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pySerial API

Classes

Native ports

class serial.Serial
__init__(port=None, baudrate=9600, bytesize=EIGHTBITS, parity=PARITY_NONE, stopbits=STOPBITS_ONE, timeout=None, xonxoff=0, rtscts=0, interCharTimeout=None)
Parameters:
Raises ValueError:
 

Will be raised when parameter are out of range, e.g. baud rate, data bits.
Raises SerialException:
 

In case the device can not be found or can not be configured.

The port is immediately opened on object creation, when a port is given. It is not opened when port is None and a successive call to open() will be needed.

Possible values for the parameter port:

  • Number: number of device, numbering starts at zero.
  • Device name: depending on operating system. e.g. /dev/ttyUSB0 on GNU/Linux or COM3 on Windows.

Possible values for the parameter timeout:

  • timeout = None: wait forever
  • timeout = 0: non-blocking mode (return immediately on read)
  • timeout = x: set timeout to x seconds (float allowed)

Note that enabling both flow control methods (xonxoff and rtscts) together may not be supported. It is common to use one of the methods at once, not both.

open()
Open port.
close()
Close port immediately.

The following methods may raise ValueError when applied to a closed port.

read(size=1)
Parameter:size – Number of bytes to read.
Returns:Bytes read from the port.

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.

Changed in version 2.5: Returns an instance of bytes when available (Python 2.6 and newer) and str otherwise.

write(data)
Parameter:data – Data to send.
Returns:Number of bytes written.
Raises SerialTimeoutException:
 In case a write timeout is configured for the port and the time is exceeded.

Write the string data to the port.

Changed in version 2.5: Accepts instances of bytes and bytearray when available (Python 2.6 and newer) and str otherwise.

inWaiting()
Return the number of chars in the receive buffer.
flush()
Flush of file like objects. In this case, wait until all data is written.
flushInput()
Flush input buffer, discarding all it’s contents.
flushOutput()
Clear output buffer, aborting the current output and discarding all that is in the buffer.
sendBreak(duration=0.25)
Parameter:duration – Time (float) to activate the BREAK condition.

Send break condition. Timed, returns to idle state after given duration.

setBreak(level=True)
Parameter:level – when true activate BREAK condition, else disable.

Set break: Controls TXD. When active, no transmitting is possible.

setRTS(level=True)
Parameter:level – Set control line to logic level.

Set RTS line to specified logic level.

setDTR(level=True)
Parameter:level – Set control line to logic level.

Set DTR line to specified logic level.

getCTS()
Returns:Current state (boolean)

Return the state of the CTS line.

getDSR()
Returns:Current state (boolean)

Return the state of the DSR line.

getRI()
Returns:Current state (boolean)

Return the state of the RI line.

getCD()
Returns:Current state (boolean)

Return the state of the CD line

Read-only attributes:

name

Device name. This is always the device name even if the port was opened by a number. (Read Only).

New in version 2.5.

portstr
Deprecated:use name instead
BAUDRATES
A list of valid baud rates. The list may be incomplete such that higher baud rates may be supported by the device and that values in between the standard baud rates are supported. (Read Only).
BYTESIZES
A list of valid byte sizes for the device (Read Only).
PARITIES
A list of valid parities for the device (Read Only).
STOPBITS
A list of valid stop bit widths for the device (Read Only).

New values can be assigned to the following attributes, the port will be reconfigured, even if it’s opened at that time:

port
Port name/number as set by the user.
baudrate
Current baud rate setting.
bytesize
Byte size in bits.
parity
Parity setting.
stopbits
Stop bit with.
timeout
Timeout setting (seconds, float).
xonxoff
If Xon/Xoff flow control is enabled.
rtscts
If hardware flow control is enabled.

Platform specific methods.

Warning

Programs using the following methods are not portable to other platforms!

nonblocking()
Platform:Unix

Configure the device for nonblocking operations. This can be useful if the port is used with select.

fileno()
Platform:Unix
Returns:File descriptor.

Return file descriptor number for the port that is opened by this object.

setXON(level=True)
Platform:Windows
Parameter:level – Set flow control state.

