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Old 01-14-2011, 06:48 PM   #1
Registered: Mar 2009
Distribution: CentOS, Fedora
Posts: 34

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Unable to type anything on "cu" serial terminal window

I'm using CentOS 5.5. I connected serial terminal using "cu". The command I had given was "cu -l /dev/ttyUSB0 -s 115200". It connects I'm seeing the target boards boot-up messages and log in screen. But I couldn't type anything on "cu" terminal window.

Any help is highly appreciated.

Thanks, S
Old 01-16-2011, 01:41 PM   #2
Registered: Jun 2003
Location: Socorro, New Mexico
Distribution: Debian ("lenny", "squeeze"), Linux Mint, XUbuntu
Posts: 218

Rep: Reputation: 24
Full duplex and half duplex

The shortest possible answer to your question is to tell you to install minicom as well
as UUCP (the thing that gives you cu).

From minicom, hit Ctrl A-Z and you get this screen:

  Minicom Command Summary                       |
OPTIO|                                                                   |   
Compi|          Commands can be called by CTRL-A <key>                   |   
Port |                                                                   |   
     |               Main Functions                  Other Functions     |   
     |                                                                   |   
     | Dialing directory..D  run script (Go)....G | Clear Screen.......C |   
     | Send files.........S  Receive files......R | cOnfigure Minicom..O |   
     | comm Parameters....P  Add linefeed.......A | Suspend minicom....J |   
     | Capture on/off.....L  Hangup.............H | eXit and reset.....X |   
     | send break.........F  initialize Modem...M | Quit with no reset.Q |   
     | Terminal settings..T  run Kermit.........K | Cursor key mode....I |   
     | lineWrap on/off....W  local Echo on/off..E | Help screen........Z |
     | Paste file.........Y                       | scroll Back........B |
     |                                                                   |
     |      Select function or press Enter for none.                     |
     |                                                                   |
     |             Written by Miquel van Smoorenburg 1991-1995           |
     |             Some additions by Jukka Lahtinen 1997-2000            |
     |             i18n by Arnaldo Carvalho de Melo 1998                 |
Minicom allows you to turn "local echo on" (E). That is why you are not seeing anything when
you type.

I cannot tell you exactly what your problem is but I can shine some light that ought to let you solve it.
A terminal emulator allows you to send data over a serial port (every key you press gets sent) and then to recieve
replies over serial port from your target, which are decoded and appear on your screen.

The target you are using is not sending replies. When you type and "see nothing", you are probably still sending
commands to your target. You are only distressed that you can't see them. Your target probably also has a
setting like "Echo On". If you turn it on in your target, then whenever you send a character (by typing) it will send
it back, and you will see it on your emulator. Until you figure out how to do this (and perhaps your target is
so dumb it does not have this feature) then you can use "Local Echo On" so that your terminal emulator (minicom)
shows what keystrokes it is sending.

We are all used to high-function systems that are full of illusions (like automatic echo of keys you press, and Windows and a mouse cursor,
the biggest illusion of all!) Your activity looks like firmware/hardware development.
There are no illusions here .... just hardware.

If you get a login screen on your target, try logging in, even though you don't "see" that you are typing anything.
If you login despite not seeing any text, then my explanation is correct. You are "sending" but the target is
not "echoing".

Last edited by pcardout; 01-16-2011 at 01:42 PM.
Old 02-11-2011, 11:59 AM   #3
LQ Newbie
Registered: Jan 2010
Posts: 5

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To see what you are typing, try using the -h flag.
"cu -l /dev/ttyUSB0 -s 115200 -h"
that allows you to see the local echo.

Now my problem is that I can't get any text back from the port. I have a test system ( no video / keyboard controls ) So I have to communicate via a serial port. I have successfully done so on windows using tera term web 3.1. I can use connect my linux machine ( running fedora 13 ) and I can communicate back and forth from fedora to windows using the program I mentioned. On Fedora I'm using "cu -l /dev/ttyS0 -s 115200 -h" and I can see text being transmitted. I believe the problem is that when I press return, it is not a normal return ( carriage return + line feed )... So when prompted on fedora to login my test system I can send it the username, but when I press return all I see is ^M and I get no prompt for a password. So this is leading me to believe that ^M isn't enough to tell the system that the username has been entered.

Am I missing something really basic?

I'm also assuming that suryaemlinux is having the same problem as myself.

Note: I've tried minicom and it does the same exact thing.
Old 02-14-2011, 04:54 PM   #4
LQ Newbie
Registered: Jan 2010
Posts: 5

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I have determined that it is only sending a LF char. How do I send a CR+LF over cu?
Old 02-18-2011, 09:19 AM   #5
LQ Newbie
Registered: Jan 2010
Posts: 5

Rep: Reputation: 0
I have discovered that cu isn't sending anything over the serial connection. I had to use a modified version of Blackbox ( a java application ) to communicate over the serial port.

I still have an unanswered question. Why can't I transmit anything over cu? I can receive data, but I can't transmit anything.
Old 04-06-2017, 05:01 PM   #6
LQ Newbie
Registered: Apr 2017
Posts: 1

Rep: Reputation: Disabled

It has been a while, but..
You are probably hit by this issue, related to cu HW flow control settings:


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