Changing The Bias Resistors On The Enet-485; Functionality Of Bias Resistors - National Instruments ENET-232 Series User Manual

Changing the Bias Resistors on the ENET-485

Functionality of Bias Resistors

© National Instruments Corporation
This section explains the functionality of the ENET-485 bias resistors and
describes the method for changing them.
A transmission line enters an indeterminate state if no nodes are
transmitting on it. This indeterminate state can cause the receivers to
receive invalid data bits from noise picked up on the cable. To prevent a
line from receiving these data bits, force the transmission line into a known
state. The ENET-485 Series contains two 620 Ω bias resistors on the
transmission line. These resistors create a voltage divider that forces the
voltage between the differential pair to be greater than 200 mV, the
threshold voltage for the receiver.
Figure 6-2 shows a transmission line using bias resistors.
Rx
Figure 6-2. Transmission Line Using Bias Resistors
The ENET-485 Series has four user-configurable bias resistors in front of
each connector. These resistors are socketed and preloaded with 620 Ω
resistors. They are connected to the receive signals of each port to maintain
a known state when the bus is idle. The connections are made as follows:
• RXD+ and CTS– are pulled up to VCC
• RXD– and CTS+ are pulled down to GND
Chapter 6
A
B
6-11 ENET-232 and ENET-485 Series User Manual
Using Your Serial Device Server
+5
620 Ω
Bias Resistor
100 Ω
Termination Resistor
620 Ω
Bias Resistor