I am trying to figure out how the calculate the CRC for very simple SDLC frames.
Using an MLT I am capturing the stream and i see some simple frames being sent out like: 0x3073F9E3 and 0x3011EDE3
From my understanding the F9E3 and EDE3 are the 2 byte checksums of the 3073 and 3011 since that is all that was in that frame.
using numerous CRC calculators and calculations I have been able to get the first byte of the checksum, but not the last byte (the F9 and the ED).
Using this calculator (http://www.zorc.breitbandkatze.de/crc.html):
Select CRC-CCITT
Change Final XOR Value to: FFFF
Check Reverse Data Bytes and reverse CRC result before Final XOR
Then type the input: %30%11
Which will give the output B8ED so the last byte is the ED.
Any ideas?
You are getting the correct crc16's (F9 F8, ED B8). I don't know why your last byte is E3 in both cases. This is perhaps a clue that the packets are not being disassembled correctly.
Related
I'm trying a few days to get the type of the CRC with the following hex stream (sniffed with wireshark):
The Hex data i sniffed:
0000001ec001075465737431323308557365726e616d650850617373776f7264d224
This should be the DATA in HEX:
0000001ec001075465737431323308557365726e616d650850617373776f7264
So the last 4 digits are the checksum, in this case d224
I used many code snippets (PHP, java), and some online checksum calcuation sites:
e.g.:
http://www.scadacore.com/field-applications/programming-calculators/online-checksum-calculator/
But I don't get the correct CRC value.
Thanks!
Update 1
Here are more hex streams with CRC included (the last 4 digits):
0000001dc001045465737409557365726e616d65310950617373776f726431cc96
0000001dc001045465737409557365726e616d65320950617373776f72643289d9
0000001dc001045465737409557365726e616d65330950617373776f726433b51c
0000001dc001045465737409557365726e616d65340950617373776f7264340347
0000001dc001045465737409557365726e616d65350950617373776f7264353f82
It appears to be the ARC CRC, polynomial 0x8005, reflected, zero initial value and no final xor, if I discard the initial 0000001d on each message, and take the CRC at the end to be put in the stream in little-endian order.
I want to send data to a TCP 105 circuit.
The following hex command is OK to send data 123:
7F30001103 313233 45D4
Here, 313233 is hex representation of 123 and 45D4 is the CRC value.
I'm in problem to obtain this 45D4 after calculating CRC. After searching for a long time on the web, I'm getting other CRC values in different standards. But those CRC values are not being accepted by my LED display circuit.
Please help me to know how is it possible to get 45D4 from 7F30001103313233.
Thanks in advance.
The command matches an algorithm called CRC-16/CMS.
$ reveng -w 16 -s 7f30001103313233d445
width=16 poly=0x8005 init=0xffff refin=false refout=false xorout=0x0000 ch
eck=0xaee7 name="CRC-16/CMS"
This is probably the correct algorithm, as you've only given one codeword (and because I've assumed that the CRC has been byte-swapped.)
To generate code that computes this CRC, see Mark Adler's crcany tool, for instance.
A bit stream 11100110 is to be transmitted using CRC method. The generator polynomial is X4+ X3 + 1.
What is the actual bit transmitted ?
Suppose the third bit from the left is inverted during the transmission. How the error is detected.
How the generator polynomial is already known to sender side as well as receiver side, please make this clear.
Solution :
Here, FCS will be 0110 since n = 4.
So actual bit transmitted is >> 11100110 0110
I am confused with the problem 2, 3. please reply my 2, 3 questions.
Thank You!
If you know how to generate the 0110, then invert the bit and generate a new CRC. You will see that it's different. On the other end when you compute the CRC of the eight bits sent, it will not match the four bit CRC sent.
The two sides agree a priori on a protocol, that includes the definition of the CRC to be used.
I am writing a computer program which utilizes input from some equipment which I seldom have availible in my office. In order to develop and test this program I am trying to use an Arduino board to simulate the communication from the actual equipment. To this effect I create datapackets on the Arduino and send them to my computeer over the serial port. The packets are formated as a header and a hexidecimal integer, representing some sensor data.
The header is supposed to contain a checksum (2's complement 256-modulo). I am however not sure how to calculate it. In the datasheet of the equipment (which communication I try to simulate), it is stated that I should first compute the sum all bytes in the packet, and then take the 256-modulo of the sum and perform a 8-bit two's complement on the result.
However, as I am a newbie to bits, bytes and serial communication, I do not understand the following:
1) Lets say that I want to send the value 5500 as two bytes (high byte and low byte). Then the high-byte is '15' and the low-byte is '7c' in hexidecimal encoding, which corresponds to 21 and 124 in decimal values. Do I then add the contributions 21 and 124 to the checksum before taking the 256-modulo, or do I have to do some bit-magic beforehand?
2) How do I perform a two's compliment?
Here is a code which should illustrate how I think. The idea is to send a packet with a header containing a byte which states the length of the packet, a byte which states the type of the packet, and a byte for the checksum. Then a two-byte integer value representing some sensor value is devided into a high-byte and a low-byte, and transmitted low-byte first.
int intVal;
byte Len = 5;
byte checksum;
byte Type = 2;
byte intValHi;
byte intValLo;
void setup(){
Serial.begin(9600);
}
void loop(){
intVal = 5500; //assume that this is a sensor value
intValHi = highByte(intVal);
intValLo = lowByte(intVal);
//how to calculate the checksum? I unsuccessfully tried the following
checksum = 0;
checksum = (Len+checksum+Type+intValHi+intValLo) % 256;
//send header
Serial.write(Len);
Serial.write(checksum);
Serial.write(Type);
//send sensor data
Serial.write(intValLo);
Serial.write(intValHi)
}
Thanks!
The first thing you should understand is that mod 256 is the same thing as looking at the bottom log(256) => 8 bits.
To understand this you have to first realize what the 'mod' operation does and how digits are represented in hardware.
Mod is the remainder after an old-school division (ie only with whole numbers).
eg 5%2 = 1
Digits in hardware are stored in 'bits' which can be interpreted as base 2 mathematics.
Thus if you want to take the mod operation of a power of 2 you don't actually have to do any math.
This is just like if you want to have the remainder of the power of 10, you just take the lower digits.
ie. 157 % 100 = 57.
This can be sped up by using the fact that bytes should overflow by themselves. This means that all you have to do to take %256 of a bunch of numbers is to add them to a single byte and the arduino will take care of the rest.
For twos compliment see this question:
What is “2's Complement”?
Is there any checksum that results in 2 digit hexadecimal?
I can only find NMEA Checksum...
references:
http://nmeachecksum.eqth.net/
http://en.wikipedia.org/wiki/NMEA_0183
I have some data file that I want to perform reverse engineering to find the kind of checksum.
Thank you in advance,
Two hex digits is one byte. You're looking for a checksum which produces one byte.
Obviously, you've got a simple additive checksum (sum the bytes of the input), and an xor of the input bytes.
It could also be a longer checksum of which only 8 bits have been taken.
It could also be some kind of CRC-8; Wikipedia knows about five kinds of standardised CRC-8.