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ttyS0 - terminal on serial connection

Terminal - ttyS0 - terminal on serial connection
setserial -q /dev/ttyS0
2012-02-08 20:44:09
User: 0disse0
2
ttyS0 - terminal on serial connection

I actually planned to do this for quite a long time, but since I haven't had any suitable client hardware, I procrastinated this.

Now, the old laptop I've got from my dad, features an RS-232 port. So, now that I had technically a client, that I could test my RS-232 connection with, I ordered a null modem cable.

There is no RS-232 outlet on my desktop computer directly on the mainboard, but theres a connector on the mainbord, where a RS-232 outlet can be attached to. The outlet will then cover up a PCI slot.

# Activating RS-232

Ok, once all cables were in place, I tried to access both RS-232 ports by reading from them directly. They're usually ttyS0 or ttyS1, depending what COM-Port it is.

From the file /proc/tty/driver/serial, information about the serial setup can be obtained.

A setserial -q /dev/ttyS0 might be usefull as well. Usually, the UART Type is 16550A, on a standard PC. Mine wasn't working though.

At leas not right from the start, when I tried to read the interface with cat /dev/ttyS0 I got the following error:

# cat /dev/ttyS0

cat: /dev/ttyS0: Input/output error

Obviously, the driver couldn't activate the hardware. Reason was, it was deactivated in BIOS. After activating RS-232 there, it worked well.

As a last action, I added myself to the uucp group, so I have user permission to the serial lines. It is not necessary for the terminal setup, but a good idea to do so, just for future projects, maybe...

# Setting up a terminal

Once the Serial line is configured and working properly, it's time to let a terminal run on that port. This is what I added to my /etc/inittab :

s0:2345:respawn:/sbin/agetty -L 38400 ttyS0

I added it quite on the top of that file, right below the 'si' statement, mingetty cannot be used for serial connections, it cannot be run in a console, too. I tried it for testing purposes, but the cosole - along with your login program - will log you out, as soon as you log in over your serial line.

'-L' means this is a local line, with no carrier signal. 38400 is the standard speed of a Linux console, it might be a bit high, I was told, but it works well. I tested that with some higher values as well (115200) and it worked too, I guess it tepends on things like cable length, etc. Last parameter, is the serial tty to listen on. The terminal type can be specified as an additional parameter at the end of the parameter list, vt102, for instance. This is sometimes required, depending on the client.

After finishing editing /etc/inittab, an init q will make the system re-read /etc/inittab and apply changes. The agetty should now be listening on ttyS0.

#Setting up a client

It's time to establish a connection and test the serial line. I use a laptop, that has an RS-232 port, so some preliminary setup is required. I tried minicom as terminal initially, but it turned out, not to be the best client. It initializes the modem, this lasts quite long, and it doesn't convey ANSI colors. So the better option is cu, it's part of the UUCP-Package.

Oh, and the serial port of that computer, has to be accessible as well, of course.

Once everything was set up, I established the connection:

cu -l ttyS0 -38400 --nostop

Pretty self explanatory, I think. The --nostop option disables XON/XOFF handling.

# root access over ttyS0

In order to become root over the serial terminal, the tty needs to be added to /etc/securetty

I appended ttyS0 to the end of the file. It is now possible, to gain root access over the serial terminal. The agetty process needs to be restarted to apply changes.

# Accessing GRUB over ttyS0

To make bootloader access possible over ttyS0, some changes to /boot/grub/menu.lst need to be done. (GRUB is the bootloader I use, I suppose LiLo has similar capabilities.)

Those are the lines, I appended to the top of my menu.lst :

serial --unit=0 --speed=38400 --word=8 --parity=no --stop=1

terminal --timeout=3 serial console

The serial command initiates the serial terminal option, --unit=0 defines our first serial connector, I my case, it's the only one I have on my machine. I used the standard Linux-Console speed, as well as the "8N1" connection strategy.

terminal defines the terminal priorities, first terminal (serial) is the standard one, the last one is the secondary terminal (console).

--timeout=3 enables a delay on both consoles, with a prompt for a keystroke. Depending on which terminal, the key is pressed, this terminal, will be used. If no key is pressed after the timeout, the standard console (in my case serial) will be used.

# Relaying Kernel output on boot

The Kernel accepts multiple console options, of which the last one, is the standard console, and the one that will be used in Single User mode. These are my Kernel options:

title Fedora Core (2.6.20-1.2316.fc5)

root (hd0,0)

kernel /vmlinuz-2.6.20-1.2316.fc5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet vga=795 console=tty0 console=ttyS0,38400

initrd /initrd-2.6.20-1.2316.fc5.img

console=tty0 is the standard console, located on the machine, i.e. monitor and keyboard.

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