diff --git a/apps/modbus/src/modbus_service.erl b/apps/modbus/src/modbus_service.erl
index a1dbad5..abab248 100644
--- a/apps/modbus/src/modbus_service.erl
+++ b/apps/modbus/src/modbus_service.erl
@@ -14,7 +14,7 @@
 
 %% API
 -export([start_link/1]).
--export([test/0]).
+-export([test/0, frame_delay/4]).
 
 %% gen_statem callbacks
 -export([init/1, handle_event/4, terminate/3, code_change/4, callback_mode/0]).
@@ -29,6 +29,15 @@
 
 -record(state, {
     port,
+    packet_id = 1,
+    queue = queue:new(),
+    is_pending = false :: boolean(),
+    %% 延迟的毫秒数
+    delay_ms = 0,
+
+    %% 未确认的消息
+    inflight = #{},
+
     access_log_pid :: pid() | undefined,
     error_log_pid :: pid() | undefined
 }).
@@ -67,10 +76,11 @@ init([AST = #ast{modbus = Modbus, devices = Devices}]) ->
     {ok, DevicePid} = modbus_device:start_link(hd(Devices)),
     lager:debug("device pid: ~p", [DevicePid]),
 
+
     %{ok, AccessLogPid} = modbus_logger:start_link(AccessLog),
     %{ok, ErrorLogPid} = modbus_logger:start_link(ErrorLog),
 
-    {ok, ?DISCONNECTED, #state{}}.
+    {ok, ?DISCONNECTED, #state{queue = queue:new(), packet_id = 1}}.
 
 %% @private
 %% @doc This function is called by a gen_statem when it needs to find out
@@ -83,15 +93,40 @@ callback_mode() ->
 %% gen_statem receives an event from call/2, cast/2, or as a normal
 %% process message, this function is called.
 
-handle_event(info, {read, SlaveId, Address}, ?CONNECTED, State = #state{port = Port}) ->
-    ReadCmd = <<?MODBUS_READ:8, SlaveId:8, Address:16>>,
-    Port !  {self(), {command, ReadCmd}},
-    {keep_state, State};
+handle_event(info, {read, SlaveId, Address}, ?CONNECTED, State = #state{packet_id = PacketId, queue = Q, is_pending = IsPending}) ->
+    %% 基于队列处理, modbus不是全双工的模式
+    NQ = queue:in({PacketId, SlaveId, Address}, Q),
+    Actions = case IsPending of
+                  true  -> [{next_event, info, read_next}];
+                  false -> []
+              end,
+    {keep_state, State#state{queue = NQ, packet_id = PacketId + 1, is_pending = false}, Actions};
 
-%% 从port读取数据
-handle_event(info, {Port, {data, Data}}, ?CONNECTED, State = #state{port = Port}) ->
-    lager:debug("port data is: ~p", [Data]),
-    {keep_state, State};
+%% 读取下一个任务
+handle_event(info, read_next, ?CONNECTED, State = #state{queue = Q, port = Port}) ->
+    case queue:out(Q) of
+        {{value, {PacketId, SlaveId, Address}}, Q2} ->
+            ReadCmd = <<?MODBUS_READ:8, PacketId:32, SlaveId:8, Address:16>>,
+            Port !  {self(), {command, ReadCmd}},
+            {keep_state, State#state{queue = Q2}};
+        {empty, Q1} ->
+            {keep_state, State#state{queue = Q1}}
+    end;
+
+%% 从port读取数据, todo 获取到的数据表示为32位整数
+handle_event(info, {Port, {data, <<?MODBUS_READ:8, PacketId:32, Val:32>>}}, ?CONNECTED, State = #state{port = Port, delay_ms = DelayMs, inflight = Inflight}) ->
+    NInflight = case maps:take(PacketId, Inflight) of
+                    error ->
+                        Inflight;
+                    {{ReceiverPid, Ref}, NMap} ->
+                        erlang:is_process_alive(ReceiverPid) andalso ReceiverPid ! {request_reply, Ref, Val},
+                        NMap
+                end,
+    lager:debug("port data is: ~p", [{PacketId, Val}]),
+    %% 读取下一条
+    erlang:start_timer(DelayMs, self(), read_next),
+
+    {keep_state, State#state{inflight = NInflight}};
 
 handle_event(_EventType, _EventContent, _StateName, State = #state{}) ->
     NextStateName = the_next_state_name,
@@ -114,12 +149,15 @@ code_change(_OldVsn, StateName, State = #state{}, _Extra) ->
 %%% Internal functions
 %%%===================================================================
 
-connect(#modbus{transport = #modbus_transport_rtu{port = Port, baudrate = Baudrate, stopbits = Stopbits, parity = Parity, timeout = Timeout}}) ->
+%% databits为8位
+connect(#modbus{transport = #modbus_transport_rtu{port = Port, baudrate = BaudRate, stopbits = StopBits, parity = Parity, timeout = Timeout}}) ->
     RealExecCmd = "",
     Port = erlang:open_port({spawn_executable, RealExecCmd}, [binary, {packet, 2}, exit_status]),
 
+    DelayMs = frame_delay(BaudRate, 8, Parity, StopBits),
+
     Len0 = byte_size(Port),
-    ConnectCmd = <<?MODBUS_CONNECT:8, Len0:8, Port/binary, Baudrate:32, Stopbits:8, Parity:8, Timeout:32>>,
+    ConnectCmd = <<?MODBUS_CONNECT:8, Len0:8, Port/binary, BaudRate:32, StopBits:8, Parity:8, Timeout:32>>,
     %% 建立连接
     Port ! {self(), {command, ConnectCmd}},
     ok;
@@ -131,4 +169,11 @@ connect(#modbus{transport = #modbus_transport_tcp{host = Host, port = Port, time
                   false ->
                       2000
               end,
-    gen_tcp:connect(Host, Port, [binary], Timeout).
\ No newline at end of file
+    gen_tcp:connect(Host, Port, [binary], Timeout).
+
+%% 计算3.5个字符的静默时间
+frame_delay(BaudRate, DataBits, Parity, StopBits) when is_integer(BaudRate), is_integer(DataBits), is_integer(Parity), is_integer(StopBits) ->
+    %% 计算1个字符的总位数
+    CharBits = 1 + DataBits + case Parity of 0 -> 0; _ -> 1 end + StopBits,
+    %% 3.5字符时间（毫秒), 预留20%的时间
+    erlang:ceil((3500 * CharBits) / BaudRate * 1.2).
\ No newline at end of file