diff --git a/lib/modbuzz/pdu.ex b/lib/modbuzz/pdu.ex
index 4c2f4f0..ca26a11 100644
--- a/lib/modbuzz/pdu.ex
+++ b/lib/modbuzz/pdu.ex
@@ -9,16 +9,7 @@ defmodule Modbuzz.PDU do
   def encode_request(struct), do: {:ok, Modbuzz.PDU.Protocol.encode(struct)}
   def encode_response(struct), do: {:ok, Modbuzz.PDU.Protocol.encode(struct)}
 
-  for {modbus_function_code, modbus_function} <- [
-        {0x01, ReadCoils},
-        {0x02, ReadDiscreteInputs},
-        {0x03, ReadHoldingRegisters},
-        {0x04, ReadInputRegisters},
-        {0x05, WriteSingleCoil},
-        {0x06, WriteSingleRegister},
-        {0x0F, WriteMultipleCoils},
-        {0x10, WriteMultipleRegisters}
-      ] do
+  for {modbus_function_code, modbus_function} <- Modbuzz.MixProject.pdu_seed() do
     req_module = Module.concat([Modbuzz.PDU, modbus_function, Req])
     res_module = Module.concat([Modbuzz.PDU, modbus_function, Res])
     err_module = Module.concat([Modbuzz.PDU, modbus_function, Err])
diff --git a/lib/modbuzz/pdu/diagnostics.ex b/lib/modbuzz/pdu/diagnostics.ex
new file mode 100644
index 0000000..cb65c74
--- /dev/null
+++ b/lib/modbuzz/pdu/diagnostics.ex
@@ -0,0 +1,110 @@
+defmodule Modbuzz.PDU.Diagnostics do
+  @moduledoc false
+
+  defmodule Req do
+    @moduledoc Modbuzz.PDU.Helper.module_one_line_doc(__MODULE__)
+
+    @type t :: %__MODULE__{
+            sub_function: 0x0000..0xFFFF,
+            data: 0x0000..0xFFFF
+          }
+
+    defstruct [:sub_function, :data]
+
+    defimpl Modbuzz.PDU.Protocol do
+      @function_code 0x08
+
+      @doc """
+          iex> req = %Modbuzz.PDU.Diagnostics.Req{
+          ...>   sub_function: 0x0000,
+          ...>   data: 0xA537
+          ...> }
+          iex> Modbuzz.PDU.Protocol.encode(req)
+          <<#{@function_code}, 0x0000::16, 0xA537::16>>
+      """
+      def encode(struct) do
+        <<@function_code, struct.sub_function::16, struct.data::16>>
+      end
+
+      @doc """
+          iex> req = %Modbuzz.PDU.Diagnostics.Req{}
+          iex> Modbuzz.PDU.Protocol.decode(req, <<#{@function_code}, 0x0000::16, 0xA537::16>>)
+          %Modbuzz.PDU.Diagnostics.Req{sub_function: 0x0000, data: 0xA537}
+      """
+      def decode(struct, <<@function_code, sub_function::16, data::16>>) do
+        %{struct | sub_function: sub_function, data: data}
+      end
+    end
+  end
+
+  defmodule Res do
+    @moduledoc Modbuzz.PDU.Helper.module_one_line_doc(__MODULE__)
+
+    @type t :: %__MODULE__{
+            sub_function: 0x0000..0xFFFF,
+            data: 0x0000..0xFFFF
+          }
+
+    defstruct [:sub_function, :data]
+
+    defimpl Modbuzz.PDU.Protocol do
+      @function_code 0x08
+
+      @doc """
+          iex> res = %Modbuzz.PDU.Diagnostics.Res{
+          ...>   sub_function: 0x0000,
+          ...>   data: 0xA537
+          ...> }
+          iex> Modbuzz.PDU.Protocol.encode(res)
+          <<#{@function_code}, 0x0000::16, 0xA537::16>>
+      """
+      def encode(struct) do
+        <<@function_code, struct.sub_function::16, struct.data::16>>
+      end
+
+      @doc """
+          iex> res = %Modbuzz.PDU.Diagnostics.Res{}
+          iex> Modbuzz.PDU.Protocol.decode(res, <<#{@function_code}, 0x0000::16, 0xA537::16>>)
+          %Modbuzz.PDU.Diagnostics.Res{
+            sub_function: 0x0000,
+            data: 0xA537
+          }
+      """
+      def decode(struct, <<@function_code, sub_function::16, data::16>>) do
+        %{struct | sub_function: sub_function, data: data}
+      end
+    end
+  end
+
+  defmodule Err do
+    @moduledoc Modbuzz.PDU.Helper.module_one_line_doc(__MODULE__)
+
+    @type t :: %__MODULE__{
+            exception_code: 0x01 | 0x03 | 0x04
+          }
+
+    defstruct [:exception_code]
+
+    defimpl Modbuzz.PDU.Protocol do
+      @error_code 0x08 + 0x80
+
+      @doc """
+          iex> err = %Modbuzz.PDU.Diagnostics.Err{exception_code: 0x01}
+          iex> Modbuzz.PDU.Protocol.encode(err)
+          <<#{@error_code}, 0x01>>
+      """
+      def encode(struct) do
+        <<@error_code, struct.exception_code>>
+      end
+
+      @doc """
+          iex> err = %Modbuzz.PDU.Diagnostics.Err{}
+          iex> Modbuzz.PDU.Protocol.decode(err, <<#{@error_code}, 0x01>>)
+          %Modbuzz.PDU.Diagnostics.Err{exception_code: 0x01}
+      """
+      def decode(struct, <<@error_code, exception_code>>) do
+        %{struct | exception_code: exception_code}
+      end
+    end
+  end
+end
diff --git a/lib/modbuzz/tcp/client.ex b/lib/modbuzz/tcp/client.ex
index 880c31b..fdd0fbb 100644
--- a/lib/modbuzz/tcp/client.ex
+++ b/lib/modbuzz/tcp/client.ex
@@ -3,7 +3,7 @@ defmodule Modbuzz.TCP.Client do
 
