Checksum for cash register packet

Posted on 2004-08-21
Last Modified: 2013-11-18
I have problem finding the right checksum for this packet.
Checksum is CRC-like type except for the table.
This is "Datecs" cash registers and the packet that is sent trough RS-232 from PC. For now, i'm looking for easy solution (not decompiling executive of the program that does the communication or disassemble the firmware to find it out).

Here it is:
Example 1:
#$0A R2;1; #$0D #$18 #$78 - This command is a "read" with subcommand 2 and article no. 1.
#$0A I0;1; #$0D #$5A #$51 - Don't remember what this command does, but it's using the same scheme

Here's the format:
"All bytes in the block are ASCII characters except the two control bytes CRC.
A sucessful transmission of a block from the ECR includes transmitting of:
- control character for beginning of a block  BEG ( 0A HEX or ^J)
- data bytes
- control symbol for end of block END ( 0D HEX or  ^M)
- 2 control bytes CRC."

So, i need the way to calculate last 2 numbers from the packet - in first case $1878 and in the second $5A51.

Here's the explanation of the manufacturers (very unclear) - looks like some CRC16, but it's not alike anything i ever see:
      The two CRC bytes are calculated according to the formula x^15 + 1. In the calculation are included all data bytes plus the byte for block end. Every byte passes through the calculation register from teh MSB to LSB.
      Three working bytes are used - S1, S0 and TR
      S1 - Most significant byte from the CRC ( it is transmitted immediatelly after END)
      S0 - Least significant byte from the CRC ( It is transmitted after S1)
      TR - the current transmitted byte in the block.

      The CRC is calculated as follows:
      1. S1 and S0 are zeroed
      2. TR is loaded with the current transmitted byte. The byte is transmitted.
      3. Points 3.1 and 3.2 are executed 8 times:
      3.1. S1, S0 and TR are shifted one bit to the left.
      3.2. If the carry bit from S1 is 1, the MSB of S1 and LSB of S0 are inverted.
      Points 2 and 3 are executed for all bytes, included in the calculation of the CRC - from the first byte after BEG up to and including byte END.
      4. TR is loaded with 0 and point 3 is executed
      5. TR is loaded with 0 and point 3 is executed
      6. Byte S1 is transmitted
      7. Byte S0 is transmitted
Question by:Dragonmen
LVL 84

Expert Comment

ID: 11862173
What do you need?
A program in some particular language to calculate S1 and S0?

Author Comment

ID: 11872875
A scheme for calculating will be good enough.
And, yes, it's S0 and S1 (checksum byte 1 and 2).
LVL 19

Accepted Solution

drichards earned 500 total points
ID: 11878289
Can someone out there figure out what I did wrong here?  I think this should be the algorithm, but I get an extra bit in my answer:

0x9878 vs. 0x1878 in the first case and
0x5AD1 vs. 0x5A51 in the second.

Interestingly, off by 0x8000 and 0x0080.

Here's my code:
  // Put these lines in main or combine into main...
  unsigned char data1[] = { 0x0A, 'R', '2', ';', '1', ';', 0x0D, 0x18, 0x78 };
  unsigned char data2[] = { 0x0A, 'I', '0', ';', '1', ';', 0x0D, 0x5A, 0x51 };
  unsigned short c1 = Checksum(data1);
  unsigned short c2 = Checksum(data2);

// Checksum computation
unsigned short Checksum(unsigned char*)
    unsigned char TR = 0;
    unsigned short checksum = 0;
    unsigned char &S0 = *((unsigned char*)&checksum);
    unsigned char &S1 = *(&S0+1);

    for ( int ii = 1; TR != 0x0d; ii++ )
        TR = data[ii];
        checksum = checksum ^ ((unsigned short)TR << 8);
        for ( int jj = 0; jj < 8; jj++ )
            if ( (checksum & 0x8000) != 0 )
                checksum = (checksum << 1) ^ 0x8001;
                checksum <<= 1;
    return checksum;
ScreenConnect 6.0 Free Trial

Want empowering updates? You're in the right place! Discover new features in ScreenConnect 6.0, based on partner feedback, to keep you business operating smoothly and optimally (the way it should be). Explore all of the extras and enhancements for yourself!


Author Comment

ID: 11886349
Really don't know, but here's one more examples:
And it returns F8CC instead of F84C. Interestly, 0x80 again ?
LVL 19

Expert Comment

ID: 11887706
Seems very strange that after that many shifts and XOR's it would be off by only 1 bit.  I must be missing something.

Author Comment

ID: 11889995
Maybe it's just an extra processing which isn't explained ?
I did this and it worked for these test samples:

if (Checksum AND $80)<>0 then
Checksum:=Checksum XOR $80 else
Checksum:=Checksum XOR $8000;

Thank you anyway

Expert Comment

ID: 12044860
As for me, _this_ works fine.

function TForm1.CheckSum(s: string): word;
var tr,z,x : byte;
_checksum : word;
_s0,_s1 : ^byte;
_s1:=pointer(integer(@_checksum) + 1);
for z:=1 to length(s) do begin
        _checksum:=_checksum xor ( tr shl 8 );
        for x:= 0 to 7 do begin
                if _checksum and $8000 <> 0 then
                _checksum:=(_checksum shl 1) xor $8001
                _checksum := _checksum shl 1;
{ /// DON'T need ??

                if (_Checksum AND $80)<>0 then
                _Checksum:=_Checksum XOR $80 else
                _Checksum:=_Checksum XOR $8000;    }

seems works ok !

Featured Post

PRTG Network Monitor: Intuitive Network Monitoring

Network Monitoring is essential to ensure that computer systems and network devices are running. Use PRTG to monitor LANs, servers, websites, applications and devices, bandwidth, virtual environments, remote systems, IoT, and many more. PRTG is easy to set up & use.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

SSL stands for “Secure Sockets Layer” and an SSL certificate is a critical component to keeping your website safe, secured, and compliant. Any ecommerce website must have an SSL certificate to ensure the safe handling of sensitive information like…
Although it can be difficult to imagine, someday your child will have a career of his or her own. He or she will likely start a family, buy a home and start having their own children. So, while being a kid is still extremely important, it’s also …
In this fourth video of the Xpdf series, we discuss and demonstrate the PDFinfo utility, which retrieves the contents of a PDF's Info Dictionary, as well as some other information, including the page count. We show how to isolate the page count in a…

825 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question