Crc16.cs

using System.Collections.Generic;

namespace SerialLogger
{
    internal static class Crc16
    {
        public static ushort Compute(IEnumerable<byte> data)
        {
            const ushort mask = 0x1021;
            ushort crc = 0;
            foreach (var b in data)
            {
                var word = b << 8;
                for (byte i = 0; i < 8; i++)
                {
                    if (((crc ^ word) & 0x8000) != 0)
                    {
                        crc <<= 1;
                        crc ^= mask;
                    }
                    else
                    {
                        crc <<= 1;
                    }

                    word <<= 1;
                }
            }
            return crc;
        }
    }
}

Program.cs

using System;
using System.ComponentModel;
using System.IO;
using System.IO.Ports;

namespace SerialLogger
{
    class Program
    {
        static void Main(string[] args)
        {
            using (var serial = new SerialPort("COM11"))
            {
                serial.Open();
                Console.WriteLine("< STARTED >");
                var stream = serial.BaseStream;
                ReadBytesAndPrintToScreen(stream);

                while (true) HandleKeys(stream);
            }
        }

        private static void HandleKeys(Stream stream)
        {
            var key = Console.ReadKey(true);
            switch (key.Key)
            {
                case ConsoleKey.Escape:
                    stream.WriteByte(/* EOT */ 0x04);
                    break;
                case ConsoleKey.Enter:
                    stream.WriteByte(/* ACK */ 0x06);
                    break;
                case ConsoleKey.Delete:
                    stream.WriteByte(/* NAK */ 0x15);
                    break;

                case ConsoleKey.NumPad1:
                    stream.WriteData("OPN000");
                    break;

                default:
                    return;
            }
            Console.WriteLine();
            Console.ForegroundColor = ConsoleColor.White;
            Console.WriteLine($"< ");
        }

        private static void ReadBytesAndPrintToScreen(System.IO.Stream stream)
        {
            var bw = new BackgroundWorker();
            bw.DoWork += (s, e) =>
            {
                while (true)
                {
                    var b = stream.ReadByte();
                    if (b >= 0x20 && b <= 0x7f)
                    {
                        Console.ForegroundColor = ConsoleColor.Blue;
                        if (b >= 'A' && b < 'Z' ||
                            b >= 'a' && b < 'z' ||
                            b >= 'a' && b < 'z')
                        { Console.Write((char)b); }
                        else
                        { Console.Write(b.ToString("X")); }
                    }
                    else
                    {
                        Console.ForegroundColor = ConsoleColor.Yellow;
                        Console.Write(b.ToString("X"));
                    }
                    Console.Write(" ");
                }
            };
            bw.RunWorkerAsync();
        }
    }
}

StreamExtensions.cs

using System.IO;
using System.Text;

namespace SerialLogger
{
    public static class StreamExtensions
    {
        public static void WriteData(this Stream stream, byte[] data)
        {
            stream.WriteByte(/* SYN */ 0x16);
            stream.Write(data, 0, data.Length);

            var crc = Crc16.Compute(data);
            stream.WriteByte((byte)(crc / 256));
            stream.WriteByte((byte)(crc % 256));

        }
        public static void WriteData(this Stream stream, string data)
        {
            stream.WriteData(Encoding.ASCII.GetBytes(data));
        }
    }
}