Re: [Tutor] Consistant Overhead Byte Stuffing (COBS) algorithm help
Michael Cotherman wrote:
> I am a noob to converting pointers in C++ to arrays in
> python, although the first time I see it done, I will
> have no problem. Can you help converting the below
> (what I think is the 'decoder' section) to python?
OK I'll bite. Your code is copying from a src buffer to a dst buffer. The src
buffer has a count byte followed by count-1 bytes of data. If the count is less
that 0xFF, a zero byte has to be inserted into the dst. So the decode loop gets
a count, copies that many bytes to the dst buffer, then optionally appends a 0.
Python doesn't have a direct equivalent to this sort of manipulation of memory
pointers and raw memory buffers. They are commonly replaced by strings or
lists. My function takes a string as an input argument, builds the output as
string fragments in a list, then builds a string to return.
Note that Python strings include an implicit length so there is no need to pass
and return a length argument.
I included a test case with the data from your previous email. Your data is
evidently hex-encoded as well as COBS encoded - in other words your strings are
hex values where each two chars represent a single byte. I have converted to
and from byte strings to feed this to the decoder.
Kent
PS Please send your replies to the whole list, not to me personally. Thanks!
def unstuff(src):
# src is a COBS compressed string
current = 0 # index into src
dst = []# a list that will build the result
while current < len(src):
# Get the count and convert it to an integer
count = ord(src[current])
# Append count-1 chars from src to dst
dst.append(src[current+1:current+count])
# Do we need to add a zero byte?
if count < 0xFF:
dst.append('\x00')
# Bump the counter and continue
if count>0:
current += count
else:
current += 1
# dst is a list of string fragments; this converts it to a single string
return ''.join(dst)
def hexToString(hexStr):
''' Convert a string of two-digit hex values to a string of bytes with
those values '''
return ''.join([chr(int(hexStr[i:i+2], 16)) for i in range(0, len(hexStr),
2)])
def stringToHex(src):
''' Convert a byte string to a string of two-digit hex values '''
return ''.join([ '%02x' % ord(s) for s in src ])
if __name__ == '__main__':
data = '0002860104DB203F0100'
print data
data = hexToString(data)
print
newData = unstuff(data)
print stringToHex(newData)
> UINT CCobsPackets::UnStuffData(unsigned char *src,
> unsigned char *dst, UINT length)
> {
> unsigned char *dstStart = dst;
> unsigned char *end = src + length;
>
> while (src < end)
> {
> int i, code = *src++;
>
> for (i=1; i {
> *dst++ = *src++;
> }
>
> if (code < 0xFF)
> {
> *dst++ = 0;
> }
> }
>
> return (UINT)(dst - dstStart);
> }
___
Tutor maillist - Tutor@python.org
http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Consistant Overhead Byte Stuffing (COBS) algorithm help
I'm not sure what the question is here. It looks like you need to write a COBS encoder / decoder in Python, maybe using your .NET code as a model. Then you can integrate that with comms code taken from miniterm. What help do you need? Kent Michael Cotherman wrote: > Hello, I am really new to python and really have not > programmed much since college, I played with it a > little now, and it seems to be a tool I feel > comfortable implementing a personal project in. > > I wish to communicate via a serial port to a device > that is using COBS. I wish to configure it and then > receive data at an interval and store it in a rrd. > The device itself receives telemetry information from > other devices, and this telemetry info is going to get > graphed and made available via a web page. > > The serial port will be com or tty, for which I > prepped by playing with pygarmin and miniterm. The > device is working and communicable? via a program > written in .NET by a friend of the friend who gave it > to me. The program has so many things I wish to change > that it would be easiest to start from scratch. I have > some of the source for bits and pieces that may be > needed. > > The data coming in/going out will be COBS encoded, > which changes/escapes all 0x00 bytes, then uses a 0x00 > byte for the framing. > > > COBS theory is explained here: > http://www.stuartcheshire.org/papers/COBSforToN.pdf > > and it looks like a version written in c is at: > http://gtk-gnutella.sourceforge.net/doxygen/cobs_8c.htm > > > I would initially be happy creating a cobs.py and then > modding the initial 1.1 release of miniterm and seeing > if I could listen to the device... The device will be > sending packets of 2-12 bytes at regular intervals > (99% will be 7 byte packets every minute or so), and I > can just move the serial cable over from the com port > with the working application to the cobs-miniterm one > to see if I am getting the right. > > Thanks in advance! > > -mike > clearwater, fl > > > > __ > Yahoo! Mail - PC Magazine Editors' Choice 2005 > http://mail.yahoo.com > ___ > Tutor maillist - Tutor@python.org > http://mail.python.org/mailman/listinfo/tutor > > ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Consistant Overhead Byte Stuffing (COBS) algorithm help
A little more info is below: With miniterm modified to output hex to the screen, here is the data coming in unformatted. (note zero bytes delimit end of packet): c:\Python23>python miniterm1.1a.py --- Miniterm --- type ESC to quit 0002860104DB203F0102860504CB1A740102860504CB1B740102860504CB1B740100 0002860504CB1B740102860504CB1B740102860504CB1B740102860504CB1B740100 0002860504CB1B740102860504CB1B740102860504CB1B740100 Here I have formatted it(one line equals a packet, currently about every 30 seconds): c:\Python23>python miniterm1.1a.py --- Miniterm --- type ESC to quit 00-02-86-05-04-CB-1B-74-01-00 00-02-86-05-04-CB-1B-74-01-00 00-02-86-01-04-DB-20-3E-01-00 00-02-86-05-04-CB-1B-74-01-00 00-02-86-05-04-CB-1B-74-01-00 00-02-86-05-04-CB-1B-74-01-00 00-02-86-05-04-CB-1B-74-01-00 00-02-86-05-04-CB-1B-74-01-00 00-02-86-05-04-CB-1B-74-01-00 00-02-86-05-04-CB-1B-74-01-00 00-02-86-05-04-CB-1B-74-01-00 00-02-86-05-04-CB-1B-74-01-00 here is what the data will look like after COBS decoding (the zero bytes are real data): 86-00-04-CB-1B-74-00 86-00-04-CB-1B-74-00 86-00-04-CB-1B-74-00 86-00-04-CB-1B-74-00 86-00-04-CB-1B-74-00 86-00-04-CB-1B-74-00 86-00-04-CB-1B-74-00 For those who are curious as to what I am doing, here is what telemetry is in the packet: packe type= 0x86 Device ID = 0x0004 Payload: Batt Volt.= 0xCB Pot Volt. = 0x1B Temp Volt.= 0x74 sw 1 = 0 sw 2 = 0 sw 3 = 0 sw 4 = 0 Btn 1 = 0 Btn 2 = 0 Yel LED = 0 Red LED = 0 Thanks, mike __ Yahoo! for Good Donate to the Hurricane Katrina relief effort. http://store.yahoo.com/redcross-donate3/ ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
[Tutor] Consistant Overhead Byte Stuffing (COBS) algorithm help
Hello, I am really new to python and really have not programmed much since college, I played with it a little now, and it seems to be a tool I feel comfortable implementing a personal project in. I wish to communicate via a serial port to a device that is using COBS. I wish to configure it and then receive data at an interval and store it in a rrd. The device itself receives telemetry information from other devices, and this telemetry info is going to get graphed and made available via a web page. The serial port will be com or tty, for which I prepped by playing with pygarmin and miniterm. The device is working and communicable? via a program written in .NET by a friend of the friend who gave it to me. The program has so many things I wish to change that it would be easiest to start from scratch. I have some of the source for bits and pieces that may be needed. The data coming in/going out will be COBS encoded, which changes/escapes all 0x00 bytes, then uses a 0x00 byte for the framing. COBS theory is explained here: http://www.stuartcheshire.org/papers/COBSforToN.pdf and it looks like a version written in c is at: http://gtk-gnutella.sourceforge.net/doxygen/cobs_8c.htm I would initially be happy creating a cobs.py and then modding the initial 1.1 release of miniterm and seeing if I could listen to the device... The device will be sending packets of 2-12 bytes at regular intervals (99% will be 7 byte packets every minute or so), and I can just move the serial cable over from the com port with the working application to the cobs-miniterm one to see if I am getting the right. Thanks in advance! -mike clearwater, fl __ Yahoo! Mail - PC Magazine Editors' Choice 2005 http://mail.yahoo.com ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
