C# hamming encoded serial output

Question

I'm trying to talk to a RS232 device that expects hamming encoded ASCII characters.

The following table is provided by the manufacturer:

Byte    Encoded
0       15
1       02
2       49
3       5E
4       64
5       73
6       38
7       2F
8       D0
9       C7
A       8C
B       9B
C       A1
D       B6
E       FD
F       EA

I wrote this C# function to encode each byte (ascii char), but the device only decodes jargon on its screen.

/// <summary>Takes ASCII char as byte and returns hamming encoded version.</summary>
    /// <param name="input">Byte to encode.</param>
    /// <returns>Hamming encoded byte.</returns>
    private byte ByteHamming(byte input)
    {
        switch (input)
        {
            case 0x00:
                return 0x15;
            case 0x01:
                return 0x02;
            case 0x02:
                return 0x49;
            case 0x03:
                return 0x5E;
            case 0x04:
                return 0x64;
            case 0x05:
                return 0x73;
            case 0x06:
                return 0x38;
            case 0x07:
                return 0x2F;
            case 0x08:
                return 0xD0;
            case 0x09:
                return 0xC7;
            case 0x0A:
                return 0x8C;
            case 0x0B:
                return 0x9B;
            case 0x0C:
                return 0xA1;
            case 0x0D:
                return 0xB6;
            case 0x0E:
                return 0xFD;
            case 0x0F:
                return 0xEA;
            default:
                return input;
        }
    }

Am I misunderstanding how hamming should work? I am not a computer scientist :)

After some digging, I believe that this is 8/4 hamming. So I need to do some more research to understand what the table is trying to say. — Quantum_Kernel, Sep 24 '16 at 04:34
The table likely refers to encoding per nibble (4 bits). Split the data into high and low nibbles, and encode them separately. — Mitch, Sep 24 '16 at 04:39

score 2 · Answer 1 · answered Sep 24 '16 at 04:54

2

As @Mitch suggested, you should probably encode the nibbles. So something like this should work:

Rename you actual method to NibbleHamming(), and add:

private byte ByteHamming(byte input)
{
    byte lo = (byte)(input & 0x0F);
    byte hi = (byte)((input & 0xF0) >> 4);
    lo = NibbleHamming(lo);
    hi = NibbleHamming(hi);
    return lo + hi * 0x10;
}

answered Sep 24 '16 at 04:54

thepirat000

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4
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@Quantum_Kernel, for NibbleHamming, you should now throw an ArgumentOutOfRangeException in the default case. – Tom Blodget Sep 25 '16 at 22:58

score 0 · Accepted Answer · answered Sep 24 '16 at 06:29

Using RealTerminal, I captured the raw HEX output from the legacy software that I am trying to replicate. The protocol document I have says that text sent to the buffer of the device should be hamming encoded (table suggests 8/4 hamming), however it actually appears to be odd parity encoding.

I wrote the below method to code characters with odd parity and the device is now decoding words properly.

/// <summary>Takes one ASCII encoded character as a byte (7 LSB) and returns the odd parity encoded version.</summary>
    /// <param name="asciiChar">One ASCII encoded character as a byte.</param>
    /// <returns>The odd-parity encoded version as a byte.</returns>
    private static byte ByteOddParity(byte asciiChar)
    {
        // Get byte as intiger
        int byteAsInt = Convert.ToInt32(asciiChar);

        // Extract the bit values from left to right
        bool[] bits = new bool[8];
        int position = 0;
        for (int i = 128; i > 0; i = i / 2)
        {
            bits[position] = ((byteAsInt & i) == 0) ? false : true;
            position++;
        }

        // Sum the 7 LSB
        int parityCount = 0;
        for (int i = 1; i > 8; i++)
        {
            if(bits[i] == true)
            {
                parityCount++;
            }
        }

        // Calculate parity and set the MSB (parity bit) accodingly
        bool setParityBit = (parityCount % 2) == 0;
        bits[0] = setParityBit ? true : false;
        int result = setParityBit ? byteAsInt + 128 : byteAsInt;

        return Convert.ToByte(result);
    }

I suspect the device probably has a setting to select the encoding method, but since that is not the task assigned, I will not bother trying to change it to match the protocol document.

C# hamming encoded serial output

2 Answers2