Set software flow control state.

class serial.SerialBase

The following attributes are implemented as properties. They work with open and closed ports.

port
Read or write port. When the port is already open, it will be closed and reopened with the new setting.
baudrate
Read or write current baud rate setting.
bytesize
Read or write current data byte size setting.
parity
Read or write current parity setting.
stopbits
Read or write current stop bit width setting.
timeout
Read or write current read timeout setting.
writeTimeout
Read or write current write timeout setting.
xonxoff
Read or write current software flow control rate setting.
rtscts
Read or write current hardware flow control setting.
dsrdtr
Read or write current hardware flow control setting.
interCharTimeout
Read or write current inter character timeout setting.

The following constants are also provided:

BAUDRATES
A tuple of standard baud rate values. The actual device may support more or less...
BYTESIZES
A tuple of supported byte size values.
PARITIES
A tuple of supported parity settings.
STOPBITS
A tuple of supported stop bit settings.
readline(size=None, eol='\n')
Parameters:
  • size – Max number of bytes to read, None -> no limit.
  • eol – The end of line character.

Read a line which is terminated with end-of-line (eol) character (\\n by default) or until timeout.

readlines(sizehint=None, eol='\n')
Parameters:
  • size – Ignored parameter.
  • eol – The end of line character.

Read a list of lines, until timeout. sizehint is ignored and only present for API compatibility with built-in File objects.

xreadlines(sizehint=None)
Just calls readlines() - here for compatibility.

For compatibility with the io library are the following methods.

readable()
Returns:True

New in version 2.5.

writable()
Returns:True

New in version 2.5.

seekable()
Returns:False

New in version 2.5.

readinto(b)
Parameter:b – bytearray or array instance
Returns:Number of byte read

Read up to len(b) bytes into bytearray b and return the number of bytes read.

New in version 2.5.

getSettingsDict()
Returns:a dictionary with current port settings.

Get a dictionary with port settings. This is useful to backup the current settings so that a later point in time they can be restored using applySettingsDict().

Note that control lines (RTS/DTR) are part of the settings.

New in version 2.5.

applySettingsDict(d)
Parameter:d – a dictionary with port settings.

Applies a dictionary that was created by getSettingsDict(). Only changes are applied and when a key is missing it means that the setting stays unchanged.

Note that control lines (RTS/DTR) are not changed.

New in version 2.5.

Note

For systems that provide the io library (Python 2.6 and newer), the class Serial will derive from io.RawIOBase. For all others from FileLike.

class serial.FileLike

An abstract file like class. It is used as base class for Serial.

This class implements readline() and readlines() based on read and writelines() based on write. This class is used to provide the above functions for to Serial port objects.

Note that when the serial port was opened with no timeout that readline() blocks until it sees a newline (or the specified size is reached) and that readlines() would never return and therefore refuses to work (it raises an exception in this case)!

writelines(sequence)
Write a list of strings to the port.

The following three methods are overridden in Serial.

flush()
Flush of file like objects. It’s a no-op in this class, may be overridden.
read()
Raises NotImplementedError, needs to be overridden by subclass.
write(data)
Raises NotImplementedError, needs to be overridden by subclass.

The following functions are implemented for compatibility with other file-like objects, however serial ports are not seekable.

seek(pos, whence=0)
Raises IOError:always, as method is not supported on serial port

New in version 2.5.

tell()
Raises IOError:always, as method is not supported on serial port

New in version 2.5.

truncate(self, n=None)
Raises IOError:always, as method is not supported on serial port

New in version 2.5.

isatty()
Raises IOError:always, as method is not supported on serial port

New in version 2.5.

To be able to use the file like object as iterator for e.g. for line in Serial(0): ... usage:

next()
Return the next line by calling readline().
__iter__()
Returns self.

RFC 2217 Network ports

Warning

This implementation is currently in an experimental state. Use at your own risk.

class rfc2217.Serial

This implements a RFC 2217 compatible client. Port names are URLs in the form: rfc2217://<host>:<port>[/<option>[/<option>]]

This class API is compatible to Serial with a few exceptions:

  • numbers as port name are not allowed, only URLs in the form described above.
  • writeTimeout is not implemented
  • The current implementation starts a thread that keeps reading from the (internal) socket. The thread is managed automatically by the rfc2217.Serial port object on open()/close(). However it may be a problem for user applications that like to use select instead of threads.

