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.