National Instruments PCI-8430, Manual

Page: 185 / 237

Why isn't the Auto232 transceiver mode on my
USB-232 working?
The Auto232 mode of operation allows you to connect to either a RS-232
DTE or DCE device using either a null-modem or straight-through cable.
The USB-232 port will automatically switch approximately every 250 ms
between DTE and DCE operating modes until at least one RS-232
receiver line (RX, CTS, DSR, RI, or DCD) has a valid voltage. Auto232 is
available on the USB-232/2 and USB-232/4, and is not available on the
USB-232 (single port).
Not all devices and applications can take advantage of this feature:
Some devices can generate valid voltages on receive lines for both
modes, even though the device itself is only DTE or DCE. Thus,
these devices may not operate correctly when using Auto232
mode.
Some applications immediately start communicating after a COM
port is open, thus not allowing enough time for the proper
operating mode to be selected.
Some cabling schemes short RI, DCD, and DSR together. While
this does not present a problem in DTE mode, these signals are
driven when in DCE mode. Thus, by shorting the signals together,
the outputs will be double (or triple) driven which may cause signal
noise or other artifacts. Use a straight-through cable instead.
If problems with Auto232 can not be resolved, set the USB-232 to the
correct mode (DTE or DCE) for your device and application. By default,
the USB-232 ports are set to DTE mode.