I am trying to write a unit test for a CRC16 checksum for a P1 meter message (and having a hard time getting it right...).
What I have is:
- A P1 meter message
- Stripped it so that I have the correct part according to the spec (see below)
- Turned it into a byte array (ASCII encoding)
- Calculate the CRC16 checksum
- Compare it with the checksum it should be
Given the fact that I posted a question on SO, the result is not OK...
Please bear with me, I will share what I have below:
The P1 message
private static readonly string telegramText =
"/KFM5KAIFA-METER\n" +
"\n" +
"1-3:0.2.8(42)\n" +
"0-0:1.0.0(170124213128W)\n" +
"0-0:96.1.1(4530303236303030303234343934333135)\n" +
"1-0:1.8.1(000306.946*kWh)\n" +
"1-0:1.8.2(000210.088*kWh)\n" +
"1-0:2.8.1(000000.000*kWh)\n" +
"1-0:2.8.2(000000.000*kWh)\n" +
"0-0:96.14.0(0001)\n" +
"1-0:1.7.0(02.793*kW)\n" +
"1-0:2.7.0(00.000*kW)\n" +
"0-0:96.7.21(00001)\n" +
"0-0:96.7.9(00001)\n" +
"1-0:99.97.0(1)(0-0:96.7.19)(000101000006W)(2147483647*s)\n" +
"1-0:32.32.0(00000)\n" +
"1-0:52.32.0(00000)\n" +
"1-0:72.32.0(00000)\n" +
"1-0:32.36.0(00000)\n" +
"1-0:52.36.0(00000)\n" +
"1-0:72.36.0(00000)\n" +
"0-0:96.13.1()\n" +
"0-0:96.13.0()\n" +
"1-0:31.7.0(003*A)\n" +
"1-0:51.7.0(005*A)\n" +
"1-0:71.7.0(005*A)\n" +
"1-0:21.7.0(00.503*kW)\n" +
"1-0:41.7.0(01.100*kW)\n" +
"1-0:61.7.0(01.190*kW)\n" +
"1-0:22.7.0(00.000*kW)\n" +
"1-0:42.7.0(00.000*kW)\n" +
"1-0:62.7.0(00.000*kW)\n" +
"0-1:24.1.0(003)\n" +
"0-1:96.1.0(4730303331303033333738373931363136)\n" +
"0-1:24.2.1(170124210000W)(00671.790*m3)\n" +
"!29ED\n";
The spec
Source can be found here
CRC is a CRC16 value calculated over the preceding characters in the data message (from “/” to “!” using the polynomial: x16 + x15 + x2 + 1).
CRC16 uses no XOR in, no XOR out and is computed with least significant bit first. The value is represented as 4 hexadecimal characters (MSB first).
Get the bytes, and do the CRC
The read message is the P1 message above, but then from '/' to '!' (including the '/' and including the '!'), as dictated by above spec.
var bytes = Encoding.ASCII.GetBytes(_readMessage);
var computeChecksum = new Crc16().ComputeChecksum(bytes);
Finally, the CRC code
Source can be found here
public class Crc16
{
const ushort polynomial = 0xA001;
ushort[] table = new ushort[256];
public ushort ComputeChecksum(byte[] bytes)
{
ushort crc = 0;
for (int i = 0; i < bytes.Length; ++i)
{
byte index = (byte)(crc ^ bytes[i]);
crc = (ushort)((crc >> 8) ^ table[index]);
}
return crc;
}
public byte[] ComputeChecksumBytes(byte[] bytes)
{
ushort crc = ComputeChecksum(bytes);
return BitConverter.GetBytes(crc);
}
public Crc16()
{
ushort value;
ushort temp;
for (ushort i = 0; i < table.Length; ++i)
{
value = 0;
temp = i;
for (byte j = 0; j < 8; ++j)
{
if (((value ^ temp) & 0x0001) != 0)
{
value = (ushort)((value >> 1) ^ polynomial);
}
else
{
value >>= 1;
}
temp >>= 1;
}
table[i] = value;
}
}
}
The unfortunate result
The checksum of the P1 telegram should be: 0x29ED, but unfortunately I calculate 0x6500.
