flow_control: CRC documentation from modbus specifications

src/tools/flow_control.py

@@ -10,7 +10,14 @@ import time
 
 DEBUG = False
 
+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 
@@ -48,11 +55,6 @@ When the 16–bit CRC (two 8–bit bytes) is transmitted in the message, the low
     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):