public class StringToByteConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return value;
    }

    //hex string to byte array
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is String)
        {
            string valueTyped = (String)value;
            try
            {
                return Enumerable.Range(0, valueTyped.Length)
                            .Where(x => x % 2 == 0)
                            .Select(x => System.Convert.ToByte(valueTyped.Substring(x, 2), 16))
                            .ToArray();
            }
            catch (Exception ex)
            {
                //clear the text box
                return new byte[] { 0x00 };
            }

        }
        return new byte[] {0x00};
    }
}

public class WProtocol
    {
        private byte[] _seq = new byte[1] { 0x00 };

        public byte[] Seq
        {
            get { return _seq; }
            set
            {
                _seq = value;
            }
        }
    }

<Grid>
            <Label Content="Sequence :" HorizontalAlignment="Left" Height="24" Margin="29,24,0,0" VerticalAlignment="Top" Width="87"/>
            <TextBox HorizontalAlignment="Left" Height="22" Margin="115,26,0,0" TextWrapping="Wrap" Text="{Binding Seq, Converter= {StaticResource StringToByteConverter},Mode=TwoWay, UpdateSourceTrigger=LostFocus}" VerticalAlignment="Top" Width="67" FontSize="9" FontFamily="Arial"/>
            <TextBox HorizontalAlignment="Left" Height="20" Margin="282,28,0,0" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="69"/>
        </Grid>