National Instruments ENET-232 Series User Manual

Also see for ENET-232 Series: Manual

Contents

National Instruments ENET-232 Series User Manual Manual pdf 52 page image

Chapter 6 Using Your Serial Device Server© National Instruments Corporation 6-11 ENET-232 and ENET-485 Series User ManualChanging the Bias Resistors on the ENET-485This section explains the functionality of the ENET-485 bias resistors anddescribes the method for changing them.Functionality of Bias ResistorsA transmission line enters an indeterminate state if no nodes aretransmitting on it. This indeterminate state can cause the receivers toreceive invalid data bits from noise picked up on the cable. To prevent aline from receiving these data bits, force the transmission line into a knownstate. The ENET-485 Series contains two 620 Ω bias resistors on thetransmission line. These resistors create a voltage divider that forces thevoltage between the differential pair to be greater than 200 mV, thethreshold voltage for the receiver.Figure 6-2 shows a transmission line using bias resistors.Figure 6-2. Transmission Line Using Bias ResistorsThe ENET-485 Series has four user-configurable bias resistors in front ofeach connector. These resistors are socketed and preloaded with 620 Ωresistors. They are connected to the receive signals of each port to maintaina 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 GND620 ΩBias Resistor+5ARxB620 ΩBias Resistor100 ΩTermination Resistor