   use GenServer
 
-  require Logger
+  alias Modbuzz.TCP.Log
 
   defmodule Transaction do
     @moduledoc false
@@ -125,11 +125,11 @@ defmodule Modbuzz.TCP.Client do
   def handle_continue(:connect, %{socket: nil} = state) do
     case gen_tcp_connect(state) do
       {:ok, socket} ->
-        Logger.debug("#{__MODULE__}: :connect succeeded.")
+        Log.debug(":connect succeeded", state)
         {:noreply, %{state | socket: socket}}
 
       {:error, reason} ->
-        Logger.error("#{__MODULE__}: :connect failed, the reason is #{inspect(reason)}.")
+        Log.error(":connect failed", reason, state)
         {:noreply, state, {:continue, :connect}}
     end
   end
@@ -154,17 +154,17 @@ defmodule Modbuzz.TCP.Client do
       {:noreply, %{state | socket: socket}}
     else
       {:connect, {:error, reason} = error} ->
-        Logger.error("#{__MODULE__}: :recall connect failed, the reason is #{inspect(reason)}.")
+        Log.error(":recall connect failed", reason, state)
         GenServer.reply(from, error)
         {:noreply, state, {:continue, :connect}}
 
       {:send, {:error, reason} = error} ->
-        Logger.error("#{__MODULE__}: :recall send failed, the reason is #{inspect(reason)}.")
+        Log.error(":recall send failed", reason, state)
         GenServer.reply(from, error)
         {:noreply, state, {:continue, :connect}}
 
       {:recv, {:error, reason} = error} ->
-        Logger.error("#{__MODULE__}: :recall recv failed, the reason is #{inspect(reason)}.")
+        Log.error(":recall recv failed", reason, state)
         GenServer.reply(from, error)
         {:noreply, state, {:continue, :connect}}
     end
@@ -195,11 +195,11 @@ defmodule Modbuzz.TCP.Client do
       {:noreply, %{state | socket: socket, transactions: transactions}}
     else
       {:connect, {:error, reason}} ->
-        Logger.error("#{__MODULE__}: :recast connect failed, the reason is #{inspect(reason)}.")
+        Log.error(":recast connect failed", reason, state)
         {:noreply, state, {:continue, :connect}}
 
       {:send, {:error, reason}} ->
-        Logger.error("#{__MODULE__}: :recast send failed, the reason is #{inspect(reason)}.")
+        Log.error(":recast send failed", reason, state)
         {:noreply, state, {:continue, :connect}}
     end
   end
@@ -223,18 +223,14 @@ defmodule Modbuzz.TCP.Client do
       {:reply, res_tuple, state}
     else
       {:send, {:error, reason}} ->
-        Logger.warning(
-          "#{__MODULE__}: :call send failed, the reason is #{inspect(reason)}, :recall."
-        )
+        Log.warning(":call send failed, :recall", reason, state)
 
         :ok = transport.close(socket)
         state = %{state | socket: nil}
         {:noreply, state, {:continue, {:recall, unit_id, request, timeout, from}}}
 
       {:recv, {:error, reason}} ->
-        Logger.warning(
-          "#{__MODULE__}: :call recv failed, the reason is #{inspect(reason)}, :recall."
-        )
+        Log.warning(":call recv failed, :recall", reason, state)
 
