Matt J. Gumbley | Website:
Blog: On The Edge of Occam's Razor

scope - Linux Serial Line Analyser


scope is a simple Linux program that allows you to capture data flowing between two devices that communicate over a 7-wire (or fewer) RS232 serial connection. To do this, it requires a special cable. This page tells you how to obtain scope and how to build it, install it, and build the cable.

Table Of Contents

1 Download Instructions

scope is currently at version 0.04. This adds --- lines in continuous mode (-c option) to distinguish between different packets. It has been tested on Red Hat 9, and also builds fine on Ubuntu Linux 5.04.

Prior to this, is was available as part of my linux-ncp package, but has proved very useful, and so I've made it into a package in its own right.

You may download it here: scope-0.04.tar.gz.

Previous versions are still available.

Then, do this:

gzip -cd scope-x.xx.tar.gz | tar xvf -
(Where x.xx is the version you downloaded).

Then read the file called README. This is displayed below.

2 The README File

Scope: A Serial Line Analyser/Logger for Linux 
    (and possibly other UNIX systems)

(C) 1999-2005 Matt J Gumbley

Scope is a general-purpose serial line monitor, capable of recording the traffic
on an RS232 cable (if provided with a suitable cable and recording computer).

Tested On...
It was written for:
 Red Hat Linux 4.2/Sparc,
 Red Hat Linux 6.0/Sparc,
 Red Hat Linux 6.2/Intel
 Red Hat Linux 9/Intel
 Ubuntu Linux 5.04/Intel
although it should also compile on other UNIXes. You may need to make changes to

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

Building and Installing
The program now uses GNU autoconf, thanks to Warren Young.
To build, use the usual configure invocations; a 'configure --help' will show
you what is supported.

Typically, you may wish to place the program in a location other than its
default. So, './configure --prefix=/wherever/you/want' is all that's needed.

Once configuration is done, type 'make', and then 'make install'.

For further information, refer to the file INSTALL, which is ripped verbatim
from some other package which uses autoconf ;-)

(Note to developers: the configure script supplied was generated with autoconf
2.13-5 on a Red Hat 6.2/Intel system)

Specific configure options for this program
There are none, yet.

How to Use Scope
Basically, scope will monitor /dev/ttyS0 and /dev/ttyS1, with output on 
standard out, at 19200 bps. Use the -d option to suppress the control/status 
line data (since that's not 100% working), and use other options to control
speed, different ports, verbosity etc. 

scope -h will give a short help file; there is no man page.

Capture the output in a file, and use this as input to some of my other
programs, such as ncpscope.

The Special Serial Cable
Scope works best when used in conjunction with a special serial cable. 

This cable goes inline between the two devices that you want to monitor, and
splits signals on the "main" line into two serial ports which are monitored
simultaneously to provide a composite view of the current line status. 

The cable I use connects to a Sun SparcStation 5, which has two serial ports,
both D25's - similar to a PC. On a Sparc 10, the two ports (A/B) are housed in a
single socket - the cable is wired as follows:

                  D25          D25       MALE D25
                   1 - FG ----- 1 
                   2 - TD ----- 2  ------ SRD 16
 3 - RD ---------  3 - RD ----- 3
                   4 - RTS ---- 4  ----- SCTS 13
 5 - CTS --------  5 - CTS ---- 5
 6 - DSR --------  6 - DSR ---- 6
 7 - GND --------  7 - GND ---- 7
 8 - DCD --------  8 - DCD ---- 8
                  20 - DTR ---- 20 ----- SDCD 12
                  22 - RI ----- 22

Basically, the first port is used to read most lines, and the second port is
used to read TD, RTS and DTR, which are read on the input lines RD, CTS and DCD

So wire the second plug:
                                2 ----------- 3
                                4 ----------- 5
                               20 ----------- 8

Standard disclaimer applies. If you blow up your systems
with this, don't blame me - it works fine (ish) for me.


* Make it work properly on Intel Linux (CRTSCTS undeclared for some reason)
  - this ioctl option is not used on Intel. Add #define CONFIG_M386 in global.h
    to make this work.
* Use select(), don't poll.
* Add pass-through mode (for Charlie Stross) - anything coming in on one port is
  echoed to another.

3 The ChangeLog File

0.04 04/11/2005
Continuous mode now outputs --- lines when there's a delay in data being
observed. Helps to distinguish between different packets.

0.03 08/10/2002
Added the following features, from a patch from Warren Young:

- It adds the -1 command line argument, which makes it only read from 
one serial port.  I needed this because I was just interested in making it show
me what a particular program was sending.

- autoconfiscation consequence #1: all the -DUNIX and -DLINUX stuff is 

- consequence #2: The old CRTSCTS check is also removed, as it doesn't 
seem necessary.  It must have been a bug in the old Linux headers, since 
that's POSIX standard, I think. (*) The code as it is now works on both 
RH6.2 and 7.3.  If you have a platform where this still breaks, there 
are better ways of handling this problem with autoconf than 
platform-specific ifdefs.  I can help you code up the necessary autoconf 
invocations if you don't want to take the time to figure it out yourself.

- consequence #3: The "byte" and similar typedefs are now based on the 
reasonably-portable u_int8_t and similar types usually found in 
sys/types.h.  If you have a platform that doesn't have these typedefs, 
again, there are ways to extend autoconf to detect that platform's sized 
int typedef style.

- I removed several skeleton files like 'src/header'.  I don't think 
they belong in a source distribution.

- Obviously the makefile structure is now different.  A lot of the code 
you had in your makefiles is replaced by autoconf.  For example, the 
"forwarding" targets so that "make clean" in the top level also does a 
clean in src.  Also things like VERSION and the install directory stuff 
already exist in autoconf/automake.

- Several of the Makefile targets weren't ported to automake, like the 
backup one.  If you still want these, just add them to  You 
might check to see if there's an equivalent already, though.  The "tar 
up the source code" target isn't needed any more, for example, because 
"make dist" will do what you want now.  (Or if it doesn't, it can be 
fixed so that it does, rather than replacing the target with a 
hand-rolled version.)

Many thanks, Warren!

(*) Matt thinks this may have been a Sparc Red Hat Linux 6.0 thing; he's not
bothered about that version, since he's on 6.2 now. If this is an issue for you,
earlier versions of scope are availabel from the project home page.

Added -w "write this string" option, which writes the given string out on the
first port, then starts monitoring. You can include returns, e.g.:
-w "AT&V\\r" which on some modems will cause a settings dump to be generated.

0.02 20/02/2002
Added fix for compile error on RH7.x, from a patch by Grant Edwards: "The
'timezone' struct has been obsoleted, so I've jettisoned it.  Passing a NULL for
the second parameter to gettimeofday is backward compatible at least as far back
as RH6.  FWIW, I verified that the same change will work under Solaris (though
asy.c doesn't compiler -- no surprise there). I've also added an "#include" or
two to get rid of some warnings about implicit declarations of exit() and

Thanks, Grant!

(C) Matt J. Gumbley 1998-2005 - All Tights Reversed