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Appendix B Serial Port Information

Termination

Because each differential pair of wires is a transmission line, you must

properly terminate the line to prevent reflections. A common method of

terminating a two-wire multidrop RS-485 network is to install terminating

resistors at each end of the multidrop network. If you daisy-chained

multiple instruments together, you need a terminating resistor at only the

first and last instruments. The terminating resistor should match the

characteristic impedance of the transmission line (typically 100 to 120 Ω).

You can order an optional DB-9 RS-485 termination connector that

contains embedded terminating resistors for easy termination from

National Instruments. For ordering information, contact National

Instruments.

Figure B-3 shows a multidrop network using terminating resistors.

Figure B-3. Multidrop Network Using Terminating Resistors

Bias Resistors

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. To do so, install two 620 Ω 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. You should install these resistors on only one node.

Note

Bias resistors are not available on the eight-port PCI-485 or eight-port PXI-8421.

MASTER

Slave 1

100 Ω

Slave 2 Slave n

100 Ω

1 2 ... 131 132 133 134 135 136 137 138 139 140 141 ... 207 208

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National Instruments AT User Manual Page 136