Carrier Detection in FreeBSD Post: 302820975

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Operating Systems BSD Carrier Detection in FreeBSD Post 302820975 by rstor on Thursday 13th of June 2013 05:54:00 PM

06-13-2013

Registered User

Carrier Detection in FreeBSD

Regarding carrier detection on FreeBSD 9.1 being run within Virtual Box:

I used a usb-serial dongle which shows up as ttyu0 on the BSD machine. Carrier detection appears to work. When I establish a null modem connection I receive a login prompt. If I login, and then unplug and plug back the cable, I get the login prompt again and the old session does not continue. This is the correct behaviour to my understanding however I am curious of the way the getty process behaves, esp. on bootup in FreeBSD.

Question:

The line in my /etc/ttys file is as follows:

Code:

ttyu0   "/usr/libexec/getty std.300"   dialup on

I then issued the command "kill -HUP 1" and restarted the system with nothing plugged into the serial port.

Reading the documentation online, it is my understanding that when the system starts up and no carrier is detected, when issuing the ps ax command, the result I am supposed to see should be something as follows:

Code:

  114 ??  I      0:00.10 /usr/libexec/getty std.300 ttyu0

However I instead see a similar output to the above but the question marks are replaced by letters. Per the documentation online it indicates that if something like this is displayed it means that "getty has completed its open on the communications port. This could indicate a problem with the cabling or a misconfigured modem, because getty should not be able to open the communications port until carrier detect has been asserted by the modem."

I figured that since nothing is connected into the serial port, perhaps the voltage levels may be floating, instead of being held at the negative voltage which would indicate no carrier. I therefore applied a negative voltage to the CD line and restarted the system. Upon restart I issuing the ps -ax command and the result was as before, letters instead of the question marks. I then manually applied a positive voltage to the CD line and then a negative voltage again. I had to do it a couple times however eventually it showed up with question marks. I then was able to toggle back and forth, positive voltage resulted in letters, negative voltage on CD line resulted in question marks.

Why is it that when a negative voltage is applied to the CD line, upon system start-up, getty indicates via the ps -ax command that it has opened the port? [i.e. letters appear instead of question marks] I would expect that with a negative voltage applied to the CD line, upon restart it would show up with ??

I need to toggle the voltage between negative to positive and then back to negative (sometimes I need to do it a couple times) for it to eventually show up with ?? and then it would toggle back and forth as I switch the voltages thereafter.

Last edited by Scott; 06-13-2013 at 07:06 PM.. Reason: Please use code tags for code, data and terminal output

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getty

getty(1M)						  System Administration Commands						 getty(1M)

NAME

       getty - set terminal type, modes, speed, and line discipline

SYNOPSIS

       /usr/lib/saf/ttymon [-h] [-t timeout] line [ speed [ type [linedisc]]]

       /usr/lib/saf/ttymon -c file

DESCRIPTION

       getty  sets terminal type, modes, speed, and line discipline. getty is a symbolic link to  /usr/lib/saf/ttymon. It is included for compati-
       bility with previous releases  for the few applications that still call getty directly.

       getty can only be executed by the super-user, (a process with the user  ID root). Initially getty prints the login prompt,  waits  for  the
       user's  login  name,  and then invokes the login command. getty attempts to adapt the system to the terminal speed by using the options and
       arguments specified on the command line.

       Without optional arguments, getty specifies the following: The  speed of the interface is set to 300 baud, either parity is  allowed,  NEW-
       LINE  characters  are  converted to carriage return-line feed, and tab expansion is performed on the standard output. getty types the login
       prompt before reading the user's name a character at a time. If a null character (or framing error)  is received, it is assumed to  be  the
       result  of  the user pressing the  BREAK key. This will cause getty to attempt the next speed in the series. The series that getty tries is
       determined by what it finds in /etc/ttydefs .

OPTIONS

       The following options are supported:

       -h	       If the -h flag is not set, a hangup will be forced by setting the speed to zero before setting the speed to the default	or
		       a specified speed.

       -t timeout      Specifies that getty should exit if the open on the line succeeds and no one types anything in  timeout seconds.

       -c file	       The -c option is no longer supported. Instead use  /usr/sbin/sttydefs -l to list the contents of the  /etc/ttydefs file and
		       perform a validity check on the file.

OPERANDS

       The following operands are supported:

       line		       The name of a  TTY line in  /dev to which getty is to attach itself. getty uses this string as the name of  a  file
			       in the /dev directory to open for reading and writing.

       speed		       The  speed argument is a label to a speed and  TTY definition in the file /etc/ttydefs. This definition tells getty
			       at what speed to run initially,	what the initial TTY settings are, and what speed to try next,	(should  the  user
			       press the  BREAK key to indicate that the speed is inappropriate). The default speed is 300 baud.

       type and linedisc       These options are obsolete and will be ignored.

FILES

       /etc/ttydefs

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWcsr			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       ct(1C), login(1), sttydefs(1M), ttymon(1M), ioctl(2), attributes(5), tty(7D)

SunOS 5.10							    14 Sep 1992 							 getty(1M)