How to send data to a serial port and see any answer?

Question

On Linux, I want to send a command string (i.e. some data) to a serial port (containing control characters), and listen to the response (which also usually might contain control characters).

How can I do this as simplest as possible on Linux? An example is appreciated!

Answer 1

All devices on Unix are mapped to a device file, the serial ports would be /dev/ttyS0 /dev/ttyS1 ... .

First have a look at the permissions on that file, lets assume you are using /dev/ttyS1.

ls -l /dev/ttyS1

You will want read.write access, if this is a shared system then you should consider the security consequences of opening it up for everyone.

chmod o+rw /dev/ttyS1

A very simple crude method to write to the file, would use the simple echo command.

echo -ne '\033[2J' > /dev/ttyS1

and to read

cat -v < /dev/ttyS1

You can have cat running in one terminal, and echo in a 2nd.

If everything is gibberish, then baud rate, bit settings might need setting before you start sending. stty will do that. !! NOTE stty will use stdin as default file descriptor to affect.

Equivalent commands.

stty 19200 < /dev/ttyS1
stty 19200 -F /dev/ttyS1

This might be enough for you to script something and log ? Not sure what you are trying to achieve.

For a more interactive, remembers your default settings approach would be to use minicom it is just a program which does everything I've mentioned so far. (similar to hyperterminal in Windows, you might be familiar).

An intermediate solution, would use a terminal program like screen which will work on a serial device.

screen /dev/ttyS1

man screen man minicom man stty for more information

Answer 2

All you have to do is open two terminals. In the first terminal you cat everything from the device, e.g.

cat /dev/ttyS0

in the other terminal, you can send arbitrary hex characters and text to the terminal e.g. as follows:

echo -e "\x7E\x03\xD0\xAF und normaler Text" > /dev/ttyS0

The echo -e command enables the interpretation of backslash escapes.

One has to make sure of course that (i) the serial settings (speed, word length, flow ctrl, etc) are correct and (ii) the serial device (on the other end) is not blocking.

Answer 3

Programs that talk to serial devices:

picocom
minicom
socat

or from shell you can do:

stty -speed 19200 < /dev/ttyS0 # sets the speed of the port
exec 99<>/dev/ttyS0 (or /dev/ttyUSB0...etc)
printf "AT\r" >&99
read answer <&99  # this reads just a CR
read answer <&99  # this reads the answer OK
exec 99>&-

Answer 4

This could be a better approach:

stty -F /dev/ttyUSB0 115200 raw -echo   #CONFIGURE SERIAL PORT
exec 3</dev/ttyUSB0                     #REDIRECT SERIAL OUTPUT TO FD 3
  cat <&3 > /tmp/ttyDump.dat &          #REDIRECT SERIAL OUTPUT TO FILE
  PID=$!                                #SAVE PID TO KILL CAT
    echo "R" > /dev/ttyUSB0             #SEND COMMAND STRING TO SERIAL PORT
    sleep 0.2s                          #WAIT FOR RESPONSE
  kill $PID                             #KILL CAT PROCESS
  wait $PID 2>/dev/null                 #SUPRESS "Terminated" output

exec 3<&-                               #FREE FD 3
cat /tmp/ttyDump.dat                    #DUMP CAPTURED DATA

Answer 5

You can read and write to a device simulataneously like so:

cat /dev/cu.usbmodem411 & cat > /dev/cu.usbmodem411

Your message is sent to the second cat from stdin, and the first cat relays the response to stdout, turning your terminal into a chatroom.

To finish up, ctrl-c, then run fg then ctrl-c again.