BACnet Router Start-up Guide
Page 9 of 27
3.3
R2 Port Jumper Settings
Gently remove the BACnet Router enclosure to access the jumpers on the unit.
3.3.1 Bias Resistors
The FieldServer bias resistors are used to keep the RS-485 bus to a known state, when there is no
transmission on the line (bus is idling), to help prevent false bits of data from being detected. The bias
resistors typically pull one line high and the other low - far away from the decision point of the logic.
In the RS-485 carrier, the bias resistor is 510 ohms which is in line with the BACnet spec. It should only
be enabled at one point on the bus (on the field port were there are very weak bias resistors of 100k).
Since there are no jumpers, many FieldServers can be put on network without running into the bias
resistor limit which is < 500 ohms.
NOTE: See
www.ni.com/support/serial/resinfo.htm
for additional pictures and notes.
Bias Resistor Jumper
Figure 4: Bias Resistor Jumper
