Why isn't the Auto232 transceiver mode on my USB-232 working?

NI Serial Hardware and Software

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.