I am trying to use minicom in Ubuntu.

To do this I first connect my USB to DB9 converter, then I type this command on terminal dmesg | grep tty to check if my USB to DB9 converter is connected properly.

I got this output:

[    0.000000] console [tty0] enabled
[   54.200503] usb 2-1.2: pl2303 converter now attached to ttyUSB0
[   76.183173] cdc_acm 1-1.1:1.2: ttyACM0: USB ACM device
[  118.363275] cdc_acm 1-1.1:1.2: ttyACM0: USB ACM device
[  190.079494] cdc_acm 1-1.1:1.2: ttyACM0: USB ACM device

So, I know my USB to DB9 converter name is ttyUSB0.

Following steps are executed in order:

  • Open the minicom by using command sudo minicom -o -s -w.
  • Open serial port setup and write serial device as /dev/ttyUSB0 and write Hardware Flow Control to NO.
  • Press Enter to exit to main menu then Esc to start minicom.

After these steps it should work fine, but my minicom is showing "offline".

Where is the mistake in my configuration? Thank you!

  • 7
    No mistake. Online/Offline status is determined by the state of the CD (carrier detect) signal, or perhaps the DTR signal. Both can be overridden. These signals were used with modems to indicate a solid connection and readiness to talk, respectively. Probably before your time.
    – lornix
    Commented Jun 30, 2014 at 19:56

5 Answers 5


Serial ports include control lines in addition to the data lines.

Some USB <-> Serial adaptors often omit the control lines.

Thus they have only "crossed over" data lines. The other serial port lines, like DTR - "Data Terminal Ready" and DCE "Data Carrier Detect" (often referred to as "Carrier Detect") are not supported.

Minicom is saying you are offline because the DCD "Carrier Detect" line is low. If you were using a MODEM, this line would be high while the MODEM was connected to the phone line. You don't mention a MODEM, but whatever is connected is either not producing a DCD signal, or the USB<->Serial adapter does not have the wiring for it.

If the data is coming though OK, you can ignore this, probably.


You can try sudo minicom -D /dev/ttyUSB0 ,this works well in my laptop. You may not concern about Online/Offline status .


The offline/online display of minicom is basically information only: it does not affect the serial port communication.

But if you are not using it with an actual modem, you might want to go to the "Modem and dialing" settings and disable the "Modem has DCD line" option. Then (I think) the online/offline indicator will follow the state of the DSR signal of the serial port, which may be more useful with non-modem devices.

  • "Modem and dialing", btw, are under "cOnfigure Minicom".
    – duanev
    Commented Feb 28, 2018 at 18:20

Once you've got the Default updated with all the proper settings, try restarting minicom with no flags. This works for me most of the time.

  • 1
    @IgorGanapolsky it means Default
    – CJBrew
    Commented Aug 7, 2017 at 11:48

I'm reading the minicom manual (man minicom), I found this paragragh:

Serial port setup
     A - Serial device
          /dev/tty1 or /dev/ttyS1 for most people.  /dev/cua<n> is still possible under linux, but not recommended any more because these devices are obsolete and
          many  newly  installed  systems with kernel 2.2.x or newer don't have them.  Use /dev/ttyS<n> instead.  You may also have /dev/modem as a symlink to the
          real device.
          If you have modems connected to two or more serial ports, you may specify all of them here in a list separated by space, comma or semicolon. When  Mini‐
          com  starts,  it  checks  the  list until it finds an available modem and uses that one. (However, you can't specify different init strings to them ..at
          least not yet.)
          To use a UNIX socket for communication the device name must be prefixed with "unix#" following by the full path and the filename of the socket.  Minicom
          will  then  try  to  connect to this socket as a client. As long as it cannot connect to the socket it stays 'offline'. As soon as the connection estab‐
          lishes, minicom goes 'online'. If the server closes the socket, minicom switches to 'offline' again.

So, I think that the online/offline status is used only with UNIX sockets. I never used this, but I made some applications with USB and DB9 and all them worked with "offline" flag. If your communication doesn't work, check the baud rate and parity are correct.

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