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3.3
Resistor and Power Jumper Information
3.3.1 Bias Resistors
The ProtoCarrier 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 - i.e. 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 Brian field port were there are very weak bias resistors of 100k). Since there are no jumpers, many
Brains can be put on network without running into the bias resistor limit which is < 500 ohms.
NOTE: See the
Termination and Bias Resistance Enote
for additional information.
ProtoCarrier Start-up Guide
10
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