CNAOB-Communications-Protocol-Notes: Difference between revisions

From *** My Personal Wiki ***
Jump to navigation Jump to search
Embeddedrf (talk | contribs)
1,247 edits
New page: == CRC CheckSum Algorithm == 4) 16-bit CRC verification code With the verification code, themaster or slave device can check if it has received the right message. Errors may occur durin...
 
No edit summary
 
(18 intermediate revisions by one other user not shown)
Line 1: Line 1:


== CRC CheckSum Algorithm ==
== Comm Port Binary Mode ==
 
Is necessary to use binary mode.  Serial port in ASCII mode chokes on null = chr(0) chars
 
*[http://www.codenewsgroups.net/group/microsoft.public.vb.general.discussion/topic6374.aspx Serial port return handling of binary data into array]
*[http://www.di-mgt.com.au/bytearrays.html extensive binary byte array in lieu of string functions handling page]
 
 
 
 
 
== Links for CRC CheckSum Algorithm ==
 
*[http://iatips.com/crc_vb.txt CRC calculation source code in VB using array lookup] - [[ModBus-CRC-Local-Version-routine-v1]]
*[http://www.planet-source-code.com/vb/scripts/ShowCode.asp?txtCodeId=6329&lngWId=10  CRC calculation source code in VB using all calculation] - [[ModBus-CRC-Local-Version-routine-2]]
 
 
== 16-bit CRC verification code Algorithm ==




4) 16-bit CRC verification code
With the verification code, themaster or slave device can check if it has received the right message. Errors may occur during the
transmission process of message, due to electronic noise or some other disturbance. The verification code can be used to check
whether the communication message in master/slave transmission has errors or not.
The 16-bit CRC verification code, placed at the end of the message frame being delivered, is calculated by the device which sends
the message. The message-receiving device will recalculate CRC of the receivedmessage to compare with the received CRC. CRC
inconsistency indicates errors.
Only 8 data bits are involved in CRC calculation, with the exclusion of start bit and end bit.
Only 8 data bits are involved in CRC calculation, with the exclusion of start bit and end bit.
Algorithmof CRC code:
Algorithm of CRC code:
1) Presetting a 16-bit register to hex FFFF (namely 1 for all bits in binary system). The register is called CRC register;
*1) Presetting a 16-bit register to hex FFFF. The register is called CRC register
2) XORing the first 8-bit binary data (the first byte of the communication message frame) with the low 8-bit of 16-bit CRC register,
 
then storing the result in
*2) XORing the first byte of the communication message frame with the low 8-bit of 16-bit CRC register.....then storing the result in CRC register
CRC register;
 
3) Right-shifting the register data by one bit (towards lower bit) and filling the highest bit with 0, then checking the shift-out bit;
*3) Right-shifting the register data by one bit and filling the highest bit with 0, then checking the shift-out bit
4) If the shift-out bit is 0, repeat step 3 (right-shifting one more bit);
 
If the shift-out bit is1, XOR the CRC register data with polynomial A001 (1010 0000 0000 0001);
*4) If the shift-out bit is 0, repeat step 3 (right-shifting one more bit);
5) Repeating step 3 and step 4 until all of the 8-bit data have been processed after 8 right-shift operations;
If the shift-out bit is 1, XOR the CRC register data with polynomial A001 (1010 0000 0000 0001) =  A001
6) Repeating step 2 to step 5 to process the next byte of the communication message frame;
 
7) When calculation procedures of the first 5 bytes in the communication message frame are completed, the 16-bit CRC verification
*5) Repeating step 3 and step 4 until all of the 8-bit data have been processed after 8 right-shift operations;
 
*6) Repeating step 2 to step 5 to process the next byte of the communication message frame;
 
*7) When calculation procedures of the first 5 bytes in the communication message frame are completed, the 16-bit CRC verification
code will be generated in the 16-bit CRC register.
code will be generated in the 16-bit CRC register.
== CRC Example Calculations: 1, 06, 40, 03,E8,  18, 22  Hex ==
*1111 1111 1111 1111 CRC
*0000 0000 0000 0001 Data 1
*----------------------------- XOR
*1111 1111 1111 1110 CRC
*0111 1111 1111 1111 CRC 32767      after right shift / shifted bit = 0
*0011 1111 1111  1111 CRC after right shift / shifted bit = 1
*1010 0000 0000  0001 polynomial = A001
*----------------------------- XOR
*1001111111111110 CRC
keep doing till 8 shifts accomplished

Latest revision as of 14:25, 30 July 2008

Comm Port Binary Mode[edit]

Is necessary to use binary mode. Serial port in ASCII mode chokes on null = chr(0) chars



Links for CRC CheckSum Algorithm[edit]


16-bit CRC verification code Algorithm[edit]

Only 8 data bits are involved in CRC calculation, with the exclusion of start bit and end bit. Algorithm of CRC code:

  • 1) Presetting a 16-bit register to hex FFFF. The register is called CRC register
  • 2) XORing the first byte of the communication message frame with the low 8-bit of 16-bit CRC register.....then storing the result in CRC register
  • 3) Right-shifting the register data by one bit and filling the highest bit with 0, then checking the shift-out bit
  • 4) If the shift-out bit is 0, repeat step 3 (right-shifting one more bit);

If the shift-out bit is 1, XOR the CRC register data with polynomial A001 (1010 0000 0000 0001) = A001

  • 5) Repeating step 3 and step 4 until all of the 8-bit data have been processed after 8 right-shift operations;
  • 6) Repeating step 2 to step 5 to process the next byte of the communication message frame;
  • 7) When calculation procedures of the first 5 bytes in the communication message frame are completed, the 16-bit CRC verification

code will be generated in the 16-bit CRC register.


CRC Example Calculations: 1, 06, 40, 03,E8, 18, 22 Hex[edit]

  • 1111 1111 1111 1111 CRC
  • 0000 0000 0000 0001 Data 1
  • ----------------------------- XOR
  • 1111 1111 1111 1110 CRC
  • 0111 1111 1111 1111 CRC 32767 after right shift / shifted bit = 0
  • 0011 1111 1111 1111 CRC after right shift / shifted bit = 1
  • 1010 0000 0000 0001 polynomial = A001
  • ----------------------------- XOR
  • 1001111111111110 CRC

keep doing till 8 shifts accomplished

Retrieved from "http://wiki.amarketplaceofideas.com/index.php?title=CNAOB-Communications-Protocol-Notes&oldid=2577"