Concept
b3 BACnet Device Configuration
The configuration for the b3 BACnet devices is simply the addition of a 120 ohm (1/4W 1%) termination resistor across the TX/RX+ and TX/RX- signal pair (assuming the automation server is located at one 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 ground connection of the shield for the complete cable segment. Connect the RET terminal (see the figure below) 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 wires from the cable segments are connected to the SHLD terminal at each b3 BACnet device.
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.
The b3 BACnet configuration supports the full recommended maximum cable length of 1200 m (4000 ft). The network does not encounter distance reductions and/or alternate configurations typical of networks requiring the addition of failsafe bias circuits.
Installation of the network in some high electrical noise environments or use of substandard cable may prompt differential noise levels on the bus sufficient to exceed the integrated failsafe noise immunity level of the transceiver. If encountered, the addition of a weak bias such as 3300 ohm pull-up (from the TX/RX+ terminal to the Bias+ terminal) and 3300 ohm pull-down (from the TX/RX- terminal to the RET terminal) can be beneficial. The 3300 ohm bias addition only adds 3.6UL to the unit load.
The TIA-485A standard specifies a maximum network load of 32UL (Unit Loads). From the TIA-485A budget of 32UL, we must subtract the Unit Load (0.5UL) for the automation server to calculate the node count:
Subtracting 0.5UL from the starting budget of 32 gives a node budget of 31.5UL.
Each of the b3 BACnet devices presents a RS-485 network load of 0.38UL.
The calculated b3 BACnet node count that consumes the remainder of the budget is 31.5UL / 0.38UL = 82 nodes.
RS-485 Communications
b3 BACnet Devices