Fix writing to serial (rs485) on windows os. (#2191)

Co-authored-by: jan iversen <[email protected]>

Author: andrew-harness

pymodbus/transport/serialtransport.py

@@ -13,7 +13,7 @@
 class SerialTransport(asyncio.Transport):
     """An asyncio serial transport."""
 
-    force_poll: bool = False
+    force_poll: bool = os.name == "nt"
 
     def __init__(self, loop, protocol, *args, **kwargs) -> None:
         """Initialize."""
@@ -29,7 +29,7 @@ def __init__(self, loop, protocol, *args, **kwargs) -> None:
 
     def setup(self) -> None:
         """Prepare to read/write."""
-        if os.name == "nt" or self.force_poll:
+        if self.force_poll:
             self.poll_task = asyncio.create_task(self.polling_task())
             self.poll_task.set_name("SerialTransport poll")
         else:
@@ -56,7 +56,7 @@ def close(self, exc: Exception | None = None) -> None:
     def write(self, data) -> None:
         """Write some data to the transport."""
         self.intern_write_buffer.append(data)
-        if not self.poll_task:
+        if not self.force_poll:
             self.async_loop.add_writer(self.sync_serial.fileno(), self.intern_write_ready)
 
     def flush(self) -> None:

test/transport/test_comm.py

@@ -179,7 +179,6 @@ async def test_serial_poll(self, client, server, use_port):
         SerialTransport.force_poll = True
         assert await client.connect()
         await asyncio.sleep(0.5)
-        SerialTransport.force_poll = False
         assert len(server.active_connections) == 1
         server_connected = list(server.active_connections.values())[0]
         test_data = b"abcd" * 1000

test/transport/test_serial.py

@@ -100,6 +100,7 @@ async def test_write_force_poll(self):
         )
         await asyncio.sleep(0)
         transport.write(b"abcd")
+        await asyncio.sleep(0.5)
         transport.close()
         SerialTransport.force_poll = False