serial/serialposix.py

#!/usr/bin/env python
#
# Python Serial Port Extension for Win32, Linux, BSD, Jython
# module for serial IO for POSIX compatible systems, like Linux
# see __init__.py
#
# (C) 2001-2015 Chris Liechti <cliechti@gmx.net>
#
# SPDX-License-Identifier:    BSD-3-Clause
#
# parts based on code from Grant B. Edwards  <grante@visi.com>:
#  ftp://ftp.visi.com/users/grante/python/PosixSerial.py
#
# references: http://www.easysw.com/~mike/serial/serial.html

import errno
import fcntl
import os
import select
import struct
import sys
import termios
import time
from serial.serialutil import *


class PlatformSpecificBase(object):
    BAUDRATE_CONSTANTS = {}

    def number_to_device(self, port_number):
        sys.stderr.write("""\
don't know how to number ttys on this system.
! Use an explicit path (eg /dev/ttyS1) or send this information to
! the author of this module:

sys.platform = %r
os.name = %r
serialposix.py version = %s

also add the device name of the serial port and where the
counting starts for the first serial port.
e.g. 'first serial port: /dev/ttyS0'
and with a bit luck you can get this module running...
""" % (sys.platform, os.name, VERSION))
        raise NotImplementedError('no number-to-device mapping defined on this platform')

    def _set_special_baudrate(self, baudrate):
        raise NotImplementedError('non-standard baudrates are not supported on this platform')

    def _set_rs485_mode(self, rs485_settings):
        raise NotImplementedError('RS485 not supported on this platform')

# try to detect the OS so that a device can be selected...
# this code block should supply a device() and set_special_baudrate() function
# for the platform
plat = sys.platform.lower()

if   plat[:5] == 'linux':    # Linux (confirmed)
    import array

    # baudrate ioctls
    TCGETS2 = 0x802C542A
    TCSETS2 = 0x402C542B
    BOTHER = 0o010000

    # RS485 ioctls
    TIOCGRS485 = 0x542E
    TIOCSRS485 = 0x542F
    SER_RS485_ENABLED        = 0b00000001
    SER_RS485_RTS_ON_SEND    = 0b00000010
    SER_RS485_RTS_AFTER_SEND = 0b00000100
    SER_RS485_RX_DURING_TX   = 0b00010000


    class PlatformSpecific(PlatformSpecificBase):
        BAUDRATE_CONSTANTS = {
            0:       0o000000,  # hang up
            50:      0o000001,
            75:      0o000002,
            110:     0o000003,
            134:     0o000004,
            150:     0o000005,
            200:     0o000006,
            300:     0o000007,
            600:     0o000010,
            1200:    0o000011,
            1800:    0o000012,
            2400:    0o000013,
            4800:    0o000014,
            9600:    0o000015,
            19200:   0o000016,
            38400:   0o000017,
            57600:   0o010001,
            115200:  0o010002,
            230400:  0o010003,
            460800:  0o010004,
            500000:  0o010005,
            576000:  0o010006,
            921600:  0o010007,
            1000000: 0o010010,
            1152000: 0o010011,
            1500000: 0o010012,
            2000000: 0o010013,
            2500000: 0o010014,
            3000000: 0o010015,
            3500000: 0o010016,
            4000000: 0o010017
        }

        def number_to_device(self, port_number):
            return '/dev/ttyS%d' % (port_number,)

        def _set_special_baudrate(self, baudrate):
            # right size is 44 on x86_64, allow for some growth
            buf = array.array('i', [0] * 64)
            try:
                # get serial_struct
                fcntl.ioctl(self.fd, TCGETS2, buf)
                # set custom speed
                buf[2] &= ~termios.CBAUD
                buf[2] |= BOTHER
                buf[9] = buf[10] = baudrate

