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