Reading serial data from ttyACM0

Mantabit · 03-12-2019, 05:16 AM

Hello everyone

I am trying to read data from a sensor which appears as /dev/ttyACM0. One my first laptop, I am able to read data via:

Code:

cat /dev/ttyACM0

If I run this command, I get an output like this (the sensor basically outputs comma seperated values seperated by newline characters):

Quote:

1,0,0,200,200,200
2,0,0,300,400,200
3,0,0,200,200,200

However, when I try the same command on another laptop, I am unable to read anything from the device, the cat command will simply get stuck and not return anything.

I setup the device in the following way on the first computer (the one where it works):

Code:

user@laptop:~$ stty -F /dev/ttyACM0
speed 3000000 baud; line = 0;
min = 0; time = 0;
-brkint -icrnl -imaxbel
-opost
-isig -icanon -iexten -echo -echoe -echok -echoctl -echoke

On the second computer, I basically set the baudrate to 3000000 and didn't change any other settings:

Code:

user@ThinkPad-T550:~$ stty -F /dev/ttyACM0
speed 3000000 baud; line = 0;
eof = ^A; min = 1; time = 0;
-brkint -icrnl -imaxbel
-opost -onlcr
-isig -icanon -iexten -echo -echoe -echok -echoctl -echoke

I guess I have to use stty to change the settings for the port on the computer on which I can't read the data?

TB0ne · 03-14-2019, 06:32 AM

Quote:

Originally Posted by Mantabit

Hello everyone
I am trying to read data from a sensor which appears as /dev/ttyACM0. One my first laptop, I am able to read data via:

Code:

cat /dev/ttyACM0

If I run this command, I get an output like this (the sensor basically outputs comma seperated values seperated by newline characters):

Code:

1,0,0,200,200,200
2,0,0,300,400,200
3,0,0,200,200,200

However, when I try the same command on another laptop, I am unable to read anything from the device, the cat command will simply get stuck and not return anything. I setup the device in the following way on the first computer (the one where it works):

Code:

user@laptop:~$ stty -F /dev/ttyACM0
speed 3000000 baud; line = 0;
min = 0; time = 0;
-brkint -icrnl -imaxbel
-opost
-isig -icanon -iexten -echo -echoe -echok -echoctl -echoke

On the second computer, I basically set the baudrate to 3000000 and didn't change any other settings:

Code:

user@ThinkPad-T550:~$ stty -F /dev/ttyACM0
speed 3000000 baud; line = 0;
eof = ^A; min = 1; time = 0;
-brkint -icrnl -imaxbel
-opost -onlcr
-isig -icanon -iexten -echo -echoe -echok -echoctl -echoke

I guess I have to use stty to change the settings for the port on the computer on which I can't read the data?

Yes, you have to get those settings correct, but there may be other issues. First, what version/distro of Linux are you using on both of those systems? What kind of device? Assuming it's a USB connected serial device. Does the output of dmesg tell you anything different between the systems? (run "sudo dmesg -c" on both systems with the device unplugged, then insert the device and run "dmesg" again). And what program is talking to this device?

Might want to install minicom since it was created to talk to serial devices. You can play with the settings, and at least see what you're getting in real-time, without the 'cat' statement.

rtmistler · 03-14-2019, 06:46 AM

Both of the different settings are output settings, so I'm not sure they're going to make much of a difference.

For instance, on the system where it works, you could add an EOF character and add the -onlcr attribute, and I bet it will continue to work.

Use the "raw" setting for stty to make the terminal go to raw mode, this is fairly common and in the library functions, there is a cfmakeraw() call. Look under the termios.h library functions.

Overall, I agree with TB0ne.

First: Do you know that it is identified as /dev/ttyACM0 on both machines? There may be other devices using that same device ID, so therefore check your system logs to verify that this device is exactly that designation on both systems.