         :ok = transport.close(socket)
         state = %{state | socket: nil}
@@ -273,9 +269,7 @@ defmodule Modbuzz.TCP.Client do
         {:noreply, %{state | transactions: transactions}}
 
       {:error, reason} ->
-        Logger.warning(
-          "#{__MODULE__}: :cast send failed, the reason is #{inspect(reason)}, :recast."
-        )
+        Log.warning(":cast send failed, :recast", reason, state)
 
         :ok = transport.close(socket)
         state = %{state | socket: nil}
@@ -316,7 +310,7 @@ defmodule Modbuzz.TCP.Client do
   def handle_info({:tcp_closed, socket}, %{socket: socket, active: true} = state) do
     %{transport: transport, transactions: transactions} = state
 
-    Logger.warning("#{__MODULE__}: transport closed.")
+    Log.warning("transport closed.", nil, state)
     :ok = transport.close(socket)
     state = %{state | socket: nil}
 
@@ -329,7 +323,7 @@ defmodule Modbuzz.TCP.Client do
         {:noreply, state, {:continue, :connect}}
 
       [{_transaction_id, transaction}] ->
-        Logger.debug("#{__MODULE__}: sent successfully but RST ACK received, :recast.")
+        Log.debug("sent successfully but RST ACK received, :recast", state)
 
         {:noreply, state,
          {:continue, {:recast, transaction.unit_id, transaction.request, transaction.from_pid}}}
@@ -338,7 +332,7 @@ defmodule Modbuzz.TCP.Client do
 
   def handle_info({:tcp_error, socket, reason}, %{socket: socket, active: true} = state) do
     %{transport: transport} = state
-    Logger.error("#{__MODULE__}: transport error, the reason is #{inspect(reason)}.")
+    Log.error("transport error", reason, state)
     :ok = transport.close(socket)
     {:noreply, %{state | socket: nil}, {:continue, :connect}}
   end
diff --git a/lib/modbuzz/tcp/log.ex b/lib/modbuzz/tcp/log.ex
new file mode 100644
index 0000000..50fff10
--- /dev/null
+++ b/lib/modbuzz/tcp/log.ex
@@ -0,0 +1,33 @@
+defmodule Modbuzz.TCP.Log do
+  @moduledoc false
+
+  require Logger
+
+  def debug(what_happend) do
+    Logger.debug(what_happend)
+  end
+
+  def debug(what_happend, state) do
+    Logger.debug(what_happend <> where(state))
+  end
+
+  def error(what_happend) do
+    Logger.error(what_happend)
+  end
+
+  def error(what_happend, reason, state) do
+    Logger.error(what_happend <> why(reason) <> where(state))
+  end
+
+  def warning(what_happend, reason, state) do
+    Logger.warning(what_happend <> why(reason) <> where(state))
+  end
+
+  defp why(nil), do: ""
+  defp why(reason), do: ", the reason is #{inspect(reason)}"
+
+  defp where(state) do
+    %{address: address, port: port} = state
+    " (address: #{inspect(address)}, port: #{inspect(port)})"
+  end
+end
diff --git a/lib/modbuzz/tcp/server.ex b/lib/modbuzz/tcp/server.ex
index 66bae35..2514909 100644
--- a/lib/modbuzz/tcp/server.ex
+++ b/lib/modbuzz/tcp/server.ex
@@ -3,7 +3,7 @@ defmodule Modbuzz.TCP.Server do
 
   use GenServer
 
-  require Logger
+  alias Modbuzz.TCP.Log
 
   @doc false
   @spec start_link(keyword()) :: GenServer.on_start()
@@ -40,7 +40,7 @@ defmodule Modbuzz.TCP.Server do
         {:noreply, %{state | listen_socket: socket}, {:continue, :accept}}
 
       {:error, reason} ->
-        Logger.error("#{__MODULE__}: :listen failed, the reason is #{inspect(reason)}.")
+        Log.error(":listen failed", reason, state)
         {:noreply, state, {:continue, :listen}}
     end
   end
@@ -65,7 +65,7 @@ defmodule Modbuzz.TCP.Server do
         )
 
       {:error, reason} ->
-        Logger.error("#{__MODULE__}: #{inspect(reason)}")
+        Log.error(":accept failed", reason, state)
     end
 
