Setup External Serial Modem on Ubuntu Server

$ dmesg | grep tty
[    0.000000] console  enabled
[    0.534814] serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[    0.632375] serial8250: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A
[    0.992562] 00:06: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
[    1.013084] 00:07: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A
 

$ sudo apt-get install setserial
 

$ sudo setserial -g /dev/ttyS0
/dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4
 

$ sudo apt-get install minicom
 

$ sudo minicom -s
 

$ minicom -c on
 

GROUP membership. By default users don't have access to the serial ports so you have to give them access. First check which group do you need to be in to access serial port ttyS0 for example.

$ sudo ls -la /dev | grep ttyS0
crw-rw----  1 root dialout   4,  64 May  2 11:05 
ttyS0
 

Now you see the user needs to be a member of the group dialout, check your the id's current group membership

$ id dhosanguid=2009(dhosang) gid=2009(dhosang) 
groups=4(adm),24(cdrom),27(sudo),33(www-data),50(staff),100(users),102(crontab),103(syslog),109(ssh),115(sambashare),116(admin),126(mysql),127(kvm),128(libvirtd),30000(shared),2009(dhosang)
 

OR

$ groups dhosangdhosang : dhosang adm cdrom sudo www-data staff 
users crontab syslog ssh sambashare admin mysql kvm libvirtd shared
 

OR

$ cat /etc/group | grep dialoutdialout:x:20:admin
 

So the user (dhosang) isn't a member of the required group (dialout) so let's add the group to the user

$ sudo adduser dhosang dialout