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reading/writing to serial port doesn't work

Hi. I use Ubuntu 16.04. In the first terminal I wait for data:

Code:

cat </dev/ttyS0

Then in the second terminal I try to send data to the serial port:

Code:

echo -e '123' > /dev/ttyS0

but data is not visible in the first terminal. I have added myself to the group dialout for /dev/ttyS0.

Additional info:
dmesg | grep ttyS

Code:

[ 0.684920] 00:06: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A

stty -F /dev/ttyS0 -a

Code:

speed 9600 baud; rows 0; columns 0; line = 0;

intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; discard = ^O;

min = 1; time = 0;

-parenb -parodd -cmspar cs8 hupcl -cstopb cread clocal -crtscts

-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon -ixoff -iuclc -ixany -imaxbel -iutf8

opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0

isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke -flusho -extproc

ifconfig -a

Code:

enp0s25  Link encap:Ethernet  HWaddr d0:50:99:5e:47:a7  

          inet addr:192.168.0.164  Bcast:192.168.0.255  Mask:255.255.255.0

          inet6 addr: 2a02:a31a:a043:6d80:8f91:68c2:9a1d:1b86/64 Scope:Global

          inet6 addr: 2a02:a31a:a043:6d80:81a4:6233:ff1f:6b9a/64 Scope:Global

          inet6 addr: fe80::1942:8c1c:9235:bbe7/64 Scope:Link

          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

          RX packets:9231 errors:0 dropped:0 overruns:0 frame:0

          TX packets:6295 errors:0 dropped:0 overruns:0 carrier:0

          collisions:0 txqueuelen:1000 

          RX bytes:9029350 (9.0 MB)  TX bytes:809689 (809.6 KB)

          Interrupt:20 Memory:f7300000-f7320000 



lo        Link encap:Local Loopback  

          inet addr:127.0.0.1  Mask:255.0.0.0

          inet6 addr: ::1/128 Scope:Host

          UP LOOPBACK RUNNING  MTU:65536  Metric:1

          RX packets:966 errors:0 dropped:0 overruns:0 frame:0

          TX packets:966 errors:0 dropped:0 overruns:0 carrier:0

          collisions:0 txqueuelen:1000 

          RX bytes:90014 (90.0 KB)  TX bytes:90014 (90.0 KB)

Why reading/writing to the serial port doesn't work ?

Welcome to LinuxQuestions.

You need a physical loopback plug attached to the serial port connector to see data. Here is a pin out of the plug wiring.

http://www.bb-elec.com/Learning-Cent...-DB9-DB25.aspx

It is also easier to use a terminal program like minicom to test the serial port. With a loopback plug the characters are displayed as soon as they are typed on the keyboard.

Is it possible to buy such connector ? My PC has a female com port so I need a male connector.

Since the IBM standard uses a male connector I think it would be difficult to find a male loopback to purchase. You probably need to purchase a female connect and a gender changer.

It depends on what you are actually trying to accomplish. If you disable hardware control you can jumper pins 2-3 to see characters as they are typed.

Gender changer is easy to get, I have also found the following connector. Is it what I need ?

What do you mean by "disable hardware control"? Could you explain it ?

probably this helps: http://tldp.org/HOWTO/Serial-HOWTO-9.html
I think that is the right connector, but you can find an interesting comment there (probably a damaged one only)

Yes, however one of the comments shows this particular model using a strange wiring layout out which may be ok.

The RS-232 uses several wires to control data flow over the data pins. Since suggested just using a jumper to connect the RX to TX pins you need to disable hardware flow control since those pins are unused and you will be able to transmit data.

https://en.wikipedia.org/wiki/RS-232

I have performed another test: I have connected to my pc usb to serial adapter which is connected with mentioned above loopback connector. Instead of ttyS0 I use ttyUSB0. It works but when I send some message:

Code:

echo -e '123' > /dev/ttyUSB0

on reading terminal

Code:

cat </dev/ttyUSBO

message is displayed multiple times and it looks like I get white spaces in a loop (infinitely). What is going on ?

Yes it should work in theory.

How are you reading/writing to the serial port. Have you given permission to your regular user to read/write to the USB adapter which is typically /dev/ttyUSB0?

Please see my answer update. I have forgotten to use ttyUSB0 instead of ttyS0

using echo and cat in the terminal is not very reliable for testing serial ports in my opinion. As suggested earlier use minicom. You need to configure the serial port but it will show characters as they are typed.

minicom with echo works fine.