                # set serial_struct
                res = fcntl.ioctl(self.fd, TCSETS2, buf)
            except IOError as e:
                raise ValueError('Failed to set custom baud rate (%s): %s' % (baudrate, e))

        def _set_rs485_mode(self, rs485_settings):
            buf = array.array('i', [0] * 8) # flags, delaytx, delayrx, padding
            try:
                fcntl.ioctl(self.fd, TIOCGRS485, buf)
                if rs485_settings is not None:
                    if rs485_settings.loopback:
                        buf[0] |= SER_RS485_RX_DURING_TX
                    else:
                        buf[0] &= ~SER_RS485_RX_DURING_TX
                    if rs485_settings.rts_level_for_tx:
                        buf[0] |= SER_RS485_RTS_ON_SEND
                    else:
                        buf[0] &= ~SER_RS485_RTS_ON_SEND
                    if rs485_settings.rts_level_for_rx:
                        buf[0] |= SER_RS485_RTS_AFTER_SEND
                    else:
                        buf[0] &= ~SER_RS485_RTS_AFTER_SEND
                    buf[1] = int(rs485_settings.delay_rts_before_send * 1000)
                    buf[2] = int(rs485_settings.delay_rts_after_send * 1000)
                else:
                    buf[0] = 0  # clear SER_RS485_ENABLED
                res = fcntl.ioctl(self.fd, TIOCSRS485, buf)
            except IOError as e:
                raise ValueError('Failed to set RS485 mode: %s' % (e,))


elif plat == 'cygwin':       # cygwin/win32 (confirmed)

    class PlatformSpecific(PlatformSpecificBase):
        BAUDRATE_CONSTANTS = {
            128000: 0x01003,
            256000: 0x01005,
            500000: 0x01007,
            576000: 0x01008,
            921600: 0x01009,
            1000000: 0x0100a,
            1152000: 0x0100b,
            1500000: 0x0100c,
            2000000: 0x0100d,
            2500000: 0x0100e,
            3000000: 0x0100f
        }

        def number_to_device(self, port_number):
            return '/dev/com%d' % (port_number + 1,)


elif plat[:7] == 'openbsd':    # OpenBSD
    class PlatformSpecific(PlatformSpecificBase):
        def number_to_device(self, port_number):
            return '/dev/cua%02d' % (port_number,)

elif plat[:3] == 'bsd' or plat[:7] == 'freebsd':
    class PlatformSpecific(PlatformSpecificBase):
        def number_to_device(self, port_number):
            return '/dev/cuad%d' % (port_number,)

elif plat[:6] == 'darwin':   # OS X
    import array
    IOSSIOSPEED = 0x80045402 #_IOW('T', 2, speed_t)

    class PlatformSpecific(PlatformSpecificBase):
        def number_to_device(self, port_number):
            return '/dev/cuad%d' % (port_number,)

        osx_version = os.uname()[2].split('.')
        # Tiger or above can support arbitrary serial speeds
        if int(osx_version[0]) >= 8:
            def _set_special_baudrate(self, baudrate):
                # use IOKit-specific call to set up high speeds
                buf = array.array('i', [baudrate])
                fcntl.ioctl(self.fd, IOSSIOSPEED, buf, 1)


elif plat[:6] == 'netbsd':   # NetBSD 1.6 testing by Erk
    class PlatformSpecific(PlatformSpecificBase):
        def number_to_device(self, port_number):
            return '/dev/dty%02d' % (port_number,)

elif plat[:4] == 'irix':     # IRIX (partially tested)
    class PlatformSpecific(PlatformSpecificBase):
        def number_to_device(self, port_number):
            return '/dev/ttyf%d' % (port_number + 1,) #XXX different device names depending on flow control

elif plat[:2] == 'hp':       # HP-UX (not tested)
    class PlatformSpecific(PlatformSpecificBase):
        def number_to_device(self, port_number):
            return '/dev/tty%dp0' % (port_number + 1,)

elif plat[:5] == 'sunos':    # Solaris/SunOS (confirmed)
    class PlatformSpecific(PlatformSpecificBase):
        def number_to_device(self, port_number):
            return '/dev/tty%c' % (ord('a') + port_number,)

