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.
Содержание ENET-232 Series
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Страница 5: ...Using Help Conventions Navigating Help Searching Help Printing Help File Topics ...
Страница 17: ...How to Use this Help File To use this help file refer to the following flow chart and the appropriate topics ...
Страница 21: ...Serial Hardware Overview PCI Kits PXI Kits USB Kits ENET Kits ExpressCard Kits PCMCIA Kits ...
Страница 24: ...PCI 485I RS 485 two port isolated RS 485 four port isolated ...
Страница 27: ...PXI 8423 RS 485 two port isolated RS 485 four port isolated ...
Страница 63: ...Save Pending Changes To save your changes to this item click Save ...
Страница 65: ...Restore Defaults To reset the fields to their default values at any time click the Defaults button ...
Страница 98: ...0 LPVOID l_SerialInterface sizeof l_SerialInterface LPDWORD l_ByteCount NULL ...
Страница 127: ...2 PORT LEDs USB Serial Hardware LEDs ...
Страница 139: ...Connecting the Cables to a Four Port PXI Serial Board ...
Страница 141: ...67 58 50 41 33 24 16 7 TXD 9 RI 9 ...
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Страница 181: ...How many serial ports can I have on my machine You can have up to 256 serial ports ...
Страница 189: ...nslookup hostname If you still cannot communicate with your device contact National Instruments ...
Страница 201: ...Glossary Prefixes Symbols A B C D E F G H I K L M P R S T U V ...
Страница 203: ...Symbols degrees ohms percent ...
Страница 204: ...A A amperes ANSI American National Standards Institute ...
Страница 205: ...B b bits B bytes baud bits per second bps bits per second ...
Страница 206: ...C C Celsius COM Computer Output Microform used in reference to a communication port CTS clear to send ...
Страница 208: ...E EIA Electronic Industries Association EMI electromagnetic interference ENET Ethernet ...
Страница 209: ...F FCC Federal Communications Commission FIFO First In First Out ft feet ...
Страница 210: ...G G ground ...
Страница 211: ...H HSI handshake input HSO handshake output Hz Hertz ...
Страница 213: ...K Kbaud kilobits per second ...
Страница 214: ...L LED light emitting diode ...
Страница 215: ...M m meters MB megabyte ...
Страница 216: ...P PC personal computer PCI Peripheral Components Interconnect ...
Страница 218: ...S s seconds SCSI Small Computer Systems Interface ...
Страница 220: ...U UART Universal Asynchronous Receiver Transmitter ...
Страница 221: ...V V volts VDC volts direct current VXI VME eXtensions for Instrumentation ...
Страница 222: ...Important Information Warranty Copyright Trademarks Patents Warning Regarding Use of NI Products Compliance ...