Can somebody point me in the right direction?
Update
@Mark Adler, I fixed the issue you found, but I still calculate the wrong checksum. I stripped all irrelevant parts below, could you be so kind to have another look? You can simply copy paste the code below if you like. Or, post back the code that does calculate the correct checksum.
Thanks a lot in advance!
class Program
{
private static readonly string telegramText =
"/KFM5KAIFA-METER\r\n" +
"\r\n" +
"1-3:0.2.8(42)\r\n" +
"0-0:1.0.0(170124213128W)\r\n" +
"0-0:96.1.1(4530303236303030303234343934333135)\r\n" +
"1-0:1.8.1(000306.946*kWh)\r\n" +
"1-0:1.8.2(000210.088*kWh)\r\n" +
"1-0:2.8.1(000000.000*kWh)\r\n" +
"1-0:2.8.2(000000.000*kWh)\r\n" +
"0-0:96.14.0(0001)\r\n" +
"1-0:1.7.0(02.793*kW)\r\n" +
"1-0:2.7.0(00.000*kW)\r\n" +
"0-0:96.7.21(00001)\r\n" +
"0-0:96.7.9(00001)\r\n" +
"1-0:99.97.0(1)(0-0:96.7.19)(000101000006W)(2147483647*s)\r\n" +
"1-0:32.32.0(00000)\r\n" +
"1-0:52.32.0(00000)\r\n" +
"1-0:72.32.0(00000)\r\n" +
"1-0:32.36.0(00000)\r\n" +
"1-0:52.36.0(00000)\r\n" +
"1-0:72.36.0(00000)\r\n" +
"0-0:96.13.1()\r\n" +
"0-0:96.13.0()\r\n" +
"1-0:31.7.0(003*A)\r\n" +
"1-0:51.7.0(005*A)\r\n" +
"1-0:71.7.0(005*A)\r\n" +
"1-0:21.7.0(00.503*kW)\r\n" +
"1-0:41.7.0(01.100*kW)\r\n" +
"1-0:61.7.0(01.190*kW)\r\n" +
"1-0:22.7.0(00.000*kW)\r\n" +
"1-0:42.7.0(00.000*kW)\r\n" +
"1-0:62.7.0(00.000*kW)\r\n" +
"0-1:24.1.0(003)\r\n" +
"0-1:96.1.0(4730303331303033333738373931363136)\r\n" +
"0-1:24.2.1(170124210000W)(00671.790*m3)\r\n" +
"!";
static void Main(string[] args)
{
var bytes = Encoding.ASCII.GetBytes(telegramText);
var computeChecksum = new Crc16().ComputeChecksum(bytes);
}
}
public class Crc16
{
const ushort polynomial = 0x8005;
ushort[] table = new ushort[256];
public ushort ComputeChecksum(byte[] bytes)
{
ushort crc = 0;
for (int i = 0; i < bytes.Length; ++i)
{
byte index = (byte)(crc ^ bytes[i]);
crc = (ushort)((crc >> 8) ^ table[index]);
}
return crc;
}
public byte[] ComputeChecksumBytes(byte[] bytes)
{
ushort crc = ComputeChecksum(bytes);
return BitConverter.GetBytes(crc);
}
public Crc16()
{
ushort value;
ushort temp;
for (ushort i = 0; i < table.Length; ++i)
{
value = 0;
temp = i;
for (byte j = 0; j < 8; ++j)
{
if (((value ^ temp) & 0x0001) != 0)
{
value = (ushort)((value >> 1) ^ polynomial);
}
else
{
value >>= 1;
}
temp >>= 1;
}
table[i] = value;
}
}
}
