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flow_control: CRC documentation from modbus specifications

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Markus Rosenstihl 2016-02-18 20:39:28 +00:00
parent 5f8f390434
commit e34103c367
1 changed files with 40 additions and 38 deletions
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src/tools/flow_control.py
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@ -10,49 +10,51 @@ import time
DEBUG = False
"""
6.2.2 CRC Generation
====================
The Cyclical Redundancy Checking (CRC) field is two bytes, containing a 16–bit binary value. The CRC value is calculated by the
transmitting device, which appends the CRC to the message. The device that receives recalculates a CRC during receipt of the
message, and compares the calculated value to the actual value it received in the CRC field. If the two values are not equal, an error
results.
The CRC is started by first preloading a 16–bit register to all 1’s. Then a process begins of applying successive 8–bit bytes of the
message to the current contents of the register. Only the eight bits of data in each character are used for generating the CRC. Start
and stop bits and the parity bit, do not apply to the CRC.
During generation of the CRC, each 8–bit character is exclusive ORed with the register contents. Then the result is shifted in the
direction of the least significant bit (LSB), with a zero filled into the most significant bit (MSB) position. The LSB is extracted and
examined. If the LSB was a 1, the register is then exclusive ORed with a preset, fixed value. If the LSB was a 0, no exclusive OR takes
place.
This process is repeated until eight shifts have been performed. After the last (eighth) shift, the next 8–bit character is exclusive ORed
with the register’s current value, and the process repeats for eight more shifts as described above. The final content of the register,
after all the characters of the message have been applied, is the CRC value.
A procedure for generating a CRC is:
1. Load a 16–bit register with FFFF hex (all 1’s). Call this the CRC register.
2. Exclusive OR the first 8–bit byte of the message with the low–order byte of the 16–bit CRC register, putting the result in the
CRC register.
3. Shift the CRC register one bit to the right (toward the LSB), zero–filling the MSB. Extract and examine the LSB.
4. (If the LSB was 0): Repeat Step 3 (another shift).
(If the LSB was 1): Exclusive OR the CRC register with the polynomial value 0xA001 (1010 0000 0000 0001).
5. Repeat Steps 3 and 4 until 8 shifts have been performed. When this is done, a complete 8–bit byte will have been
processed.
6. Repeat Steps 2 through 5 for the next 8–bit byte of the message. Continue doing this until all bytes have been processed.
7. The final content of the CRC register is the CRC value.
8. When the CRC is placed into the message, its upper and lower bytes must be swapped as described below.
Placing the CRC into the Message
When the 16–bit CRC (two 8–bit bytes) is transmitted in the message, the low-order byte will be transmitted first, followed by the high-
order byte.
"""
crc_test = "\x0d\x01\x00\x62\x00\x33"
crc_expected = 0xddd
def crc(message):
"""
(from "Modbus_over_serial_line_V1_02.pdf" at http://www.modbus.org)
6.2.2 CRC Generation
====================
The Cyclical Redundancy Checking (CRC) field is two bytes, containing a 16–bit binary value. The CRC value is calculated by the
transmitting device, which appends the CRC to the message. The device that receives recalculates a CRC during receipt of the
message, and compares the calculated value to the actual value it received in the CRC field. If the two values are not equal, an error
results.
The CRC is started by first preloading a 16–bit register to all 1’s. Then a process begins of applying successive 8–bit bytes of the
message to the current contents of the register. Only the eight bits of data in each character are used for generating the CRC. Start
and stop bits and the parity bit, do not apply to the CRC.
During generation of the CRC, each 8–bit character is exclusive ORed with the register contents. Then the result is shifted in the
direction of the least significant bit (LSB), with a zero filled into the most significant bit (MSB) position. The LSB is extracted and
examined. If the LSB was a 1, the register is then exclusive ORed with a preset, fixed value. If the LSB was a 0, no exclusive OR takes
place.
This process is repeated until eight shifts have been performed. After the last (eighth) shift, the next 8–bit character is exclusive ORed
with the register’s current value, and the process repeats for eight more shifts as described above. The final content of the register,
after all the characters of the message have been applied, is the CRC value.
A procedure for generating a CRC is:
1. Load a 16–bit register with FFFF hex (all 1’s). Call this the CRC register.
2. Exclusive OR the first 8–bit byte of the message with the low–order byte of the 16–bit CRC register, putting the result in the
CRC register.
3. Shift the CRC register one bit to the right (toward the LSB), zero–filling the MSB. Extract and examine the LSB.
4. (If the LSB was 0): Repeat Step 3 (another shift).
(If the LSB was 1): Exclusive OR the CRC register with the polynomial value 0xA001 (1010 0000 0000 0001).
5. Repeat Steps 3 and 4 until 8 shifts have been performed. When this is done, a complete 8–bit byte will have been
processed.
6. Repeat Steps 2 through 5 for the next 8–bit byte of the message. Continue doing this until all bytes have been processed.
7. The final content of the CRC register is the CRC value.
8. When the CRC is placed into the message, its upper and lower bytes must be swapped as described below.
Placing the CRC into the Message
When the 16–bit CRC (two 8–bit bytes) is transmitted in the message, the low-order byte will be transmitted first, followed by the high-
order byte.
"""
crc = 0xffff # step 1
for byte in message:
if type(byte) == type(str):