1#!/usr/bin/env python
2
3# RS485 support
4#
5# This file is part of pySerial. https://github.com/pyserial/pyserial
6# (C) 2015 Chris Liechti <[email protected]>
7#
8# SPDX-License-Identifier:    BSD-3-Clause
9
10"""\
11The settings for RS485 are stored in a dedicated object that can be applied to
12serial ports (where supported).
13NOTE: Some implementations may only support a subset of the settings.
14"""
15
16from __future__ import absolute_import
17
18import time
19import serial
20
21
22class RS485Settings(object):
23    def __init__(
24            self,
25            rts_level_for_tx=True,
26            rts_level_for_rx=False,
27            loopback=False,
28            delay_before_tx=None,
29            delay_before_rx=None):
30        self.rts_level_for_tx = rts_level_for_tx
31        self.rts_level_for_rx = rts_level_for_rx
32        self.loopback = loopback
33        self.delay_before_tx = delay_before_tx
34        self.delay_before_rx = delay_before_rx
35
36
37class RS485(serial.Serial):
38    """\
39    A subclass that replaces the write method with one that toggles RTS
40    according to the RS485 settings.
41
42    NOTE: This may work unreliably on some serial ports (control signals not
43          synchronized or delayed compared to data). Using delays may be
44          unreliable (varying times, larger than expected) as the OS may not
45          support very fine grained delays (no smaller than in the order of
46          tens of milliseconds).
47
48    NOTE: Some implementations support this natively. Better performance
49          can be expected when the native version is used.
50
51    NOTE: The loopback property is ignored by this implementation. The actual
52          behavior depends on the used hardware.
53
54    Usage:
55
56        ser = RS485(...)
57        ser.rs485_mode = RS485Settings(...)
58        ser.write(b'hello')
59    """
60
61    def __init__(self, *args, **kwargs):
62        super(RS485, self).__init__(*args, **kwargs)
63        self._alternate_rs485_settings = None
64
65    def write(self, b):
66        """Write to port, controlling RTS before and after transmitting."""
67        if self._alternate_rs485_settings is not None:
68            # apply level for TX and optional delay
69            self.setRTS(self._alternate_rs485_settings.rts_level_for_tx)
70            if self._alternate_rs485_settings.delay_before_tx is not None:
71                time.sleep(self._alternate_rs485_settings.delay_before_tx)
72            # write and wait for data to be written
73            super(RS485, self).write(b)
74            super(RS485, self).flush()
75            # optional delay and apply level for RX
76            if self._alternate_rs485_settings.delay_before_rx is not None:
77                time.sleep(self._alternate_rs485_settings.delay_before_rx)
78            self.setRTS(self._alternate_rs485_settings.rts_level_for_rx)
79        else:
80            super(RS485, self).write(b)
81
82    # redirect where the property stores the settings so that underlying Serial
83    # instance does not see them
84    @property
85    def rs485_mode(self):
86        """\
87        Enable RS485 mode and apply new settings, set to None to disable.
88        See serial.rs485.RS485Settings for more info about the value.
89        """
90        return self._alternate_rs485_settings
91
92    @rs485_mode.setter
93    def rs485_mode(self, rs485_settings):
94        self._alternate_rs485_settings = rs485_settings
95