1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
|
""" CRC is calculated over slave id + PDU.
Most code is taken from: https://github.com/pyhys/minimalmodbus/blob/e99f4d74c83258c6039073082955ac9bed3f2155/minimalmodbus.py # NOQA
"""
import struct
def generate_look_up_table():
""" Generate look up table.
:return: List
"""
poly = 0xA001
table = []
for index in range(256):
data = index << 1
crc = 0
for _ in range(8, 0, -1):
data >>= 1
if (data ^ crc) & 0x0001:
crc = (crc >> 1) ^ poly
else:
crc >>= 1
table.append(crc)
return table
look_up_table = generate_look_up_table()
def get_crc(msg):
""" Return CRC of 2 byte for message.
>>> assert get_crc(b'\x02\x07') == struct.unpack('<H', b'\x41\x12')
:param msg: A byte array.
:return: Byte array of 2 bytes.
"""
register = 0xFFFF
for byte_ in msg:
try:
val = struct.unpack('<B', byte_)[0]
# Iterating over a bit-like objects in Python 3 gets you ints.
# Because fuck logic.
except TypeError:
val = byte_
register = \
(register >> 8) ^ look_up_table[(register ^ val) & 0xFF]
# CRC is little-endian!
return struct.pack('<H', register)
def add_crc(msg):
""" Append CRC to message.
:param msg: A byte array.
:return: Byte array.
"""
return msg + get_crc(msg)
def validate_crc(msg):
""" Validate CRC of message.
:param msg: Byte array with message with CRC.
:raise: CRCError.
"""
if not struct.unpack('<H', get_crc(msg[:-2])) ==\
struct.unpack('<H', msg[-2:]):
raise CRCError('CRC validation failed.')
class CRCError(Exception):
""" Valid error to raise when CRC isn't correct. """
pass
|
