Talking to a usb device (serial terminal) - Python

> So my question is this - what is the easiest way to interface to this
> "serial" device?
>

http://pyserial.sourceforge.net/

or perhaps

http://pyusb.berlios.de/

blaine wrote:

> So my question is this - what is the easiest way to interface to
> this "serial" device?
>
> I don't imagine a straight read() and write() command to
> /dev/ttyusb0 is the most efficient (if it even works)

It doesn't only work, it's the preferred way (if you don't use
advanced wrappers like pyserial). For the basics see

http://www.easysw.com/~mike/serial/serial.html

> especially since we would need to set a baud rate.

What is the relationship between read/write, the baud rate, and
efficiency? The serial port is configured using POSIX terminal
interface (termios).

Regards,


Björn

--
BOFH excuse #89:

Electromagnetic energy loss

On Mar 3, 12:31 pm, Bjoern Schliessmann <usenet-
mail-0306.20.chr0n...@spamgourmet.com> wrote:
> It doesn't only work, it's the preferred way (if you don't use
> advanced wrappers like pyserial). For the basics see
>
> http://www.easysw.com/~mike/serial/serial.html
[...]
> What is the relationship between read/write, the baud rate, and
> efficiency? The serial port is configured using POSIX terminal
> interface (termios).

Thank you so much for that reference link, it was very helpful. I now
understand the difference in the modules - termios and fctnl.

As far as PySerial goes - were trying to stick to built-in modules
since cross compiling to an arm processor is being a little bit of a
pain for us. Thank you for the suggestion though!

Thanks!
Blaine

On 2008-03-03, blaine <frikker@gmail.com> wrote:

> As far as PySerial goes - were trying to stick to built-in
> modules since cross compiling to an arm processor is being a
> little bit of a pain for us.

pyserial is pure python, so I don't see how it's going to be
any more painful than something you write yourself. If you
want something that's more of a transparent object wrapper
aroudn the Posix serial interface, there's PosixSerial.py (upon
which pyserial's posix support is based):

ftp://ftp.visi.com/users/grante/python/PosixSerial.py

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Grant Edwards grante Yow! Everybody gets free
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> pyserial is pure python, so I don't see how it's going to be
> any more painful than something you write yourself. If you
> want something that's more of a transparent object wrapper
> aroudn the Posix serial interface, there's PosixSerial.py (upon
> which pyserial's posix support is based):
>
> ftp://ftp.visi.com/users/grante/python/PosixSerial.py
>
> --
> Grant Edwards grante Yow! Everybody gets free
> at BORSCHT!
> visi.com

Oh - good point. There wouldn't be any compilation issues with the C
api. Nevermind!

I do have a question though. In the termios module, I am attempting
to set the baud rate to 921600, which is what we use with
'cutecom' (because minicom does not have this as an option) and is
what the supplier recommends. When I try to set the rate using
termios.tcsetattr(), I get an Argument Error. I do not get this when
I use an existing baud rate, such as termios.B9600. Is there a way I
can get around this? We would like to use it at full speed.

Thank you,
Blaine

On 2008-03-03, blaine <frikker@gmail.com> wrote:

> I do have a question though. In the termios module, I am attempting
> to set the baud rate to 921600, which is what we use with
> 'cutecom' (because minicom does not have this as an option) and is
> what the supplier recommends. When I try to set the rate using
> termios.tcsetattr(), I get an Argument Error.

What values are you using for ispeed and ospeed? Those values
are special (and rather arbitrary) constant bit-patterns and
not simply integers like 9600 or 115200.

> I do not get this when I use an existing baud rate, such as
> termios.B9600. Is there a way I can get around this? We would
> like to use it at full speed.

It looks like the fastest speed supported by python termios on
Linux is B460800 (uses a constant of 0x1004). If you look in
/usr/include/..., baud rates do go up to 921600 (which uses a
constant of 0x1007).

Try using the appropriate constant from /usr/include/... (for
the target platform, of course).