     {:noreply, state, {:continue, :accept}}
diff --git a/lib/modbuzz/tcp/server/socket_handler.ex b/lib/modbuzz/tcp/server/socket_handler.ex
index a884b46..bc6718d 100644
--- a/lib/modbuzz/tcp/server/socket_handler.ex
+++ b/lib/modbuzz/tcp/server/socket_handler.ex
@@ -3,7 +3,7 @@ defmodule Modbuzz.TCP.Server.SocketHandler do
 
   use GenServer, restart: :temporary
 
-  require Logger
+  alias Modbuzz.TCP.Log
 
   def start_link(args) do
     GenServer.start_link(__MODULE__, args)
@@ -11,6 +11,8 @@ defmodule Modbuzz.TCP.Server.SocketHandler do
 
   def init(args) do
     transport = Keyword.fetch!(args, :transport)
+    address = Keyword.fetch!(args, :address)
+    port = Keyword.fetch!(args, :port)
     socket = Keyword.fetch!(args, :socket)
     data_source = Keyword.fetch!(args, :data_source)
     timeout = Keyword.get(args, :timeout, 5000)
@@ -18,6 +20,8 @@ defmodule Modbuzz.TCP.Server.SocketHandler do
     {:ok,
      %{
        transport: transport,
+       address: address,
+       port: port,
        socket: socket,
        data_source: data_source,
        timeout: timeout
@@ -52,7 +56,7 @@ defmodule Modbuzz.TCP.Server.SocketHandler do
         {:stop, reason, state}
 
       {:error, reason} ->
-        Logger.error("#{__MODULE__}: #{inspect(reason)}")
+        Log.error(":recv failed", reason, state)
         :ok = transport.close(socket)
         {:stop, reason, state}
     end
@@ -66,7 +70,7 @@ defmodule Modbuzz.TCP.Server.SocketHandler do
       end
     catch
       :exit, {:noproc, mfa} ->
-        Logger.error("#{__MODULE__}: `#{data_source}` not found. (mfa is #{inspect(mfa)})")
+        Log.error("`#{data_source}` not found. (mfa is #{inspect(mfa)})")
         Modbuzz.PDU.to_error(request)
     end
   end
diff --git a/lib/modbuzz/tcp/server/socket_handler_supervisor.ex b/lib/modbuzz/tcp/server/socket_handler_supervisor.ex
index fc51758..8c9780a 100644
--- a/lib/modbuzz/tcp/server/socket_handler_supervisor.ex
+++ b/lib/modbuzz/tcp/server/socket_handler_supervisor.ex
@@ -13,9 +13,9 @@ defmodule Modbuzz.TCP.Server.SocketHandlerSupervisor do
       {Modbuzz.TCP.Server.SocketHandler,
        [
          transport: transport,
-         socket: socket,
          address: address,
          port: port,
+         socket: socket,
          data_source: data_source
        ]}
     )
diff --git a/mix.exs b/mix.exs
index c170baa..fa46e5d 100644
--- a/mix.exs
+++ b/mix.exs
@@ -29,6 +29,19 @@ defmodule Modbuzz.MixProject do
     ]
   end
 
+  def pdu_seed() do
+    [
+      {0x01, ReadCoils},
+      {0x02, ReadDiscreteInputs},
+      {0x03, ReadHoldingRegisters},
+      {0x04, ReadInputRegisters},
+      {0x05, WriteSingleCoil},
+      {0x06, WriteSingleRegister},
+      {0x0F, WriteMultipleCoils},
+      {0x10, WriteMultipleRegisters}
+    ]
+  end
+
   # Run "mix help deps" to learn about dependencies.
   defp deps do
     [
@@ -75,9 +88,10 @@ defmodule Modbuzz.MixProject do
       docs: [
         main: "readme",
         extras: ["README.md"],
-        nest_modules_by_prefix: [
-          Modbuzz.PDU
-        ]
+        nest_modules_by_prefix:
+          for {_modbus_function_code, modbus_function} <- pdu_seed() do
+            Module.concat([Modbuzz.PDU, modbus_function])
+          end
       ]
     ]
   end
diff --git a/test/modbuzz/pdu_test.exs b/test/modbuzz/pdu_test.exs
index 4ffee4c..a775548 100644
--- a/test/modbuzz/pdu_test.exs
+++ b/test/modbuzz/pdu_test.exs
@@ -2,16 +2,7 @@ defmodule Modbuzz.PDUTest do
   use ExUnit.Case
 
   for type <- [Req, Res, Err],
-      modbus_function <- [
-        ReadCoils,
-        ReadDiscreteInputs,
-        ReadHoldingRegisters,
-        ReadInputRegisters,
-        WriteSingleCoil,
-        WriteSingleRegister,
-        WriteMultipleCoils,
-        WriteMultipleRegisters
-      ] do
+      {_modbus_function_code, modbus_function} <- Modbuzz.MixProject.pdu_seed() do
     doctest Module.concat([Modbuzz.PDU.Protocol.Modbuzz.PDU, modbus_function, type])
   end
 end