elif plat[:3] == 'aix':      # AIX
    class PlatformSpecific(PlatformSpecificBase):
        def number_to_device(self, port_number):
            return '/dev/tty%d' % (port_number,)

else:
    class PlatformSpecific(PlatformSpecificBase):
        pass

# whats up with "aix", "beos", ....
# they should work, just need to know the device names.


# load some constants for later use.
# try to use values from termios, use defaults from linux otherwise
TIOCMGET  = getattr(termios, 'TIOCMGET', 0x5415)
TIOCMBIS  = getattr(termios, 'TIOCMBIS', 0x5416)
TIOCMBIC  = getattr(termios, 'TIOCMBIC', 0x5417)
TIOCMSET  = getattr(termios, 'TIOCMSET', 0x5418)

#TIOCM_LE = getattr(termios, 'TIOCM_LE', 0x001)
TIOCM_DTR = getattr(termios, 'TIOCM_DTR', 0x002)
TIOCM_RTS = getattr(termios, 'TIOCM_RTS', 0x004)
#TIOCM_ST = getattr(termios, 'TIOCM_ST', 0x008)
#TIOCM_SR = getattr(termios, 'TIOCM_SR', 0x010)

TIOCM_CTS = getattr(termios, 'TIOCM_CTS', 0x020)
TIOCM_CAR = getattr(termios, 'TIOCM_CAR', 0x040)
TIOCM_RNG = getattr(termios, 'TIOCM_RNG', 0x080)
TIOCM_DSR = getattr(termios, 'TIOCM_DSR', 0x100)
TIOCM_CD  = getattr(termios, 'TIOCM_CD', TIOCM_CAR)
TIOCM_RI  = getattr(termios, 'TIOCM_RI', TIOCM_RNG)
#TIOCM_OUT1 = getattr(termios, 'TIOCM_OUT1', 0x2000)
#TIOCM_OUT2 = getattr(termios, 'TIOCM_OUT2', 0x4000)
if hasattr(termios, 'TIOCINQ'):
    TIOCINQ = termios.TIOCINQ
else:
    TIOCINQ = getattr(termios, 'FIONREAD', 0x541B)
TIOCOUTQ   = getattr(termios, 'TIOCOUTQ', 0x5411)

TIOCM_zero_str = struct.pack('I', 0)
TIOCM_RTS_str = struct.pack('I', TIOCM_RTS)
TIOCM_DTR_str = struct.pack('I', TIOCM_DTR)

TIOCSBRK  = getattr(termios, 'TIOCSBRK', 0x5427)
TIOCCBRK  = getattr(termios, 'TIOCCBRK', 0x5428)

CMSPAR = 0o10000000000 # Use "stick" (mark/space) parity


class Serial(SerialBase, PlatformSpecific):
    """\
    Serial port class POSIX implementation. Serial port configuration is 
    done with termios and fcntl. Runs on Linux and many other Un*x like
    systems.
    """

    def open(self):
        """\
        Open port with current settings. This may throw a SerialException
        if the port cannot be opened."""
        if self._port is None:
            raise SerialException("Port must be configured before it can be used.")
        if self.is_open:
            raise SerialException("Port is already open.")
        self.fd = None
        # open
        try:
            self.fd = os.open(self.portstr, os.O_RDWR|os.O_NOCTTY|os.O_NONBLOCK)
        except OSError as msg:
            self.fd = None
            raise SerialException(msg.errno, "could not open port %s: %s" % (self._port, msg))
        #~ fcntl.fcntl(self.fd, fcntl.F_SETFL, 0)  # set blocking

        try:
            self._reconfigure_port()
        except:
            try:
                os.close(self.fd)
            except:
                # ignore any exception when closing the port
                # also to keep original exception that happened when setting up
                pass
            self.fd = None
            raise
        else:
            self.is_open = True
        if not self._dsrdtr:
            self._update_dtr_state()
        if not self.rtscts:
            self._update_rts_state()
        self.reset_input_buffer()


