
MS/TP Data and Shield Connections
The cable shield drain wire is connected to each MNB device to provide a ground reference to the isolated RS-485 interface in each of the MNB devices. This shield connection on each MNB device is not a local ground connection. The shield drain wire from the MS/TP bus cable must only connect to the specific terminal designated to receive the shield for the MS/TP bus. The terminal label/text reference varies a little in the device documentation between the MNB devices. In all cases, it will be a third screw terminal in a set of three on a terminal block identified for MS/TP communications.
For more information, see MS/TP Data and Shield Connections .
MNB BACnet Devices
MNB Configuration 1: Dual End-point Bias (MNB Source)
MNB Configuration 2: Dual End-point Bias (External Supply Source)
MNB Configuration 3: Mid-point Bias
Power Supply Selection for MNB Configuration 1, 2 and 3
MNB Configuration 4: Single End-point Bias (Automation Server Provided Source)
MS/TP Data and Shield Connections