Concept
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.
The automation server should be configured with bias resistors (510 ohm) to allow termination to be applied to each end of the network containing the MNB devices. AS-P servers and AS-B servers provide a 5 V bias voltage supply, which supports the use of 120 ohm termination resistors at the two ends of the bus.
The requirement for bias resistors to maintain an idle state voltage greater than 200 mV, and the bus current created by the termination resistors, presents a limitation on the compliant wire length due to idle state voltage drop in the wire. Unlike networks that do not require bias and the dual end-point bias configurations, when delivering the bias from a single end, the wire size will have a significant effect on the network distance over which the bias level can travel and maintain compliance. This is due to the voltage divider network setup by the wire resistance and the termination resistance. The bias is inducing a continuous DC current flow of about 5 mA and this current induces a continuous escalating voltage drop as you move down the cable away from the bias location toward the far end termination where the worst case bias voltage will be seen. See table below for recommended wire sizes and maximum distance.
Connect a 510 ohm bias resistor from the TX/RX+ terminal to the Bias+ terminal on the automation server (see the figure below). Connect another 510 ohm bias resistor from the TX/RX- terminal to the RET terminal on the automation server.
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 termination resistor across the + and - data lines on the last node at the far end of the bus.
Connect the shield drain wire directly to earth terminal rail in the panel with the automation server. This is the only direct ground connection of the shield for the complete segment. Connect the RET terminal on the automation server to the ground rail in the panel using a 12 AWG (3.31 mm²) to 18 AWG (0.82 mm²) wire. 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 alternate RS-485 terminal block connections for the different automation server models.
The example diagram below shows the RS-485 Com B connections on the AS-P and AS-B servers. The guidelines are the same for Com A. When failsafe bias resistors are required on the Com A network, the pull-up voltage is obtained from the Bias+ terminal.
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