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I am trying to use my 9 pin serial port to connect my automotive ECU to my computer (megasquirt controller). I've done this before with Ubuntu 10.04 but now that I've just upgraded to 12.04 it simply will not connect. I didn't have any issue before. Now I simply cannot open this port.

Using chmod I get this:

~$ chmod o+rw /dev/ttyS0
chmod: changing permissions of `/dev/ttyS0': Operation not permitted
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I suggest you should just double-check if you are a member of the dialout group, let us suppose your username is joeuser, do the following:

groups joeuser

..this will list all the groups you belong to. If you don't belong to the dialout group then add yourself to it, for example:

sudo gpasswd --add joeuser dialout

You then need to log out and log back in again for it to be effective. Then see if it fixes your problem.

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Emphasis on logout/login and trying again. – AlexanderN Feb 6 '13 at 20:16
I want to remark that log out and log back in is the key to get this to work. – albfan Jun 30 '13 at 11:31

You need to be in the uucp group too. Worked for me on Ubuntu 14.04 64bit.

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