Bias Resistors; Figure 8: Bias Resistor Dip Switches - Siemens ProtoAir FPA-W44 Startup Manual

4.3

Bias Resistors

To enable Bias Resistors, move both the BIAS- and BIAS+ dip switches to the right as shown in
Figure 8.
The ProtoAir 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.
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 (for example, on the field port were there are very weak bias resistors of 100k). Since
there are no jumpers, many gateways can be put on the network without running into the bias resistor
limit which is < 500 ohms.
NOTE: See www.ni.com/support/serial/resinfo.htm
NOTE: The R1 and R2 DIP Switches apply settings to the respective serial port.
NOTE: If the gateway is already powered on, DIP switch settings will not take effect unless the
unit is power cycled.

Figure 8: Bias Resistor DIP Switches

for additional pictures and notes.
Page 13 of 66
Siemens ProtoAir Start-up Guide
R1 Bias Resistor DIP
Switches (2 and 3)
R2 Bias Resistor DIP
Switches (2 and 3)