using System;

public class Program

{

    public static void Main()

    {

        byte[] packetArrayBytes = new byte[2];

        uint intA = 0xF;   // Allowed range is 0-15   (0x0-0xF)

        uint intB = 0xABC; // Allowed range is 0-4095 (0x0-0xFFF)

​

        // Need to convert from uint to bytes:

        byte[] bytesA = BitConverter.GetBytes(intA);

        byte[] bytesB = BitConverter.GetBytes(intB);

        byte a = bytesA[0]; // a is 0x0F

        byte b = bytesB[1]; // b is 0x0A

        int c = 0x00;      // c is 0x00

        int d = bytesB[0]; // d is 0xBC

​

        // Mask out 4 least significant bits of a,

        // then shift 4 bits left to place them in the most significant bits (of the byte),

        // then OR them into c.

        c |= (a & 0x0F) << 4;  // c is now 0xF0

​

        // Mask out 4 least significant bits of b,

        // then OR them into c.

        c |= b & 0x0F;   // c is now 0xFA

        packetArrayBytes[0] = (Byte)c;

        packetArrayBytes[1] = (Byte)d;

        Console.WriteLine(BitConverter.ToString(packetArrayBytes));

    }

}