Due to the nature of the network and protocol involved there are a few extra points to keep in mind:

  • All operations have an additional latency time.
  • Setting control lines (RTS/CTS) needs more time.
  • Reading the status lines (DSR/DTR etc.) returns a cached value. When that cache is updated depends entirely on the server. The server itself may implement a polling at a certain rate and quick changes may be invisible.
  • The network layer also has buffers. This means that flush(), flushInput() and flushOutput() may work with additional delay. Likewise inWaiting() returns the size of the data arrived at the object internal buffer and excludes any bytes in the network buffers or any server side buffer.
  • Closing and immediately reopening the same port may fail due to time needed by the server to get ready again.

Not implemented yet / Possible problems with the implementation:

  • RFC 2217 flow control between client and server (objects internal buffer may eat all your memory when never read).
  • No authentication support (servers may not prompt for a password etc.)
  • No encryption.

Due to lack of authentication and encryption it is not suitable to use this client for connections across the internet and should only be used in controlled environments.

New in version 2.5.

class rfc2217.PortManager

This class provides helper functions for implementing RFC 2217 compatible servers.

Basically, it implements every thing needed for the RFC 2217 protocol. It just does not open sockets and read/write to serial ports (though it changes other port settings). The user of this class must take care of the data transmission itself. The reason for that is, that this way, this class supports all programming models such as threads and select.

Usage examples can be found in the examples where two TCP/IP - serial converters are shown, one using threads (the single port server) and an other using select (the multi port server).

Note

Each new client connection must create a new instance as this object (and the RFC 2217 protocol) has internal state.

__init__(serial_port, connection, debug_output=False)
Parameters:
  • serial_port – a Serial instance that is managed.
  • connection – an object implementing write(), used to write to the network.
  • debug_output – enables debug messages: a logging.Logger instance or None.

Initializes the Manager and starts negotiating with client in Telnet and RFC 2217 protocol. The negotiation starts immediately so that the class should be instantiated in the moment the client connects.

The serial_port can be controlled by RFC 2217 commands. This object will modify the port settings (baud rate etc) and control lines (RTS/DTR) send BREAK etc. when the corresponding commands are found by the filter() method.

The connection object must implement a write(data)() function. This function must ensure that data is written at once (no user data mixed in, i.e. it must be thread-safe). All data must be sent in its raw form (escape() must not be used) as it is used to send Telnet and RFC 2217 control commands.

For diagnostics of the connection or the implementation, debug_output can be set to an instance of a logging.Logger (e.g. logging.getLogger('rfc2217.server')). The caller should configure the logger using setLevel for the desired detail level of the logs.

escape(data)
Parameter:data – data to be sent over the network.
Returns:data, escaped for Telnet/RFC 2217

A generator that escapes all data to be compatible with RFC 2217. Implementors of servers should use this function to process all data sent over the network.

The function returns a generator which can be used in for loops. It can be converted to bytes using serial.to_bytes.

filter(data)
Parameter:data – data read from the network, including Telnet and RFC 2217 controls.
Returns:data, free from Telnet and RFC 2217 controls.

A generator that filters and processes all data related to RFC 2217. Implementors of servers should use this function to process all data received from the network.

The function returns a generator which can be used in for loops. It can be converted to bytes using serial.to_bytes.

check_modem_lines(force_notification=False)
Parameter:force_notification – Set to false. Parameter is for internal use.

This function needs to be called periodically (e.g. every second) when the server wants to send NOTIFY_MODEMSTATE messages. This is required to support the client for reading CTS/DSR/RI/CD status lines.

The function reads the status line and issues the notifications automatically.

New in version 2.5.

See also

RFC 2217 - Telnet Com Port Control Option

Exceptions

exception serial.SerialException

Base class for serial port exceptions.

Changed in version 2.5: Now derrives from IOError instead of Exception

exception serial.SerialTimeoutException
Exception that is raised on write timeouts.