    def _reconfigure_port(self):
        """Set communication parameters on opened port."""
        if self.fd is None:
            raise SerialException("Can only operate on a valid file descriptor")
        custom_baud = None

        vmin = vtime = 0                # timeout is done via select
        if self._inter_character_timeout is not None:
            vmin = 1
            vtime = int(self._inter_character_timeout * 10)
        try:
            orig_attr = termios.tcgetattr(self.fd)
            iflag, oflag, cflag, lflag, ispeed, ospeed, cc = orig_attr
        except termios.error as msg:      # if a port is nonexistent but has a /dev file, it'll fail here
            raise SerialException("Could not configure port: %s" % msg)
        # set up raw mode / no echo / binary
        cflag |=  (termios.CLOCAL|termios.CREAD)
        lflag &= ~(termios.ICANON|termios.ECHO|termios.ECHOE|termios.ECHOK|termios.ECHONL|
                     termios.ISIG|termios.IEXTEN) #|termios.ECHOPRT
        for flag in ('ECHOCTL', 'ECHOKE'): # netbsd workaround for Erk
            if hasattr(termios, flag):
                lflag &= ~getattr(termios, flag)

        oflag &= ~(termios.OPOST|termios.ONLCR|termios.OCRNL)
        iflag &= ~(termios.INLCR|termios.IGNCR|termios.ICRNL|termios.IGNBRK)
        if hasattr(termios, 'IUCLC'):
            iflag &= ~termios.IUCLC
        if hasattr(termios, 'PARMRK'):
            iflag &= ~termios.PARMRK

        # setup baud rate
        try:
            ispeed = ospeed = getattr(termios, 'B%s' % (self._baudrate))
        except AttributeError:
            try:
                ispeed = ospeed = self.BAUDRATE_CONSTANTS[self._baudrate]
            except KeyError:
                #~ raise ValueError('Invalid baud rate: %r' % self._baudrate)
                # may need custom baud rate, it isn't in our list.
                ispeed = ospeed = getattr(termios, 'B38400')
                try:
                    custom_baud = int(self._baudrate) # store for later
                except ValueError:
                    raise ValueError('Invalid baud rate: %r' % self._baudrate)
                else:
                    if custom_baud < 0:
                        raise ValueError('Invalid baud rate: %r' % self._baudrate)

        # setup char len
        cflag &= ~termios.CSIZE
        if self._bytesize == 8:
            cflag |= termios.CS8
        elif self._bytesize == 7:
            cflag |= termios.CS7
        elif self._bytesize == 6:
            cflag |= termios.CS6
        elif self._bytesize == 5:
            cflag |= termios.CS5
        else:
            raise ValueError('Invalid char len: %r' % self._bytesize)
        # setup stop bits
        if self._stopbits == STOPBITS_ONE:
            cflag &= ~(termios.CSTOPB)
        elif self._stopbits == STOPBITS_ONE_POINT_FIVE:
            cflag |=  (termios.CSTOPB)  # XXX same as TWO.. there is no POSIX support for 1.5
        elif self._stopbits == STOPBITS_TWO:
            cflag |=  (termios.CSTOPB)
        else:
            raise ValueError('Invalid stop bit specification: %r' % self._stopbits)
        # setup parity
        iflag &= ~(termios.INPCK|termios.ISTRIP)
        if self._parity == PARITY_NONE:
            cflag &= ~(termios.PARENB|termios.PARODD)
        elif self._parity == PARITY_EVEN:
            cflag &= ~(termios.PARODD)
            cflag |=  (termios.PARENB)
        elif self._parity == PARITY_ODD:
            cflag |=  (termios.PARENB|termios.PARODD)
        elif self._parity == PARITY_MARK and plat[:5] == 'linux':
            cflag |=  (termios.PARENB|CMSPAR|termios.PARODD)
        elif self._parity == PARITY_SPACE and plat[:5] == 'linux':
            cflag |=  (termios.PARENB|CMSPAR)
            cflag &= ~(termios.PARODD)
        else:
            raise ValueError('Invalid parity: %r' % self._parity)
        # setup flow control
        # xonxoff
        if hasattr(termios, 'IXANY'):
            if self._xonxoff:
                iflag |=  (termios.IXON|termios.IXOFF) #|termios.IXANY)
            else:
                iflag &= ~(termios.IXON|termios.IXOFF|termios.IXANY)
        else:
            if self._xonxoff:
                iflag |=  (termios.IXON|termios.IXOFF)
            else:
                iflag &= ~(termios.IXON|termios.IXOFF)
        # rtscts
        if hasattr(termios, 'CRTSCTS'):
            if self._rtscts:
                cflag |=  (termios.CRTSCTS)
            else:
                cflag &= ~(termios.CRTSCTS)
        elif hasattr(termios, 'CNEW_RTSCTS'):   # try it with alternate constant name
            if self._rtscts:
                cflag |=  (termios.CNEW_RTSCTS)
            else:
                cflag &= ~(termios.CNEW_RTSCTS)
        # XXX should there be a warning if setting up rtscts (and xonxoff etc) fails??

