You read from the parallel device by passing an ioextpar to the device
with cmd_read set in io_command, the number of bytes to be read set in
io_Length and the address of the read buffer set in io_Data.

   #define READ_BUFFER_SIZE 256
   /* Reserve SIZE bytes of storage */
   char ParallelReadBuffer[READ_BUFFER_SIZE];

   ParallelIO->IOPar.io_Length   = READ_BUFFER_SIZE;
   ParallelIO->IOPar.io_Data     = (APTR)&ParallelReadBuffer[0];
   ParallelIO->IOPar.io_Command  = CMD_READ;
   DoIO((struct IORequest *)ParallelIO);

If you use this example, your task will be put to sleep waiting until the
parallel device reads 256 bytes (or terminates early).  Early termination
can be caused by error conditions.