National Instruments PCI-8430, Manual

Page: 112 / 237

USB-485 Programmatically Controlled Bias
Resistors
On the USB-485 hardware, there are programmatically controlled
onboard bias resistors. In addition, the USB-485 two and four-port
hardware has user-configurable socketed bias resistors. By default, the
USB-485 hardware uses the programmatically controlled bias resistors,
which 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 +3.3 V
RXD– and CTS+ are pulled down to GND
The follwing figure shows a USB transmission line using bias resistors.
USB-485 Transmission Line Using Bias Resistors
The values of the programmatically controlled bias resistors have been
calculated such that there is a voltage of at least 200 mV between the
differential pair. Rather than using the programmatically controlled bias
resistors, you can load custom values of resistors into the sockets in front
of each connector or use external bias resistors. If these resistors are
installed, make sure the programmatically controlled bias resistors are
disabled.
Note Using custom bias resistors on the USB-485 two port may
violate the USB suspend current specification.
Note The series 25 resistors in the figure above are for short-
circuit protection and do not impact serial communication.