National Instruments ENET-232 Series, Manual

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USB Kits
The USB serial hardware gives you a variety of solutions for serial
communication. The USB-232 hardware works with the RS-232 interface
standard, and the USB-485 hardware works with the RS-422 and RS-485
interface standards. You can use the RS-232 ports for serial
communication up to distances of 50 ft. You can connect the RS-485
ports to up to 31 devices using serial cable lengths up to 4,000 ft.
Additionally, the USB serial hardware is available in one-port, two-port,
and four-port versions. All versions use DB-9 connectors. Throughout this
help file,
USB serial hardware refers to all versions of the USB serial
hardware.
The USB-232 two and four-port hardware supports three hardware
transceiver modes supporting both DTE and DCE configurations. For
more information about transceiver control modes, refer to
USB-232
Transceiver Modes .
The RS-485 hardware supports four hardware transceiver control modes
for reliable communication with two and four-wire devices. For more
information about transceiver control modes, refer to
RS-485 Transceiver
Control . In addition, the USB-485 hardware supports programmatic
control of the bias resistors. For more information, refer to USB-485
Programmatically Controlled Bias Resistors .
All USB serial hardware uses standard 16550-compatible UARTs
(Universal Asynchronous Receiver/Transmitters) for complete
compatibility with standard PC COM ports. The serial hardware contains
FIFO (First-In-First-Out) and USB buffers to reduce susceptibility to
system latency for faster transmission rates. Full Plug and Play
compatibility allows switchless configuration and installation. For more
information about the serial hardware specifications and operating
conditions, refer to the
NI Serial Hardware Guide .