Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #44248 > unrolled thread

Fixing escaped characters python-xbee

Back to article view | Back to comp.lang.python

Contents

  Fixing escaped characters python-xbee pabloblo85@gmail.com - 2013-04-24 00:33 -0700
    Re: Fixing escaped characters python-xbee MRAB <python@mrabarnett.plus.com> - 2013-04-24 11:53 +0100
    Re: Fixing escaped characters python-xbee Peter Otten <__peter__@web.de> - 2013-04-24 13:49 +0200
    Re: Fixing escaped characters python-xbee Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-04-24 20:29 -0400
      Re: Fixing escaped characters python-xbee pabloblo85@gmail.com - 2013-04-26 01:17 -0700

#44248 — Fixing escaped characters python-xbee

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...

[toc] | [next] | [standalone]

#44253

On 24/04/2013 08:33, pabloblo85@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]

[toc] | [prev] | [next] | [standalone]

#44261

pabloblo85@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...

As you cannot change the old string you have to compose a new one. I think 
the simplest approach is to always read one byte, and if it's the escape 
marker read another one and decode it. The decoded bytes/chars are then 
stored in a list and finally joined:

# Python 2
# untested
def read_nbytes(ser, n):
    accu = []
    for i in xrange(n):
        b = ser.read(1)
        if b == "\x7d":
            b = chr(ord(ser.read(1)) ^ 0x20)
        accu.append(b)
    return "".join(accu)

b = read_nbytes(ser, 4)
data = struct.unpack('<f', b)[0]

[toc] | [prev] | [next] | [standalone]

#44307

On Wed, 24 Apr 2013 00:33:30 -0700 (PDT), pabloblo85@gmail.com declaimed
the following in gmane.comp.python.general:

> 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

	Why read 4 at a time if you need to detect the escape marker...

PSEUDO_CODE -- UNTESTED:

	for c in ser.read():	#presumes it will function as an iterator
		if ord(c) == 0x7D:
			c =chr(ord(ser.read(1)) ^ 0x20)
		#do something with c (save to a list for later joining as a
string?)
		#probably need some condition to exit the read loop too
> def logical_xor(str1, str2):
>     return bool(str1) ^ bool(str2)
>
	bool() returns True or False based on the argument... Any non-empty
string will be True. Instead what you want is to x-or the bits of the
character itself.
-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
        wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/

[toc] | [prev] | [next] | [standalone]

#44396

> 
> the following in gmane.comp.python.general:
> 
> 
> 
> > 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
> 
> 
> 
> 	Why read 4 at a time if you need to detect the escape marker...
> 
> 
> 
> PSEUDO_CODE -- UNTESTED:
> 
> 
> 
> 	for c in ser.read():	#presumes it will function as an iterator
> 
> 		if ord(c) == 0x7D:
> 
> 			c =chr(ord(ser.read(1)) ^ 0x20)
> 
> 		#do something with c (save to a list for later joining as a
> 
> string?)
> 
> 		#probably need some condition to exit the read loop too
> 
> > def logical_xor(str1, str2):
> 
> >     return bool(str1) ^ bool(str2)
> 
> >
> 
> 	bool() returns True or False based on the argument... Any non-empty
> 
> string will be True. Instead what you want is to x-or the bits of the
> 
> character itself.
> 
> -- 

It works! Thank you so much. Now I can go ahead with my work!

[toc] | [prev] | [standalone]

Back to top | Article view | comp.lang.python

csiph-web