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Kalle Richter
Asked: 2018-08-22 19:27:38 +0800 CST

How to take a serial port connection via USB online?

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I'd like be able to establish a connection to a headless Ubuntu 18.04 server from a Ubuntu 18.04 laptop via serial console in order to avoid having a graphic card consuming energy in the server.

The motherboard is an ASRock X99 Extreme3. Its UEFI has an option to enable/disable the serial port which I set to enable - smart as I am - and to select 3F8h/IRQ4 or 3E8h/IRQ4 for the "Serial Port Address" where I left the initially selected 3F8h/IRQ4 without knowing what the values stand for.

On the server, I connected a RS232 to D-Sub connector (DB9) to the serial port of the motherboard and plugged in a gender changer. It translates

-------------                 -----------
\ 5 4 3 2 1 /       to       / 5 4 3 2 1 \
 \ 9 8 7 6 /                /   9 8 7 6   \
  ---------                 ---------------

The laptop doesn't have a serial port, so I use a Serial-USB adapter which causes

[ 4303.030466] pl2303 3-1.1:1.0: pl2303 converter detected
[ 4303.031471] usb 3-1.1: pl2303 converter now attached to ttyUSB0

to be printed in dmesg after it's plugged-in.
On the server side the output of dmesg | grep tty is

[ 0.218803] printk: console [tty0] enabled
[ 2.079815] 00:03: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A

I read about the possibility to specify a serial console to GRUB and related kernel parameters, however I don't find any clear statement whether that's necessary or not to be able to connect after the system booted (I don't need to be able to connect to GRUB's console for now).

When I try to connect to the server using 

sudo minicom -D /dev/ttyUSB0

or putty, I see 

CTRL-A Z for help | 115200 8N1 | NOR | Minicom 2.7.1 | VT102 | Offline | ttyUSB0

in the footer for the former and both terminals don't take any input. I payed attention to match the device file to connect to with the one printed in dmesg.

I'm suspicious that I don't have to install or configure anything on the server side except turning on the serial port in the UEFI. Can that be correct?

Maybe the BAUD rate isn't negotiated automatically and needs to be configured, but I have no idea how to figure out the correct value. I ran the configuration for minicom, but could only select default values because I don't find a good explanations - in all tutorials the connection just magically works after minicom -s apparently. Maybe the connection works and I'm missing the final step to connect.

I added the desktop user account and root to the groups dialout and uucp.

I can provide all necessary information for both server and laptop.

usb uefi serial-port

2 Answers

  1. Some good reading is available from Official Ubuntu Documentation:

    Alternative serial client: screen

    GNU screen actually is a very capable serial terminal.

    To install:

    sudo apt-get install screen

    To start:

    sudo screen /dev/ttyUSB0 n

    Some additional tips:
    1. adjust the n to what you need:

    • Start without a number on the first try; it might auto-negotiate flawlessly
    • then use 115200, 57600, 38400, 19200, 9600, 4800, 2400, 1200 and stop when it starts working.
    • If you're a purist and don't like using sudo for this or need to hand this out to users not having access to sudo, add them to the dialout group.

    To end the session, use Ctrl-A or Shift-K.

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  2. [ 2.079815] 00:03: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A

    The link above that the kernel detected in 16550 UART which provides a ttyS0 port and some resources allocated to it ( 0x3f8 IO address and IRQ 4 ).

    On the server side, Make sure

    1. The Serial already connected to USB Port. check it with following :

      $lsusb

      The Output should be like :

      Bus 003 Device 001: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port

    2. The default permissions for ttyS* are the following :

      crw-rw---- 1 root dialout ... /dev/ttyS0

      Yo're already a member of dialout group with add :

       $sudo adduser $USER dialout

      Let the Minicom can access the ttyUSB* using :

      $sudo chmod 0777 /dev/ttyUSB0
      crwxrwxrwx 1 root dialout ... /dev/ttyS0

    Next Step, Setting up the Minicom should be like :

    $sudo minicom -s
    +-----[configuration]------+                                     
    | Filenames and paths      |                                     
    | File transfer protocols  |                                     
    | Serial port setup        |                                     
    | Modem and dialing        |                                     
    | Screen and keyboard      |
    | Save setup as dfl        |
    | Save setup as..          |
    | Exit                     |
    | Exit from Minicom        |
    +--------------------------+

    Choose "Serial port Setup" menu and Then do the configuration as needed and save setup as dfl. Finally restart your Minicom.

        +-----------------------------------------------------------------------+
    | A -    Serial Device      : /dev/ttyUSB0                              |
    | B - Lockfile Location     : /var/lock                                 |
    | C -   Callin Program      :                                           |
    | D -  Callout Program      :                                           |
    | E -    Bps/Par/Bits       : 115200 8N1                                |
    | F - Hardware Flow Control : Yes                                       |
    | G - Software Flow Control : No                                        |
    |                                                                       |
    |    Change which setting?                                              |
    +-----------------------------------------------------------------------+
            | Screen and keyboard      |
            | Save setup as dfl        |
            | Save setup as..          |
            | Exit                     |
            | Exit from Minicom        |
            +--------------------------+

    On the Laptop side, open Serial client like screen or putty :

    1. Use screen serial :

      $screen /dev/ttyUSB0 115200, cs8, -parenb, -cstopb

      Or

    2. Use putty serial :

      $sudo putty /dev/ttyUSB0 -serial -sercfg 115200,8,n,1,N

    Hope this helps.

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