1.  Should I use CONFIG_MODVERSIONS when compiling a Linux kernel?

Short answer:  No.  Long answer: MODVERSIONS allows you to safely
use a module on multiple kernel versions.  However, making comedi
work with CONFIG_MODVERSIONS is a real pain.  Sometimes it works,
sometimes it doesn't.


2. I get the error message:

Makefile:14: /usr/src/linux/.config: No such file or directory
make: *** No rule to make target `/usr/src/linux/.config'.  Stop.


It means you don't have the file /usr/src/linux/.config.  This file
contains all the configuration information about your linux kernel,
which comedi needs in order to compile correctly.  Some distributions
don't contain this file, so you will need to create your own, by
compiling your own kernel.  This file is automatically created
when you run 'make config', the first step in compiling a kernel.


3. When compiling programs that use libcomedi, I get error message:

/usr/lib/libcomedi.a(range.o): In function `comedi_from_phys':
range.o(.text+0x169): undefined reference to `floor'


This error messsage indicates that the linker cannot resolve the
reference to floor(), which is a function in the math library
that is used in the function comedi_from_phys().  This only
happens when you link statically.  Add '-lm' to your linking
command.


4. Someone on the mailing list told me to look in the kernel log.
How do I do that?

Kernel log messages are continuously written to the file
/var/log/messages.  In addition, a shortcut to get recent log
messages is the 'dmesg' command, which will print the last 4-16 kB
(depending on the kernel) of kernel log messages.


5. I think I found a bug: I get Oops messages in the kernel log.

Oops messages always indicate a bug.  If you are not using the
latest version of Comedi, please upgrade and try again, since
these types of bugs are usually quickly fixed.  Look at the file
BUG_REPORTING in the Linux kernel source, follow the instructions,
and send the information to David Schleef <ds@schleef.org>.


6. Could you please add a search engine to the mailing list archive?

No.  Please use generic web search engines such as Google, where you
can add the search term "site:stm.lbl.gov" to limit the search to
the site where the Comedi archives are located.


7. I apply a signal to channel 0, but I see the signal on other
channels.

Typically, this is an electrical problem that can come from multiple
sources.  First, any inputs that are not specifically driven or
grounded will tend to float around, measuring random electrical
signals in their environment -- the strongest of which are usually
nearby channels, 50/60 Hz line noise, or the A/D converter itself.


8. An FFT of measured data shows lots of sharp peaks.

Yep.  Not much that can be done about that.  Well, actually, there is.
Peaks near multiples of 50 or 60 Hz are electromagnetic coupling from
the power mains.  Almost all the time, this indicates a ground loop
in the measurement path.  Peaks that are multiples of 100 or 120 Hz
on photodetectors are because of the room lights.  Peaks in the 100-1000
Hz range, with harmonics, are sometimes stray fields from electric
motors.  Strong, sharp peaks in the 1-100 kHz range are typically due
to the horizontal refresh of CRT displays (like your computer monitor).
Less strong peaks can be due to electronics in a computer or other
device.

However, a sharp peak in an FFT does not always indicate a problem.
Remember that you always have noise, and that some of it will be
periodic.  You just have to make sure that the particular type of
noise that you have does not affect your analysis.


9. Ok, I'm getting good results.  What should I look at to get great
results?

 - Make sure that all unused inputs are grounded.  Electrically floating
   wires act as antennas, picking up stray electrical signals, and often
   strongly coupling them into the signals you actually want to measure.

 - Get a flat panel monitor.  CRT displays emit strong magnetic fields
   oscillating at the horizontal refresh rate.  It is most easily
   observable in a Fourier transform of an input signal.

 - Calibrate your board.

 - Read up on ground loops.  Make sure that all your electrical
   shielding has exactly one path to a common ground.

 - If you use a laptop for data acquisition, make sure it is grounded
   properly.  Laptops usually are not grounded, and you generally 
   don't want your data acquisition cable to also be your grounding
   wire.

 -