diff --git a/commhandler/CHANGELOG.md b/commhandler/CHANGELOG.md
index d8de98f..d5c2feb 100644
--- a/commhandler/CHANGELOG.md
+++ b/commhandler/CHANGELOG.md
@@ -5,6 +5,18 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.5.1] - 2025-12-17
+
+## Changed
+
+- updated minimal dependency versions, maintain Python 3.9 compatibility
+
+### Fixed
+
+- include `pyserial` in required dependencies, fixes error when serial device is created
+- configure Modbus byte/word order according to SGr specification
+
+
 ## [0.5.0] - 2025-10-27
 
 ### Added
diff --git a/commhandler/pyproject.toml b/commhandler/pyproject.toml
index adfe53e..982c52c 100644
--- a/commhandler/pyproject.toml
+++ b/commhandler/pyproject.toml
@@ -17,17 +17,17 @@ maintainers = [
     { name = "Matthias Krebs" }
 ]
 dependencies = [
-    "Jinja2>=3.1.0,<4.0.0",
-    "jmespath>=1.0.0,<2.0.0",
+    "Jinja2>=3.1.6,<4.0.0",
+    "jmespath>=1.0.1,<2.0.0",
     "jsonata-python>=0.6.0,<1.0.0",
     "parsel>=1.10.0,<1.11.0",
-    "pymodbus>=3.7.0,<3.8.0",
+    "pymodbus[serial]>=3.7.4,<3.8.0",
     "gmqtt>=0.7.0,<1.0.0",
-    "xsdata>=25.0.0,<26.0.0",
-    "xmlschema>=4.1.0,<5.0.0",
-    "aiohttp>=3.12.0,<4.0.0",
-    "certifi",
-    "cachetools>=6.2.0,<7.0.0",
+    "xsdata>=25.7.0,<26.0.0",
+    "xmlschema>=4.2.0,<5.0.0",
+    "aiohttp>=3.13.2,<4.0.0",
+    "certifi>=25.11.12",
+    "cachetools>=6.2.4,<7.0.0",
     "sgr-specification>=2.2.0,<3.0.0"
 ]
 
diff --git a/commhandler/requirements-dev.txt b/commhandler/requirements-dev.txt
index 74785c0..7cc448e 100644
--- a/commhandler/requirements-dev.txt
+++ b/commhandler/requirements-dev.txt
@@ -1,2 +1,2 @@
-pytest>=8.4.0,<9.0.0
-pytest-asyncio>=1.0.0,<2.0.0
+pytest>=8.4.2,<9.0.0
+pytest-asyncio>=1.1.1,<1.2.0
diff --git a/commhandler/requirements-doc.txt b/commhandler/requirements-doc.txt
index b925000..81cd4c3 100644
--- a/commhandler/requirements-doc.txt
+++ b/commhandler/requirements-doc.txt
@@ -1,3 +1,3 @@
-sphinx>=7.4.0,<8.0.0
-sphinx-autoapi>=3.6.0,<4.0.0
-sphinx-rtd-theme>=3.0.0,<4.0.0
+sphinx>=7.4.7,<8.0.0
+sphinx-autoapi>=3.6.1,<4.0.0
+sphinx-rtd-theme>=3.0.2,<4.0.0
diff --git a/commhandler/requirements.txt b/commhandler/requirements.txt
index f26bc79..9d84aa5 100644
--- a/commhandler/requirements.txt
+++ b/commhandler/requirements.txt
@@ -1,12 +1,12 @@
-Jinja2>=3.1.0,<4.0.0
-jmespath>=1.0.0,<2.0.0
+Jinja2>=3.1.6,<4.0.0
+jmespath>=1.0.1,<2.0.0
 jsonata-python>=0.6.0,<1.0.0
 parsel>=1.10.0,<1.11.0
-pymodbus>=3.7.0,<3.8.0
+pymodbus[serial]>=3.7.4,<3.8.0
 gmqtt>=0.7.0,<1.0.0
-xsdata>=25.0.0,<26.0.0
-xmlschema>=4.1.0,<5.0.0
-aiohttp>=3.12.0,<4.0.0
-certifi
-cachetools>=6.2.0,<7.0.0
+xsdata>=25.7.0,<26.0.0
+xmlschema>=4.2.0,<5.0.0
+aiohttp>=3.13.2,<4.0.0
+certifi>=25.11.12
+cachetools>=6.2.4,<7.0.0
 sgr-specification>=2.2.0,<3.0.0
diff --git a/commhandler/src/sgr_commhandler/driver/modbus/modbus_client_async.py b/commhandler/src/sgr_commhandler/driver/modbus/modbus_client_async.py
index 622c5af..5cb6412 100644
--- a/commhandler/src/sgr_commhandler/driver/modbus/modbus_client_async.py
+++ b/commhandler/src/sgr_commhandler/driver/modbus/modbus_client_async.py
@@ -30,9 +30,10 @@ def __init__(self, endianness: BitOrder, addr_offset: int, client: ModbusBaseCli
         self._lock = threading.Lock()
         self._client: ModbusBaseClient = client
         self._byte_order: Endian = (
-            Endian.BIG
-            if endianness is None or endianness == BitOrder.BIG_ENDIAN
-            else Endian.LITTLE
+            Endian.LITTLE
+            if endianness
+            in {BitOrder.CHANGE_BYTE_ORDER, BitOrder.CHANGE_BIT_ORDER}
+            else Endian.BIG
         )
         self._word_order: Endian = (
             Endian.LITTLE
@@ -261,10 +262,10 @@ def __init__(self, ip: str, port: int, endianness: BitOrder = BitOrder.BIG_ENDIA
             AsyncModbusTcpClient(
                 host=ip,
                 port=port,
-                timeout=1,
+                timeout=1.0,
                 retries=0,
-                reconnect_delay=5000,
-                reconnect_delay_max=30000
+                reconnect_delay=1.0,
+                reconnect_delay_max=30.0
             )
         )
         """
diff --git a/specification/requirements-dev.txt b/specification/requirements-dev.txt
index 2e3e6c0..e5c5bdd 100644
--- a/specification/requirements-dev.txt
+++ b/specification/requirements-dev.txt
@@ -1 +1 @@
-xsdata[cli]>=25.0.0,<26.0.0
+xsdata[cli]>=25.7.0,<26.0.0