R2 Port Jumper Settings; Rs-485 Port; Figure 5: Bias Resistor Jumper - MSA FieldServer QuickServer FS-QS-1011 Startup Manual

Equipment Setup
2.3

R2 Port Jumper Settings

Gently remove the QuickServer enclosure to access the jumpers on the unit.
2.3.1

RS-485 Port

NOTE: The following Sections only apply to QuickServer models: FS-QS-1011 and FS-QS-1211.
Bias Resistors
Bias Resistor Jumper

Figure 5: Bias Resistor Jumper

The QuickServer 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.
QuickServer Start-up Guide 13