How to calculate this CRC using Python?

Question

I need to calculate this CRC using Python for the communication with Aurora (ABB) solar inverter.

This is the document: http://www.drhack.it/images/PDF/AuroraCommunicationProtocol_4_2.pdf in the last page there are the instructions to calculate the CRC, i need to do that in python.

The message that i have is

MESSAGE_GRID_VOLTAGE = bytes.fromhex("023b010000000000")

The results should be:

CRC_L = FF

CRC_H = 2C

Then i need to send the message complete with the CRC like this:

MESSAGE_GRID_VOLTAGE = bytes.fromhex("023b010000000000ff2c")

How can i do that in python? Thanks!

Here is the code that i tried:

message = "023b010000000000"

BccLo= int ("FF",16)
BccHi= int("FF", 16)

New = int(message, 16)

New = New ^ BccLo
Tmp=New << 4
New=Tmp ^ New
Tmp=New >> 5
BccLo=BccHi
BccHi= New ^ Tmp
Tmp=New << 3
BccLo=BccLo ^ Tmp
Tmp=New >> 4
BccLo=BccLo ^ Tmp

CRC_L = ~BccLo
CRC_H = ~BccHi

Answer 1

According to the cited document, the algorithm is actually a standard 16 Bit CCITT CRC. This can be calculated with crcmod.

Here you go:

import crcmod

# this is a standard CCITT CRC even if it does not look like
# (crcmod applies xorOut to initCrc, so initCrc is in reality 0xffff, not 0)
_CRC_FUNC = crcmod.mkCrcFun(0x11021, initCrc=0, xorOut=0xffff)

data = bytearray.fromhex("023b010000000000")
crc = _CRC_FUNC(data)
data.append(crc & 0xff)
data.append(((crc >> 8) & 0xff))

print (data.hex())

Output: 023b010000000000ff2c

Answer 2

You need to apply that algorithm to each byte of your message. A slight complication is that the algorithm given in the Aurora PDF file assumes the calculation is being performed with 8 bit unsigned arithmetic. To handle that in Python we can use a bitmask of 0xff. Here's a slightly optimized version of that code.

def crc_16(msg):
    lo = hi = 0xff
    mask = 0xff
    for new in msg:
        new ^= lo
        new ^= (new << 4) & mask
        tmp = new >> 5
        lo = hi
        hi = new ^ tmp
        lo ^= (new << 3) & mask
        lo ^= new >> 4
    lo ^= mask
    hi ^= mask
    return hi << 8 | lo

# Test

msg = bytes.fromhex("023b010000000000")
out = crc_16(msg)
hi, lo = out >> 8, out & 0xff
print('{:04x} = {:02x} {:02x}'.format(out, hi, lo))

output

2cff = 2c ff

---

The above code works, but there are simpler ways to calculate CRCs. And we can use a table to speed up the process, if you need to calculate a lot of CRCs.

As the Wikipedia Cyclic redundancy check article mentions, CRC algorithms are usually specified in terms of a polynomial encoded as a hexadecimal number. Here's a function that does that using the reversed polynomial representation.

def crc_16_CCITT(msg):
    poly = 0x8408
    crc = 0xffff
    for byte in msg:
        for _ in range(8):
            if (byte ^ crc) & 1:
                crc = (crc >> 1) ^ poly
            else:
                crc >>= 1
            byte >>= 1
    return crc ^ 0xffff

To speed things up, we can compute a table.

def make_crc_table():
    poly = 0x8408
    table = []
    for byte in range(256):
        crc = 0
        for bit in range(8):
            if (byte ^ crc) & 1:
                crc = (crc >> 1) ^ poly
            else:
                crc >>= 1
            byte >>= 1
        table.append(crc)
    return table

table = make_crc_table()

def crc_16_fast(msg):
    crc = 0xffff
    for byte in msg:
        crc = table[(byte ^ crc) & 0xff] ^ (crc >> 8)
    return crc ^ 0xffff

# Test

msg = bytes.fromhex("023b010000000000")
out = crc_16_fast(msg)
hi, lo = out >> 8, out & 0xff
print('{:04x} = {:02x} {:02x}'.format(out, hi, lo))

If you like, you can print the table & paste it into your script, so that you don't have to compute the table every time you run the script.