National Instruments PCI-8430, Manual

Page: 111 / 237

Bias Resistors
An RS-485 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, the transmission line can be forced
into a known state by installing two bias resistors at one node on the
transmission line. Doing so creates a voltage divider that forces the
voltage between the differential pair to be greater than 200 mV, the
threshold voltage for the receiver.
The following figure shows a transmission line using bias resistors.
Transmission Line Using Bias Resistors
On the PCI RS-485 two and four-port serial cards, PXI RS-485 two and
four-port serial cards, and ENET RS-485 two and four-port serial cards,
there are four user-configurable bias resistors in front of each connector.
These resistors are socketed and pre-loaded with 620 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 +5 V
RXD– and CTS+ are pulled down to GND
Rather than using two 620 resistors at one node, you can increase the
value of the resistors and put them at every node. For instance, if there
are eight nodes in a system, you can use 4.7 k resistors at each node to
effectively achieve the same result.
If your application needs termination, an optional external RS-485 DB-9
terminator that provides 120 is available.