From 794978239eba2c2aaf9d4e040f2227fccae0db3a Mon Sep 17 00:00:00 2001
From: Benjamin Braatz <bb@bbraatz.eu>
Date: Tue, 16 Feb 2021 11:30:43 +0100
Subject: [PATCH] Retry modbus send if no answer

---
 gmodbus/transport.py | 56 ++++++++++++++++++++++++--------------------
 1 file changed, 31 insertions(+), 25 deletions(-)

diff --git a/gmodbus/transport.py b/gmodbus/transport.py
index 8405eec..89678b5 100644
--- a/gmodbus/transport.py
+++ b/gmodbus/transport.py
@@ -168,29 +168,35 @@ class SerialClient(ClientInterface):
     return value
 
   async def __send_message(self, message: bytes) -> List[int]:
-    await self.__port.write(message)
-
-    # Check exception ADU (which is shorter than all other responses) first.
-    exception_adu_size = 5
-    response_error_adu = b''
-    try:
-        response_error_adu = await asyncio.wait_for(self.__port.read(exception_adu_size), timeout=1.0)
-    except asyncio.TimeoutError:
-        print("modbus read timed out")
-    rtu.raise_for_exception_adu(response_error_adu)
-
-    expected_response_size = \
+    success = False
+    while not success:
+      success = True
+
+      await self.__port.write(message)
+
+      # Check exception ADU (which is shorter than all other responses) first.
+      exception_adu_size = 5
+      response_error_adu = b''
+      try:
+        response_error_adu = await asyncio.wait_for(self.__port.read(exception_adu_size), timeout=0.1)
+        rtu.raise_for_exception_adu(response_error_adu)
+      except asyncio.TimeoutError:
+        print("modbus read timed out for response_error_adu")
+        exception_adu_size = 0
+
+      expected_response_size = \
         expected_response_pdu_size_from_request_pdu(message[1:-2]) + 3
-    response_remainder = b''
-    try:
-        response_remainder = await asyncio.wait_for(self.__port.read(expected_response_size - exception_adu_size), timeout=1.0)
-    except asyncio.TimeoutError:
-        print("modbus read timed out")
-
-    response = response_error_adu + response_remainder
-    if len(response) < expected_response_size:
-      return []
-    result = rtu.parse_response_adu(response, message)
-    if not isinstance(result, list):
-      return [result]
-    return result
+      response_remainder = b''
+      try:
+        response_remainder = await asyncio.wait_for(self.__port.read(expected_response_size - exception_adu_size), timeout=0.1)
+      except asyncio.TimeoutError:
+        print("modbus read timed out for response_remainder, retrying")
+        success = False
+
+      response = response_error_adu + response_remainder
+      if len(response) < expected_response_size:
+        return []
+      result = rtu.parse_response_adu(response, message)
+      if not isinstance(result, list):
+        return [result]
+      return result
-- 
2.34.1