95 lines
3.2 KiB
Python
95 lines
3.2 KiB
Python
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#!/usr/bin/env python
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# RS485 support
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#
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# This file is part of pySerial. https://github.com/pyserial/pyserial
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# (C) 2015 Chris Liechti <cliechti@gmx.net>
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#
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# SPDX-License-Identifier: BSD-3-Clause
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"""\
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The settings for RS485 are stored in a dedicated object that can be applied to
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serial ports (where supported).
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NOTE: Some implementations may only support a subset of the settings.
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"""
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from __future__ import absolute_import
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import time
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import serial
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class RS485Settings(object):
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def __init__(
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self,
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rts_level_for_tx=True,
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rts_level_for_rx=False,
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loopback=False,
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delay_before_tx=None,
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delay_before_rx=None):
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self.rts_level_for_tx = rts_level_for_tx
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self.rts_level_for_rx = rts_level_for_rx
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self.loopback = loopback
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self.delay_before_tx = delay_before_tx
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self.delay_before_rx = delay_before_rx
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class RS485(serial.Serial):
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"""\
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A subclass that replaces the write method with one that toggles RTS
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according to the RS485 settings.
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NOTE: This may work unreliably on some serial ports (control signals not
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synchronized or delayed compared to data). Using delays may be
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unreliable (varying times, larger than expected) as the OS may not
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support very fine grained delays (no smaller than in the order of
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tens of milliseconds).
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NOTE: Some implementations support this natively. Better performance
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can be expected when the native version is used.
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NOTE: The loopback property is ignored by this implementation. The actual
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behavior depends on the used hardware.
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Usage:
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ser = RS485(...)
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ser.rs485_mode = RS485Settings(...)
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ser.write(b'hello')
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"""
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def __init__(self, *args, **kwargs):
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super(RS485, self).__init__(*args, **kwargs)
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self._alternate_rs485_settings = None
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def write(self, b):
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"""Write to port, controlling RTS before and after transmitting."""
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if self._alternate_rs485_settings is not None:
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# apply level for TX and optional delay
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self.setRTS(self._alternate_rs485_settings.rts_level_for_tx)
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if self._alternate_rs485_settings.delay_before_tx is not None:
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time.sleep(self._alternate_rs485_settings.delay_before_tx)
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# write and wait for data to be written
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super(RS485, self).write(b)
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super(RS485, self).flush()
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# optional delay and apply level for RX
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if self._alternate_rs485_settings.delay_before_rx is not None:
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time.sleep(self._alternate_rs485_settings.delay_before_rx)
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self.setRTS(self._alternate_rs485_settings.rts_level_for_rx)
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else:
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super(RS485, self).write(b)
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# redirect where the property stores the settings so that underlying Serial
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# instance does not see them
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@property
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def rs485_mode(self):
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"""\
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Enable RS485 mode and apply new settings, set to None to disable.
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See serial.rs485.RS485Settings for more info about the value.
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"""
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return self._alternate_rs485_settings
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@rs485_mode.setter
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def rs485_mode(self, rs485_settings):
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self._alternate_rs485_settings = rs485_settings
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