Can't deserialize my xml file in proper way. Always get null

Question

I have problem with deserialize XML my xml code is:

<?xml version="1.0" encoding="UTF-8"?>
<Plc name="Test" vendor="S" >
  <connection name="Plc" model="S2" address="192.168.0.1" pooling="1.0" slot="0" rack="0" active="true" frameLength="200">
    <tag name="DBTimeMean"                   address="DB100.DBD628.0" />
    <tag name="DBTimeMax1"                   address="DB100.DBD632.0" />
    <tag name="DBTimeMax2"                   address="DB100.DBD636.0" />
    <tag name="DBTimeMin"                    address="DB100.DBD640.0" />
    <tag name="MachineStatusID"              address="DB100.DBD644.0" />
  </connection>
</Plc>

and this is my object class :

 public class PlcPointFormat
    {
        [XmlRoot(ElementName = "tag")]
        public class Tag
        {
            [XmlAttribute(AttributeName = "name")]
            public string Name { get; set; }
            [XmlAttribute(AttributeName = "address")]
            public string Address { get; set; }
          
        }

        [XmlRoot(ElementName = "connection")]
        public class Connection
        {
            [XmlElement(ElementName = "tag")]
            public ObservableCollection<Tag> Tag { get; set; }
            [XmlAttribute(AttributeName = "name")]
            public string Name { get; set; }
            [XmlAttribute(AttributeName = "model")]
            public string Model { get; set; }
            [XmlAttribute(AttributeName = "address")]
            public string Address { get; set; }
            [XmlAttribute(AttributeName = "pooling")]
            public string Pooling { get; set; }
            [XmlAttribute(AttributeName = "slot")]
            public string Slot { get; set; }
            [XmlAttribute(AttributeName = "rack")]
            public string Rack { get; set; }
            [XmlAttribute(AttributeName = "active")]
            public string Active { get; set; }
            [XmlAttribute(AttributeName = "frameLength")]
            public string FrameLength { get; set; }
        }

        [XmlRoot(ElementName = "Plc")]
        public class Plc
        {
            [XmlElement(ElementName = "connection")]
            public Connection Connection { get; set; }
            [XmlAttribute(AttributeName = "name")]
            public string Name { get; set; }
            [XmlAttribute(AttributeName = "vendor")]
            public string Vendor { get; set; }
        }

        private static XmlSerializer getSerializer()
        {
            if (formatDataListXmlSerializer == null)
            {
                formatDataListXmlSerializer = new XmlSerializer(typeof(PlcPointFormat));
            }
            return formatDataListXmlSerializer;
        }
        public string SerializeToXML()
        {
            XmlSerializer xsSubmit = getSerializer();
            var xml = "";

            using (var sww = new StringWriter())
            {
                using (XmlWriter writer = XmlWriter.Create(sww, new XmlWriterSettings() { Indent = true }))
                {
                    xsSubmit.Serialize(writer, this);
                    xml = sww.ToString();
                }
            }

            return xml;
        }

        private static XmlSerializer formatDataListXmlSerializer = null;

      
        public static PlcPointFormat CreateFromXML(string xml)
        {
            //PlcPointFormat ret;

            XmlSerializer serializer = getSerializer();

            try
            {
                StringReader reader = new StringReader(xml);
                var ret = serializer.Deserialize(reader);
                reader.Close();
            }
            catch
            {
                //ret = new PlcPointFormat();
            }

            return null;
        }
    }

Every time i try to invoke method CreateFromXML with xml above as parameter I got null. Do you have any suggestions? I tried to specify encoding it doesn't make any affect. I am using .net 4.6 .2 in this project and System.XMl.Serialization namespace for deserialization and serialization.