On 24/04/2013 08:33, pablobl...@gmail.com wrote:
I am using a XBee to receive data from an arduino network.
But they have AP=2 which means escaped characters are used when a 11 or 13
appears (and some more...)
When this occurs, XBee sends 7D and inmediatly XOR operation with char and 0x20.
I am trying to recover the original character in python but I don't know ho to
do it.
I tried something like this:
read = ser.read(4) #Read 4 chars from serial port
for x in range (0,4):
if(toHex(read[x]) != '7d'): #toHex converts it to hexadecimal
just for checking purposes
if(x < 3):
read[x] = logical_xor(read[x+1], 20) #XOR
for y in range (x+1,3):
read[y] = read[y+1]
read[3] = ser.read()
else:
read[x] = logical_xor(ser.read(), 20) #XOR
data = struct.unpack('<f', read)[0]
logical_xor is:
def logical_xor(str1, str2):
return bool(str1) ^ bool(str2)
I check if 7D character is in the first 3 chars read, I use the next char to
convert it, if it is the 4th, I read another one.
But I read in python strings are inmutables and I can't change their value once
they have one.
What would you do in this case? I started some days ago with python and I don't
know how to solve this kind of things...
You could try converting to a list, which is mutable.
# Python 3
read = list(ser.read(4))
pos = 0
try:
while True:
pos = read.index(0x7D, pos)
del read[pos]
read.extend(ser.read())
read[pos] ^= 0x20
except ValueError:
# There are no (more) 0x7D in the data.
pass
data = struct.unpack('<f', bytes(read))[0]
# Python 2
read = [ord(c) for c in ser.read(4)]
pos = 0
try:
while True:
pos = read.index(0x7D, pos)
del read[pos]
read.append(ord(ser.read()))
read[pos] ^= 0x20
except ValueError:
# There are no (more) 0x7D in the data.
pass
data = struct.unpack('<f', b"".join(chr(c) for c in read))[0]
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