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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 ULTRIX

dc

dc(4)							     Kernel Interfaces Manual							     dc(4)

Name
       dc - serial line/mouse/keyboard

Syntax
       device	 dc0  at ibus?	vector dcintr

Description
       The  serial line controller provides four ports, with modem control on two of the ports.  The DECstation 3100 and DECstation 2100 only pro-
       vide partial modem control.  The DECstation 5000 provides full modem control. The ports are used as follows:

       Port	 Usage
       0	 Graphics device keyboard at 4800 BPS
       1	 Mouse or tablet at 4800 BPS
       2	 Communications port 1 (w/modem control)/local terminal
       3	 Communications port 2 (w/modem control)/local terminal

       Each communication port from the serial line controller behaves as described in and can be set to run at any of 16 speeds.  For the  encod-
       ing, see

       When  a	graphics device is not being used as the system console, communications port 2 becomes the system console.  In this configuration,
       the port can only be used at 9600 BPS and no modem control is supported.

       The serial line driver operates in interrupt-per-character mode (all pending characters are flushed from the silo on each interrupt).

Restrictions
       Speed must be set to 9600 BPS on the console port and 4800 BPS on ports used by graphics devices.  The serial  line  driver  enforces  this
       restriction; that is, changing speeds with the command may not always work on these ports.

Files
       console terminal

       local terminal

       local terminal

See Also
       console(4), devio(4), tty(4), ttys(5), MAKEDEV(8)

								       RISC								     dc(4)
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