1 files changed, 94 insertions, 0 deletions
diff --git a/serial/rs485.py b/serial/rs485.py
new file mode 100644
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+++ b/serial/rs485.py
@@ -0,0 +1,94 @@
+#!/usr/bin/env python
+
+# RS485 support
+#
+# This file is part of pySerial. https://github.com/pyserial/pyserial
+# (C) 2015 Chris Liechti <cliechti@gmx.net>
+#
+# SPDX-License-Identifier:    BSD-3-Clause
+
+"""\
+The settings for RS485 are stored in a dedicated object that can be applied to
+serial ports (where supported).
+NOTE: Some implementations may only support a subset of the settings.
+"""
+
+from __future__ import absolute_import
+
+import time
+import serial
+
+
+class RS485Settings(object):
+    def __init__(
+            self,
+            rts_level_for_tx=True,
+            rts_level_for_rx=False,
+            loopback=False,
+            delay_before_tx=None,
+            delay_before_rx=None):
+        self.rts_level_for_tx = rts_level_for_tx
+        self.rts_level_for_rx = rts_level_for_rx
+        self.loopback = loopback
+        self.delay_before_tx = delay_before_tx
+        self.delay_before_rx = delay_before_rx
+
+
+class RS485(serial.Serial):
+    """\
+    A subclass that replaces the write method with one that toggles RTS
+    according to the RS485 settings.
+
+    NOTE: This may work unreliably on some serial ports (control signals not
+          synchronized or delayed compared to data). Using delays may be
+          unreliable (varying times, larger than expected) as the OS may not
+          support very fine grained delays (no smaller than in the order of
+          tens of milliseconds).
+
+    NOTE: Some implementations support this natively. Better performance
+          can be expected when the native version is used.
+
+    NOTE: The loopback property is ignored by this implementation. The actual
+          behavior depends on the used hardware.
+
+    Usage:
+
+        ser = RS485(...)
+        ser.rs485_mode = RS485Settings(...)
+        ser.write(b'hello')
+    """
+
+    def __init__(self, *args, **kwargs):
+        super(RS485, self).__init__(*args, **kwargs)
+        self._alternate_rs485_settings = None
+
+    def write(self, b):
+        """Write to port, controlling RTS before and after transmitting."""
+        if self._alternate_rs485_settings is not None:
+            # apply level for TX and optional delay
+            self.setRTS(self._alternate_rs485_settings.rts_level_for_tx)
+            if self._alternate_rs485_settings.delay_before_tx is not None:
+                time.sleep(self._alternate_rs485_settings.delay_before_tx)
+            # write and wait for data to be written
+            super(RS485, self).write(b)
+            super(RS485, self).flush()
+            # optional delay and apply level for RX
+            if self._alternate_rs485_settings.delay_before_rx is not None:
+                time.sleep(self._alternate_rs485_settings.delay_before_rx)
+            self.setRTS(self._alternate_rs485_settings.rts_level_for_rx)
+        else:
+            super(RS485, self).write(b)
+
+    # redirect where the property stores the settings so that underlying Serial
+    # instance does not see them
+    @property
+    def rs485_mode(self):
+        """\
+        Enable RS485 mode and apply new settings, set to None to disable.
+        See serial.rs485.RS485Settings for more info about the value.
+        """
+        return self._alternate_rs485_settings
+
+    @rs485_mode.setter
+    def rs485_mode(self, rs485_settings):
+        self._alternate_rs485_settings = rs485_settings