# Checksum for cash register packet

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
LVL 5
###### Who is Participating?

Commented:
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;
}
else
{
checksum <<= 1;
}
}
}
return checksum;
}
0

Commented:
What do you need?
A program in some particular language to calculate S1 and S0?
0

Author Commented:
A scheme for calculating will be good enough.
And, yes, it's S0 and S1 (checksum byte 1 and 2).
0

Author Commented:
Really don't know, but here's one more examples:
0X0A,'0','0','5','6','9','0',';',0X0D,0XF8,0X4C
And it returns F8CC instead of F84C. Interestly, 0x80 again ?
0

Commented:
Seems very strange that after that many shifts and XOR's it would be off by only 1 bit.  I must be missing something.
0

Author Commented:
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
0

Commented:
As for me, _this_ works fine.

function TForm1.CheckSum(s: string): word;
var tr,z,x : byte;
_checksum : word;
_s0,_s1 : ^byte;
begin
tr:=0;
_checksum:=0;
_s0:=pointer(@_checksum);
_s1:=pointer(integer(@_checksum) + 1);
for z:=1 to length(s) do begin
tr:=ord(s[z]);
_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
else
_checksum := _checksum shl 1;
{ /// DON'T need ??

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

seems works ok !
0
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

All Courses

From novice to tech pro — start learning today.