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As an alternative to DoIO() you can use an asynchronous I/O request to
transmit the command.  Asynchronous requests are initiated with SendIO().
Your task can continue to execute while the device processes the command.
You can occasionally do a CheckIO() to see if the I/O has completed.  The
write request in this example will be processed while the example
continues to run:

    SerialIO->IOSer.io_Length   = -1;
    SerialIO->IOSer.io_Data     = (APTR)"Save the whales! ";
    SerialIO->IOSer.io_Command  = CMD_WRITE;
    SendIO((struct IORequest *)SerialIO);

    printf("CheckIO %lx\n",CheckIO((struct IORequest *)SerialIO));
    printf("The device will process the request in the background\n");
    printf("CheckIO %lx\n",CheckIO((struct IORequest *)SerialIO));
    WaitIO((struct IORequest *)SerialIO);  /* Remove message and cleanup */

Most applications will want to wait on multiple signals.  A typical
application will wait for menu messages from Intuition at the same time as
replies from the serial device.  The following fragment demonstrates
waiting for one of three signals.  The Wait() will wake up if the read
request ever finishes, or if the user presses Ctrl-C or Ctrl-F from the
Shell.  This fragment may be inserted into the above complete example.

    /* Precalculate a wait mask for the CTRL-C, CTRL-F and message
     * port signals.  When one or more signals are received,
     * Wait() will return.  Press CTRL-C to exit the example.
     * Press CTRL-F to wake up the example without doing anything.
     * NOTE: A signal may show up without an associated message!

    WaitMask = SIGBREAKF_CTRL_C|
                 1L << SerialMP->mp_SigBit;

    SerialIO->IOSer.io_Command  = CMD_READ;
    SerialIO->IOSer.io_Length   = READ_BUFFER_SIZE;
    SerialIO->IOSer.io_Data     = (APTR)&SerialReadBuffer[0];

    printf("Sleeping until CTRL-C, CTRL-F, or serial input\n");

    while (1)
           Temp = Wait(WaitMask);
           printf("Just woke up (YAWN!)\n");

           if (SIGBREAKF_CTRL_C & Temp)

           if (CheckIO(SerialIO) ) /* If request is complete... */
               WaitIO(SerialIO);   /* clean up and remove reply */
               printf("%ld bytes received\n",SerialIO->IOSer.io_Actual);
    AbortIO(SerialIO);  /* Ask device to abort request, if pending */
    WaitIO(SerialIO);   /* Wait for abort, then clean up */

   WaitIO() vs. Remove().
   The WaitIO() function is used above, even if the request is already
   known to be complete.  WaitIO() on a completed request simply removes
   the reply and cleans up.  The Remove() function is not acceptable for
   clearing the reply port; other messages may arrive while the function
   is executing.

 High Speed Operation 
 Use Of BeginIO() With The Serial Device 
 Ending A Read Or Write Using Termination Characters 
 Using Separate Read And Write Tasks