diff --git a/Modbus.Net/Modbus.Net.Modbus/ModbusRtuProtocolReceiver.cs b/Modbus.Net/Modbus.Net.Modbus/ModbusRtuProtocolReceiver.cs
index dec919e..5a0ac32 100644
--- a/Modbus.Net/Modbus.Net.Modbus/ModbusRtuProtocolReceiver.cs
+++ b/Modbus.Net/Modbus.Net.Modbus/ModbusRtuProtocolReceiver.cs
@@ -32,7 +32,7 @@ namespace Modbus.Net.Modbus
                     }
                     else if (receiveBytes.Length == 6)
                     {
-                        if (receiveBytes[1] == 1 || receiveBytes[1] == 2)
+                        if (receiveBytes[1] == 1 || receiveBytes[1] == 3)
                         {
                             writeContent = null;
                             return new ReceiveDataDef()
diff --git a/Modbus.Net/Modbus.Net/Linker/ProtocolReceiver.cs b/Modbus.Net/Modbus.Net/Linker/ProtocolReceiver.cs
index 009aa0f..c586d65 100644
--- a/Modbus.Net/Modbus.Net/Linker/ProtocolReceiver.cs
+++ b/Modbus.Net/Modbus.Net/Linker/ProtocolReceiver.cs
@@ -46,8 +46,12 @@ namespace Modbus.Net
                 var decBytes = BytesDecact(content);
                 var explainContent = DataExplain(decBytes);
                 var returnBytes = DataProcess(explainContent);
-                var extBytes = BytesExtend(returnBytes);
-                return extBytes;
+                if (returnBytes != null)
+                {
+                    var extBytes = BytesExtend(returnBytes);
+                    return extBytes;
+                }
+                return null;
             }
             else
             {
diff --git a/Modbus.Net/Modbus.Net/README.md b/Modbus.Net/Modbus.Net/README.md
index 9cffc9c..46bcdcb 100644
--- a/Modbus.Net/Modbus.Net/README.md
+++ b/Modbus.Net/Modbus.Net/README.md
@@ -61,7 +61,7 @@ Manage several types of Protocol to a same calling interface.
 
 ### Machine
 
-Shows the Hardware PLC or other types of machine and implement a high level send and receive api.
+Shows the Hardware DCS or other types of machine and implement a high level send and receive api.
 
 ### Job