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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 DEBIAN

modbus_rtu_set_serial_mode

MODBUS_RTU_SET_SERIA(3) 					 Libmodbus Manual					   MODBUS_RTU_SET_SERIA(3)

NAME

       modbus_rtu_set_serial_mode - set the serial mode

SYNOPSIS

       int modbus_rtu_set_serial_mode(modbus_t *ctx);

DESCRIPTION

       The modbus_rtu_set_serial_mode() function shall set the selected serial mode:

       MODBUS_RTU_RS232
	   the serial line is set for RS232 communication. RS-232 (Recommended Standard 232) is the traditional name for a series of standards for
	   serial binary single-ended data and control signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data
	   Circuit-terminating Equipment). It is commonly used in computer serial ports

       MODBUS_RTU_RS485
	   the serial line is set for RS485 communication. EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the electrical
	   characteristics of drivers and receivers for use in balanced digital multipoint systems. This standard is widely used for
	   communications in industrial automation because it can be used effectively over long distances and in electrically noisy environments.

       This function is only supported on Linux kernels 2.6.28 onwards.

RETURN VALUE

       The modbus_set_serial_mode() function shall return 0 if successful. Otherwise it shall return -1 and set errno to one of the values defined
       below.

ERRORS

       EINVAL
	   The current libmodbus backend is not RTU.

       ENOTSUP
	   The function is not supported on your platform.

       If the call to ioctl() fails, the error code of ioctl will be returned.

AUTHORS

       The libmodbus documentation was written by Stephane Raimbault <stephane.raimbault@gmail.com[1]>

NOTES

	1. stephane.raimbault@gmail.com
	   mailto:stephane.raimbault@gmail.com

libmodbus 3.0.3 						    05/26/2012						   MODBUS_RTU_SET_SERIA(3)
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