[mdlug] Serial port monitoring

[Aaron Kulkis]

Drew wrote:
> At 10:24 PM 1/31/08, Robert Meier wrote:
> 
>> The underlying hardware, if access is supported,
>> should be available via ioctl(2).
>> Start by looking at tty_ioctl(4), including TIOCM_CD.
>> It is probable you will have to load some additional
>> CONFIG_SERIAL_8250 kernel modules.
>> It is possible you will have to recompile your kernel,
>> as IIRC tty_ioctl support is no longer standard.
>>
>> To interpret much of the above man pages you will need to download and
>> understand the spec sheet for the 8251 Universal Asynchronous Receiver
>> Transmitter that is the canonical hardware implementation of RS-232.
> 
>      I'm looking at the man page for tty_ioctl(4), from which it 
> looks like I need
> to write a C program or something which can #include and call the function,
> executing "ioctl(fd, TIOCMGET, &modeminfo);" then "modeminfo &= TIOCM_CAR;"
> 
>      Most of the info that I need (eg, values for TIOCMGET and TIOCM_CAR)
> should be in the header files, and I could theoretically write the 
> program without
> ever knowing what these constants are.
> 
>      Finding a value for fd (the serial port handle) is a bit more 
> problematic: For
> all TRUE returns, it is anticipated (in fact part of the definition 
> of a valid TRUE
> return) that the serial port will be in use, grabbed by VMware. I found that
> PalmOS allows multiple programs to open the (same) serial port; but 
> I'm not sure
> about whether Linux does.
> 
> 
>>>> * If there is no actual modem, but just a straight serial connection
>>>>    using pins 2, 3, and ground, can reading Carrier Detect in that
>>>>    manner be relied upon to return an OFF status?
>> No.
> 
>      Then I need to find some other way to determine whether the serial
> port is connected *and* refrain from using it from VMs for any serial work
> (eg, Palm apps)...
> 
> 
> 
>> If your goal is security,
>> then you might consider wiring the carrier detect line to a parallel
>> port pin to monitor it.
> 
> 
>      I might look into the parallel pin method a bit more anyway, but my goal
> doesn't have anything to do with security.
> 
>      What I'm after is something that will automatically keep count of how many
> minutes I've been connected to Netzero dialup during the month, without my
> having to remember to do anything special before starting it up. I only get 10
> hours a month, and they don't seem willing to provide a way to determine how
> much time is left.

YOu can get DSL for like $13 a month now through Verizon/AOL.