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Full Discussion: Serial Lines Explained
Top Forums UNIX for Dummies Questions & Answers Serial Lines Explained Post 302912183 by Corona688 on Wednesday 6th of August 2014 11:56:56 AM
Old 08-06-2014
That's only half the story though. Serial ports in UNIX aren't just telecommunications, they're session control, job control, raw interaction.

I believe this all stems from the way terminals used to be used... You would login through a terminal, and that terminal would belong to you. Any processes you created would know what your terminal was(it would be your "controlling termina"). The kernel would know which processes were allowed to control it(foreground processes) or not allowed (background processes). Hitting ctrl-C would send SIGINT to processes belonging to your terminal. When you logged out, the group of processes belonging to that terminal would be killed.

Logging in through a remote teletype was almost the same, since it was an extension of the serial port. Modems had a few more signals, UNIX would know when a modem hung up for instance.

Having a "controlling terminal" does several things. For one thing, it means that all programs you run will be able to find out what your terminal is(they inherit their controlling terminal from you), and get direct access to it if they want it (by opening /dev/tty). This means that, even when you bury ssh in a 9-deep pipe chain, it can still go directly to the source and ask for your password. Or if /dev/tty can't be opened, they know there's no human there to ask for a password and just give up. If you ever see an error like "No tty present and no askpass program specified", that's what it means.
Last edited by Corona688; 08-06-2014 at 01:08 PM..
 

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LEARN ABOUT OSF1

iscc

iscc(7) 						 Miscellaneous Information Manual						   iscc(7)

NAME

       iscc - serial communications interface

SYNOPSIS

       controller  iscc0  at  isa0

DESCRIPTION

       An  onboard  Zilog  8530  DUART	provides  AXPvme-based Single Board Computers with console (CON) and auxiliary (AUX) serial communications
       ports.  Both serial communications ports can operate at baud rates from B50 to B38400,  excluding B134 (see tty(7)).

       On AXPvme Single Board Computers configured with a graphics head as the system console, the CON serial communications port (/dev/tty01)	is
       available for use by application programs as an additional communications port.

       In  the	absence  of a graphics head, the CON serial communications port (/dev/console) becomes the system console. When used as the serial
       system console, the CON serial communications port always operates at 9600 BPS, 8-bits, and no parity.

       Regardless of system console, the AUX serial communications port (/dev/tty00) is always available for use by application programs.

RESTRICTIONS

       Data communications equipment attached to the CON serial communications port must be set to 9600 BPS, 8-bits, no parity when configured	as
       the serial system console. The iscc terminal driver enforces this restriction.

       No modem control is supported for either the CON or AUX serial communications ports.

FILES

       console terminal local terminal local terminal

RELATED INFORMATION

       console(7), devio(7), tty(7), MAKEDEV(8) delim off

																	   iscc(7)
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