--
Grant Edwards grante Yow! Please come home with
at me ... I have Tylenol!!
visi.com

>
> It looks like the fastest speed supported by python termios on
> Linux is B460800 (uses a constant of 0x1004). If you look in
> /usr/include/..., baud rates do go up to 921600 (which uses a
> constant of 0x1007).
>
> Try using the appropriate constant from /usr/include/... (for
> the target platform, of course).
>
> --
> Grant Edwards grante Yow! Please come home with
> at me ... I have Tylenol!!
> visi.com
Thank you for your response. I can't seem to find what you're
referencing in /usr/include. I found termios.h - but it does not
define any constants (I can include it if you wish). I did find
termios.c in the Python module file, which only defines constants up
to :
[...]
{"B2400", B2400},
{"B4800", B4800},
{"B9600", B9600},
{"B19200", B19200},
{"B38400", B38400},
#ifdef B57600
{"B57600", B57600},
#endif
#ifdef B115200
{"B115200", B115200},
#endif
#ifdef B230400
{"B230400", B230400},

I could recompile python, I imagine, with additional constants but
this wouldn't be ideal - although it is a possible solution.

the only reference to a baud rate in termios.h is here:
/* Return the output baud rate stored in *TERMIOS_P. */
extern speed_t cfgetospeed (__const struct termios *__termios_p)
__THROW;

/* Return the input baud rate stored in *TERMIOS_P. */
extern speed_t cfgetispeed (__const struct termios *__termios_p)
__THROW;

/* Set the output baud rate stored in *TERMIOS_P to SPEED. */
extern int cfsetospeed (struct termios *__termios_p, speed_t __speed)
__THROW;

/* Set the input baud rate stored in *TERMIOS_P to SPEED. */
extern int cfsetispeed (struct termios *__termios_p, speed_t __speed)
__THROW;

the termios.h and const.h in the linux/ folder aren't much help. I'm
sure I'm just looking at the wrong file.

Thank you again for all your help!
Blaine

On 2008-03-04, blaine <frikker@gmail.com> wrote:

>> Try using the appropriate constant from /usr/include/... (for
>> the target platform, of course).
>>
>> --
>> Grant Edwards grante Yow! Please come home with
>> at me ... I have Tylenol!!
>> visi.com
> Thank you for your response. I can't seem to find what you're
> referencing in /usr/include. I found termios.h - but it does not
> define any constants (I can include it if you wish).

$ grep -r B921600 /usr/include
/usr/include/asm/termbits.h:#define B921600 0010007
/usr/include/bits/termios.h:#define B921600 0010007

> I could recompile python, I imagine, with additional constants but
> this wouldn't be ideal - although it is a possible solution.

termios.c in Python definitely needs to be updated.

> the only reference to a baud rate in termios.h is here:

It's in a file that's included in termios.h. See above.

--
Grant Edwards grante Yow! Here I am at the flea
at market but nobody is buying
visi.com my urine sample bottles ...

> It looks like the fastest speed supported by python termios on
> Linux is B460800 (uses a constant of 0x1004). If you look in
> /usr/include/..., baud rates do go up to 921600 (which uses a
> constant of 0x1007).
>
> Try using the appropriate constant from /usr/include/... (for
> the target platform, of course).
>
> --
> Grant Edwards grante Yow! Please come home with
> at me ... I have Tylenol!!
> visi.com

I want to thank you SO MUCH for all your help. Here are my issues
that I overcame (sanity check):
1. Why did we have to use 0x1007, instead of 0x10007 that our grep
command returns?
2. PySerial works beautifully. Thank you for the suggestion. What I
had to do was add this to the PySerial source root in serialpostix.py,
after the import termios:

termios.B921600 = 0x1007

because PySerial looks for the actual baud rate in termios (via
getattr()) which does not exist. PySerial actually defines the
following baud rates, but termios does not handle it:
#default values, may be overriden in subclasses that do not
support all values
BAUDRATES =
(50,75,110,134,150,200,300,600,1200,1800,2400,4800,9600,

19200,38400,57600,115200,230400,460800,500000,576000,921600,

1000000,1152000,1500000,2000000,2500000,3000000,3500000,4000000)
.... so now I can pass 921600 as a parameter to PySerial!

So my next question is - I would like to contribute to the python
source tree by updating termios.h to handle the higher baud rates by
default. Is this a good opportunity for me to submit a patch? I've
never done this before but have always been interested in doing so.

Thanks again!
Blaine