Change #define SERIAL_DEBUG to 1 in serialport.h.
After recompiling DOSBox you can add debugging parameters to the serial port config string i.e. serial1=modem dbgtr. The output will be captured as file in the CAPTURE directory.

More sort of a general question on Dosbox compiling:

Where do I place the precompiled sdl.dll and sdl_net.dll libraries when I'm compiling Dosbox?

Right now, I'm failing on command ./configure :

checking for sdl-config... no checking for SDL - version >= 1.2.0... no *** The sdl-config script installed by SDL could not be […]
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checking for sdl-config... no
checking for SDL - version >= 1.2.0... no
*** The sdl-config script installed by SDL could not be found
*** If SDL was installed in PREFIX, make sure PREFIX/bin is in
*** your path, or set the SDL_CONFIG environment variable to the
*** full path to sdl-config.
configure: error: *** SDL version 1.2.0 not found!

Perhaps I just need to set a PATH variable? I've never worked with a UNIX shell before, so this is new to me.[/quote]

You need the SDL-devel package, not the dll files for compiling

Really? Per the wiki page at http://dosbox.sourceforge.net/wiki/index.php? … =BuildingDOSBox, I thought I didn't have to.

When working on Windows, you might want to save a lot of effort and just use the SDL (and optionally SDL_net) .DLL files included with the latest official DOSBox release, as compiling SDL with full options (e.g. DirectX support) is a bit involved.

Is the wiki incorrect?

(I have an older build with serial debugging enabled on my homepage)

Rudd - you might want to try this to ease the installation of SDL (if you are using MSYS/MinGW to compile):

Topic 14743