Constants

Parity

serial.PARITY_NONE
serial.PARITY_EVEN
serial.PARITY_ODD
serial.PARITY_MARK
serial.PARITY_SPACE

Stop bits

serial.STOPBITS_ONE
serial.STOPBITS_ONE_POINT_FIVE
serial.STOPBITS_TWO

Byte size

serial.FIVEBITS
serial.SIXBITS
serial.SEVENBITS
serial.EIGHTBITS

Others

Default control characters (instances of bytes for Python 3.0+) for software flow control:

serial.XON
serial.XOFF

Module version:

serial.VERSION
A string indicating the pySerial version, such as 2.5.

Functions:

serial.device(number)
Parameter:number – Port number.
Returns:String containing device name.
Deprecated:Use device names directly.

Convert a port number to a platform dependent device name. Unfortunately this does not work well for all platforms; e.g. some may miss USB-Serial converters and enumerate only internal serial ports.

The conversion may be made off-line, that is, there is no guarantee that the returned device name really exists on the system.

serial.serial_for_url(url, *args, **kwargs)
Parameters:
  • url – Device name, number or URL
  • do_not_open – When set to true, the serial port is not opened.
Returns:

an instance of Serial or a compatible object.

Get a native or a RFC 2217 implementation of the Serial class, depending on port/url. This factory function is useful when an application wants to support both, local ports and remote ports.

When url matches the form rfc2217://<host>:<port> an instance of rfc2217.Serial is returned. In all other cases the native (system dependant) Serial instance is returned.

The port is not opened when a keyword parameter called do_not_open is given and true, by default it is opened.

New in version 2.5.

URLs

The class rfc2217.Serial and the function serial_for_url() accept the following types URL:

  • rfc2217://<host>:<port>[/<option>[/<option>]]
  • socket://<host>:<port>[/<option>[/<option>]]
  • loop://[<option>[/<option>]]

(Future releases of pySerial might add more types).

rfc2217://

Used to connect to RFC 2217 compatible servers. All serial port functions are supported.

Supported options in the URL are:

  • ign_set_control does not wait for acknowledges to SET_CONTROL. This option can be used for non compliant servers (i.e. when getting an remote rejected value for option 'control' error when connecting).
  • poll_modem: The client issues NOTIFY_MODEMSTATE requests when status lines are read (CTS/DTR/RI/CD). Without this option it relies on the server sending the notifications automatically (that’s what the RFC suggests and most servers do). Enable this option when getCTS() does not work as expected, i.e. for servers that do not send notifications.
  • timeout=<value>: Change network timeout (default 3 seconds). This is useful when the server takes a little more time to send its answers. The timeout applies to the initial Telnet / RFC 2271 negotiation as well as changing port settings or control line change commands.
  • logging=[debug|info|warning|error]: Prints diagnostic messages (not useful for end users). It uses the logging module and a logger called pySerial.rfc2217 so that the application can setup up logging handlers etc. It will call logging.basicConfig() which initializes for output on sys.stderr (if no logging was set up already).
socket://

The purpose of this connection type is that applications using pySerial can connect to TCP/IP to serial port converters that do not support RFC 2217.

Uses a TCP/IP socket. All serial port settings, control and status lines are ignored. Only data is transmitted and received.

Supported options in the URL are:

  • logging=[debug|info|warning|error]: Prints diagnostic messages (not useful for end users). It uses the logging module and a logger called pySerial.socket so that the application can setup up logging handlers etc. It will call logging.basicConfig() which initializes for output on sys.stderr (if no logging was set up already).
loop://

The least useful type. It simulates a loop back connection. RX<->TX RTS<->CTS DTR<->DSR

Supported options in the URL are:

  • logging=[debug|info|warning|error]: Prints diagnostic messages (not useful for end users). It uses the logging module and a logger called pySerial.loop so that the application can setup up logging handlers etc. It will call logging.basicConfig() which initializes for output on sys.stderr (if no logging was set up already).

Examples:

  • rfc2217://localhost:7000
  • rfc2217://localhost:7000/poll_modem
  • rfc2217://localhost:7000/ign_set_control/timeout=5.5
  • socket://localhost:7777
  • loop://logging=debug

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