        # buffer
        # vmin "minimal number of characters to be read. 0 for non blocking"
        if vmin < 0 or vmin > 255:
            raise ValueError('Invalid vmin: %r ' % vmin)
        cc[termios.VMIN] = vmin
        # vtime
        if vtime < 0 or vtime > 255:
            raise ValueError('Invalid vtime: %r' % vtime)
        cc[termios.VTIME] = vtime
        # activate settings
        if [iflag, oflag, cflag, lflag, ispeed, ospeed, cc] != orig_attr:
            termios.tcsetattr(self.fd, termios.TCSANOW, [iflag, oflag, cflag, lflag, ispeed, ospeed, cc])

        # apply custom baud rate, if any
        if custom_baud is not None:
            self._set_special_baudrate(custom_baud)

        if self._rs485_mode is not None:
            self._set_rs485_mode(self._rs485_mode)

    def close(self):
        """Close port"""
        if self.is_open:
            if self.fd is not None:
                os.close(self.fd)
                self.fd = None
            self.is_open = False

    #  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -

    @property
    def in_waiting(self):
        """Return the number of bytes currently in the input buffer."""
        #~ s = fcntl.ioctl(self.fd, termios.FIONREAD, TIOCM_zero_str)
        s = fcntl.ioctl(self.fd, TIOCINQ, TIOCM_zero_str)
        return struct.unpack('I',s)[0]

    # select based implementation, proved to work on many systems
    def read(self, size=1):
        """\
        Read size bytes from the serial port. If a timeout is set it may
        return less characters as requested. With no timeout it will block
        until the requested number of bytes is read.
        """
        if not self.is_open: raise portNotOpenError
        read = bytearray()
        while len(read) < size:
            try:
                ready,_,_ = select.select([self.fd],[],[], self._timeout)
                # If select was used with a timeout, and the timeout occurs, it
                # returns with empty lists -> thus abort read operation.
                # For timeout == 0 (non-blocking operation) also abort when there
                # is nothing to read.
                if not ready:
                    break   # timeout
                buf = os.read(self.fd, size-len(read))
                # read should always return some data as select reported it was
                # ready to read when we get to this point.
                if not buf:
                    # Disconnected devices, at least on Linux, show the
                    # behavior that they are always ready to read immediately
                    # but reading returns nothing.
                    raise SerialException('device reports readiness to read but returned no data (device disconnected or multiple access on port?)')
                read.extend(buf)
            except OSError as e:
                # this is for Python 3.x where select.error is a subclass of OSError
                # ignore EAGAIN errors. all other errors are shown
                if e.errno != errno.EAGAIN:
                    raise SerialException('read failed: %s' % (e,))
            except select.error as e:
                # this is for Python 2.x
                # ignore EAGAIN errors. all other errors are shown
                # see also http://www.python.org/dev/peps/pep-3151/#select
                if e[0] != errno.EAGAIN:
                    raise SerialException('read failed: %s' % (e,))
        return bytes(read)

