CRC-8 checksum calculation

[Dirk Bubley]

Hello FC community,
at the moment it try to find my way to the CRC checksum operation.

In a RS232 serial byte transmission I have as an example the following byte sequence:
0x7E 0x9F 0x4C 0xA1 0xA8 0x5D 0x55 0x00 0x11 0xE8 0x23 0x21 0x1B 0x00 0xF4

0xF4 the last byte is a CRC checksum made with a polynomial X^8 + X^2 + X + 1 and the underlined byte group. From what I read and learned this polynomial should look like the following bit sequence: 100000111 (9Bits)

In the intranet I found the following CRC-8 checksum calculator: http://smbus.org/faq/crc8Applet.htm

If I drop in the above byte sequence in the CRC-8 calculator ( 4CA1A85D550011E823211B00 ) I get exactly 0xF4 as a result. 
If I work with only one byte and the CRC-8 calculator. For example I take the first underlined byte( 0x4C ) and put it in the calculator,
I get 0xE3 as a result. So far so good. From my knowledge this checksum is made with the following math. Starting with 0x4c = 0b01001100 = 0b1001100

100110000000000 “Adding 8 times zero from the a polynomial”
100000111 The polynomial (9Bit)
000110111000000 XOR result
>>>100000111 Again the polynomial
000010111111000 XOR result
>>>>100000111 Again the polynomial
>>>>00111100100 XOR result
>>>>>>100000111 Again the polynomial
>>>>>>011100011 XOR final result 0xE3!! Bingo!!

My problem now is how can I apply / create the above CRC checksum calculation / workflow in Flow Code. I´m using FC4. Every learning help is aprechiated.

Best regards Dirk

[Dirk Bubley]

...the underlined Byte Group is:

0x4C 0xA1 0xA8 0x5D 0x55 0x00 0x11 0xE8 0x23 0x21 0x1B 0x00

BR

Dirk

[Spanish_dude]

Code: Select all

// unsigned char rs232_byte, contains value 0x4C from RS232

unsigned long temp = 0;
unsigned long poly = 0x107;// 0001 0000 0111

temp = rs232_byte << 8; // adding 8 zeros
temp = temp ^ (poly << 7); // XOR 1
temp = temp ^ (poly << 4); // XOR 2
temp = temp ^ (poly << 3); // XOR 3
temp = temp ^ poly; // XOR 4
temp = temp & 0xFF; // clear unwanted bits

unsigned char CRC = temp;

Try this and let me know if it outputs the correct CRC.

- Nicolas

EDIT: Didn't see this post is a month old, sorry.

[Dirk Bubley]

Hello Nicolas,

many thanks for the Input.

Unfortunatly the calculation is giving me always 0xCF as a CRC result.
No matter what start value i have.

BR

Dirk

Note: The FC file is made with FC6