How to view the DB9 to USB port？？

ls -la /dev/ttyS* will also give the owner & group

eg (these ports don't actually exist on my system, but the dev nodes are there anyway (I think it maybe a legacy thing)

crw-rw-rw- 1 root dialout 4, 68 May 9 11:49 /dev/ttyS4
crw-rw-rw- 1 root dialout 4, 69 May 9 11:50 /dev/ttyS5
crw-rw-rw- 1 root dialout 4, 70 May 9 11:50 /dev/ttyS6
crw-rw-rw- 1 root dialout 4, 71 May 9 11:50 /dev/ttyS7
crw-rw-rw- 1 root dialout 4, 72 May 9 11:50 /dev/ttyS8
crw-rw-rw- 1 root dialout 4, 73 May 9 11:50 /dev/ttyS9

In this example root is the owner and dialout is the group.
As the owner of th ttyS0-3 are dialout you will have to add yourself to the dial out group, if you have not already. The HOWTO docs I linked to earlier will explain this better.
Another thing you may need to consider is the "flow control" of the port. Usually for simple communications this is set to none. That's why using a GUI serial terminal maybe easier as the setup of the serial port is easier. If the default "flow control" is something other than "none" say "hardware" the serial port may not send until the correct control lines are set.

Hi，ALittleOffTheRails！
Yes, I also encountered this question. Obviously, the machine I bought has only one DB9 port, but the terminal found ttys0-ttys3, a total of four.
The port queried on my machine is the same as yours, "root" is the owner and "dialout" is the group.
So do I need to do this: group my operation users into "dialout"?
I will carefully check the howto document you sent, although the content is difficult for me to understand.

There is also the setting of "flow control". Where can I set this setting? This is the first time I heard this word. There is an introduction to the "howto" document, isn't it? Damn it, I'll check the document right away

Thank you very much. I'll try again

Enter the following command, replacing <username> with the name of your account.

Hi !!  Tommy
I have added my user name "linuxcnc" to the group "dialout"
But I do a loopback test: that is, short circuit the TX and Rx of DB9.
Then the terminal sends a command, and the communication still fails.
Do you have any clue?
thank you!

If the user name is LinuxCNC then First, check if the serial port is enabled in BIOS.

Hi Tommy！
I entered the BIOS and checked the serial port settings. I put the photos below.
It doesn't look like a problem, does it? It seems to have been turned on

The second picture shows the config for the Serial ATA drive interfaces, not the Serial comms port. Did we make a silly ? lol

Actually on a serious not dmesg is reporting 1 serial comms port and you are only stating one physical port. I have a similar issue with my laptop, physically there are no ports on the laptop itself, but there is a serial port reported by dmesg. I think that it maybe for use when an external docking station is in use. I thought it mayhave related to either the bluetooth interface or the WAN port, turning them off in the bios has not affect.

Have you access to an 'scope that could look at the TX waveform from the serial port, even an LED and resistor could work whilst transmitting at a low baud rate, if you can see the LED flashing data is being sent, if it doesn't flash there is an issue. It could be that the port hardware is in someway damaged. Another thing is the actual DB9 port soldered to the motherboard or is the connection via ribbon cable. If via a ribbon cable is that plugged in and plugged in the correct way ?

hi ！ guy
Excuse me, am I taking pictures in the wrong place? Or is the configuration wrong? In fact, I don't understand ATA and so on. I'm very strange to these. LOL~
Can you guide me? thank you

Serial ATA is the connection form your hard drive or solid state drive, SSD, to the computer. It has nothing to do with serial communications. I noticed in the first picture there is a reference to Super I/O, if this has a sub menu it may have some items regarding the serial comms.

BTW what model of computer motherboard are you using, this could be of some help.

In BIOS, I found that the motherboard model is h81m-s2ph, which may be a very old motherboard? Because I bought this industrial computer online. I'm just doing application level
I also took photos and posted the information of super io. It seems that there is serial port information in it. I also took photos of the back of my computer
Please check, thank you