    def write(self, data):
        """Output the given byte string over the serial port."""
        if not self.is_open: raise portNotOpenError
        d = to_bytes(data)
        tx_len = len(d)
        if self._write_timeout is not None and self._write_timeout > 0:
            timeout = time.time() + self._write_timeout
        else:
            timeout = None
        while tx_len > 0:
            try:
                n = os.write(self.fd, d)
                if timeout:
                    # when timeout is set, use select to wait for being ready
                    # with the time left as timeout
                    timeleft = timeout - time.time()
                    if timeleft < 0:
                        raise writeTimeoutError
                    _, ready, _ = select.select([], [self.fd], [], timeleft)
                    if not ready:
                        raise writeTimeoutError
                else:
                    # wait for write operation
                    _, ready, _ = select.select([], [self.fd], [], None)
                    if not ready:
                        raise SerialException('write failed (select)')
                d = d[n:]
                tx_len -= n
            except SerialException:
                raise
            except OSError as v:
                if v.errno != errno.EAGAIN:
                    raise SerialException('write failed: %s' % (v,))
        return len(data)

    def flush(self):
        """\
        Flush of file like objects. In this case, wait until all data
        is written.
        """
        if not self.is_open: raise portNotOpenError
        termios.tcdrain(self.fd)

    def reset_input_buffer(self):
        """Clear input buffer, discarding all that is in the buffer."""
        if not self.is_open: raise portNotOpenError
        termios.tcflush(self.fd, termios.TCIFLUSH)

    def reset_output_buffer(self):
        """\
        Clear output buffer, aborting the current output and discarding all
        that is in the buffer.
        """
        if not self.is_open: raise portNotOpenError
        termios.tcflush(self.fd, termios.TCOFLUSH)

    def send_break(self, duration=0.25):
        """\
        Send break condition. Timed, returns to idle state after given
        duration.
        """
        if not self.is_open: raise portNotOpenError
        termios.tcsendbreak(self.fd, int(duration/0.25))

    def _update_break_state(self):
        """\
        Set break: Controls TXD. When active, no transmitting is possible.
        """
        if self._break_state:
            fcntl.ioctl(self.fd, TIOCSBRK)
        else:
            fcntl.ioctl(self.fd, TIOCCBRK)

    def _update_rts_state(self):
        """Set terminal status line: Request To Send"""
        if self._rts_state:
            fcntl.ioctl(self.fd, TIOCMBIS, TIOCM_RTS_str)
        else:
            fcntl.ioctl(self.fd, TIOCMBIC, TIOCM_RTS_str)

    def _update_dtr_state(self):
        """Set terminal status line: Data Terminal Ready"""
        if self._dtr_state:
            fcntl.ioctl(self.fd, TIOCMBIS, TIOCM_DTR_str)
        else:
            fcntl.ioctl(self.fd, TIOCMBIC, TIOCM_DTR_str)

    @property
    def cts(self):
        """Read terminal status line: Clear To Send"""
        if not self.is_open: raise portNotOpenError
        s = fcntl.ioctl(self.fd, TIOCMGET, TIOCM_zero_str)
        return struct.unpack('I',s)[0] & TIOCM_CTS != 0

    @property
    def dsr(self):
        """Read terminal status line: Data Set Ready"""
        if not self.is_open: raise portNotOpenError
        s = fcntl.ioctl(self.fd, TIOCMGET, TIOCM_zero_str)
        return struct.unpack('I',s)[0] & TIOCM_DSR != 0

    @property
    def ri(self):
        """Read terminal status line: Ring Indicator"""
        if not self.is_open: raise portNotOpenError
        s = fcntl.ioctl(self.fd, TIOCMGET, TIOCM_zero_str)
        return struct.unpack('I',s)[0] & TIOCM_RI != 0

