crc16 help

[beto]

Dear Group!

I have a device I'm trying to connect to, not very successfully... I
need to compute a 16bit CRC and would greatly appreciate some help. I
have an example written in C, and would like some help translating it to
PBP. This is the only information I have on the CRC. Any help would be
appreciated


Thanks for any help.

/************************************************** *********************
********
* crc16
*
* DESCRIPTION:
* This function computes a 16 bit CRC value for an array of bytes as
specified
* . The value that is returned by this
* function needs to be inverted before it gets placed into the packet.
The CRC
* goes at the end of the packet, low byte first. For example:
*
* crc = ~crc16(0xFFFF, msgbuf, length);
*
*
* INPUTS:
* crc - the value to start computing the CRC with. This allows for
chaining
* together of CRC values computed for multiple arrays of data. The
* initial value should be FFFFh (#defined as CRC_INIT).
* ptr - the pointer to the array of bytes to compute the CRC for
* length - the length of the data array
*
* OUTPUTS:
* returns - the computed 16 bit crc value ****/
#define unit16_t unsigned short
#define unit8_t unsigned char

uint16_t crc16(uint16_t crc, uint8_t *ptr, int length)
{
auto uint16_t i;
while(length--)
{
crc = crc ^ (uint16_t) *ptr++;
for(i=0;i<8;i++)
{
if(crc & 0x0001)
crc = (crc >> 1) ^ 0x8408;
else
crc >>= 1;
}
}
return crc;
}

And this is my ripped code in pbp:

for j = 0 to lenght
crc=crc^(send_data[j]*$ffff)
for i=0 to 7
if (crc & $0001) then
crc=(crc>>1)^$8408
else
crc=crc>>1
endif
next i
next j
return

but doesn't work

[Salutatous]

http://www.picbasic.co.uk/forum/showthread.php?t=316&highlight=rs485

[beto]

your link has lot of info (more to confuse me), I found a lot of info and ways to do CRC, Witch is correct?
CRC-16 0xBB3D
CRC-16 (Sick) 0x56A6
CRC-CCITT (0x0000) 0x31C3
CRC-CCITT (0xFFFF) 0x29B1
CRC-CCITT (0x1D0F) 0xE5CC
CRC-DNP 0x82EA
CRC-32 0xCBF43926
these are CRC results from the SAME string "123456789" look at:
http://www.lammertbies.nl/comm/info/crc-calculation.html

[Salutatous]

i use this in a modbus network and its work, try differents if you have pb!
good luck
..................
GoSub Calcul_CRC16
BufTx[Nbr-2]=CRC16.LowByte
BufTx[Nbr-1]=CRC16.HighByte
SerOut2 InOut,BR,[STR BufTx\Nbr]
..................
Calcul_CRC16:
CRC16=$FFFF
For i=0 to Nbr-3
CRC16=CRC16^BufTx[i]
For j=1 to 8
IF CRC16.Bit0=1 Then
CRC16=$A001^(CRC16>>1)
Else
CRC16=CRC16>>1
EndIF
Next j
Next i
Return

[beto]

Tnx Salutatous I try w/other values! because there is no more info about CRC