flow_control: CRC documentation from modbus specifications

src/tools/flow_control.py

@@ -10,26 +10,33 @@ 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.
+crc_test = "\x0d\x01\x00\x62\x00\x33"
+crc_expected = 0xddd
 
-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: 
+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 
@@ -43,16 +50,11 @@ A procedure for generating a CRC is:
      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. 
-"""
+    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):
     crc = 0xffff  # step 1
     for byte in message:
         if type(byte) == type(str):