    @property
    def cd(self):
        """Read terminal status line: Carrier Detect"""
        if not self.is_open: raise portNotOpenError
        s = fcntl.ioctl(self.fd, TIOCMGET, TIOCM_zero_str)
        return struct.unpack('I',s)[0] & TIOCM_CD != 0

    # - - platform specific - - - -

    @property
    def out_waiting(self):
        """Return the number of bytes currently in the output buffer."""
        #~ s = fcntl.ioctl(self.fd, termios.FIONREAD, TIOCM_zero_str)
        s = fcntl.ioctl(self.fd, TIOCOUTQ, TIOCM_zero_str)
        return struct.unpack('I',s)[0]

    def nonblocking(self):
        """internal - not portable!"""
        if not self.is_open: raise portNotOpenError
        fcntl.fcntl(self.fd, fcntl.F_SETFL, os.O_NONBLOCK)

    def fileno(self):
        """\
        For easier use of the serial port instance with select.
        WARNING: this function is not portable to different platforms!
        """
        if not self.is_open: raise portNotOpenError
        return self.fd

    def set_input_flow_control(self, level=True):
        """\
        Manually control flow - when software flow control is enabled.
        This will send XON (true) or XOFF (false) to the other device.
        WARNING: this function is not portable to different platforms!
        """
        if not self.is_open: raise portNotOpenError
        if enable:
            termios.tcflow(self.fd, termios.TCION)
        else:
            termios.tcflow(self.fd, termios.TCIOFF)

    def set_output_flow_control(self, enable):
        """\
        Manually control flow of outgoing data - when hardware or software flow
        control is enabled.
        WARNING: this function is not portable to different platforms!
        """
        if not self.is_open: raise portNotOpenError
        if enable:
            termios.tcflow(self.fd, termios.TCOON)
        else:
            termios.tcflow(self.fd, termios.TCOOFF)



class PosixPollSerial(Serial):
    """\
    Poll based read implementation. Not all systems support poll properly.
    However this one has better handling of errors, such as a device
    disconnecting while it's in use (e.g. USB-serial unplugged).
    """

    def read(self, size=1):
        """\
        Read size bytes from the serial port. If a timeout is set it may
        return less characters as requested. With no timeout it will block
        until the requested number of bytes is read.
        """
        if self.fd is None: raise portNotOpenError
        read = bytearray()
        poll = select.poll()
        poll.register(self.fd, select.POLLIN|select.POLLERR|select.POLLHUP|select.POLLNVAL)
        if size > 0:
            while len(read) < size:
                # print "\tread(): size",size, "have", len(read)    #debug
                # wait until device becomes ready to read (or something fails)
                for fd, event in poll.poll(self._timeout*1000):
                    if event & (select.POLLERR|select.POLLHUP|select.POLLNVAL):
                        raise SerialException('device reports error (poll)')
                    #  we don't care if it is select.POLLIN or timeout, that's
                    #  handled below
                buf = os.read(self.fd, size - len(read))
                read.extend(buf)
                if ((self._timeout is not None and self._timeout >= 0) or 
                    (self._inter_character_timeout is not None and self._inter_character_timeout > 0)) and not buf:
                    break   # early abort on timeout
        return bytes(read)


if __name__ == '__main__':
    s = Serial(0,
                 baudrate=19200,        # baud rate
                 bytesize=EIGHTBITS,    # number of data bits
                 parity=PARITY_EVEN,    # enable parity checking
                 stopbits=STOPBITS_ONE, # number of stop bits
                 timeout=3,             # set a timeout value, None for waiting forever
                 xonxoff=0,             # enable software flow control
                 rtscts=0,              # enable RTS/CTS flow control
               )
    s.setRTS(1)
    s.setDTR(1)
    s.flushInput()
    s.flushOutput()
    s.write('hello')
    sys.stdout.write('%r\n' % s.read(5))
    sys.stdout.write('%s\n' % s.inWaiting())
    del s