Serial loop back test doesnt work

raypen · 05-29-2008, 03:20 PM

I have been trying to perform a serial loop back test of my
serial port (ttyS0) on a machine running Slackware 12.0.

I have inserted one end of a null modem cable into the serial
port and have shorted pins 2 & 3 on the other end. I then issue the
following commands:

on tty1

Code:

cat < /dev/ttyS0

on tty2

Code:

echo "This is a serial line/port test" > /dev/ttyS0

Expecting to see the result of the echo command on tty1, I get
nothing but a blinking cursor and some odd jibberish scrolling
by so fast I can't read it. As a matter of fact, the blinking
cursor and scrolling occur when I enter cat < /dev/ttyS0.

What am I doing wrong?

michaelk · 05-29-2008, 04:36 PM

Would be easier to use a terminal program such as minicom. Any alphanumeric character typed in should be echoed back (visible) on the screen which means the loopback is working. The program must be configured not to echo characters which is typically the default.

raypen · 05-29-2008, 05:10 PM

I suppose I should have mentioned it, but I tried minicom and
it would not work properly. Not sure, but I get a blinking cursor
at the bottom of the window similar to the tty1 output. I tried
to exit and it hung. Had to kill its process from another tty.

Anyway, answers in these forums say the best way to do the loopback
test is as outlined above.

jailbait · 05-29-2008, 05:29 PM

Quote:

Originally Posted by raypen

I suppose I should have mentioned it, but I tried minicom and
it would not work properly. Not sure, but I get a blinking cursor
at the bottom of the window similar to the tty1 output. I tried
to exit and it hung. Had to kill its process from another tty.

From that clue I would say that the data is not being received at tty1. Take a look at the possibility that the data is not actually being transmitted or that it is being transmitted and the null modem cable is not sending it back.

Could you run a test with a different cable where you connect the serial ports of two different computers?

---------------------
Steve Stites

raypen · 05-30-2008, 11:41 PM

Recently ran across this which is a decent explanation of effects
of shorting the pins:

Quote:

Another test is to jumper the transmit and receive pins (pins 2 and 3 of either the 25-pin or 9-pin connector) of a test serial port. Then send something to each port (from the PCs keyboard) and see if it gets sent back. If it does it's likely the port with the jumper on it. Then remove the jumper and verify that nothing gets sent back.

Note that if "echo" is set (per stty) then a jumper creates an infinite loop. Bytes that pass thru the jumper go into the port and come right back out of the other pin back to the jumper. Then they go back in and out again and again. Whatever you send to the port repeats itself forever (until you interrupt it by removing the jumper, etc.). This may be a good way to test it as the repeating test messages halt when the jumper is removed.

I think this answers my question!

littos · 01-07-2012, 07:06 AM

Ahhhh!

I just love you guys! Thanks for sharing information also after you find the soulution to your problems!

I had a simmilar problem when trying to loop back on the /dev/tts/1 on a Linksys WRT54g, running DD-WRT. I just couldnt get things to work, but after reading this thread I sent the router a:
"root@DD-WRT:~# stty -F /dev/tts/1 -echo"

and now things seems to work. I might need to remove some new line echo also, but that shouls be docunented in the stty documentation, I think.

Tanks for sending me down the right trail!

/Anders