Here is an example of how to use the command (perhaps you should read
its <command>man</command> page now):
<screen>
-/usr/sbin/comedi_config /dev/comedi0 ni_atmio 0x260,3
+PATH=/sbin:/usr/sbin:/usr/local/sbin:$PATH
+comedi_config /dev/comedi0 labpc-1200 0x260,3
</screen>
This command says that the “file”
<filename>/dev/comedi0</filename> can be used to access the &comedi;
-device that uses the <parameter>ni_atmio</parameter> driver, and that
+device that uses the <parameter>labpc-1200</parameter> board, and that
you give it two run-time parameters (<literal>0x260</literal> and
<literal>3</literal>). More parameters are possible, for example to
discriminate between two or more identical cards in your system.
</para>
<para>
If you want to have the board configured in this way every time you
-boot, put the line above into a start-up script file of your Linux
+boot, put the lines above into a start-up script file of your Linux
system (for example, the
-<filename>/etc/rc.d/rc.local</filename> file), or in the system-wide
-&comedi; configuration file <filename>/etc/comedi.conf</filename>.
-You can, of course, also run this command at a command prompt.
+<filename>/etc/rc.d/rc.local</filename> file), or for PCMCIA
+boards the appropriate place is the /etc/pcmcia/comedi script.
+For non-PCMCIA boards, you can also arrange to have your driver
+loaded and comedi_config run with by adding a few lines
+to /etc/modules.conf (see the INSTALL file for the comedi
+kernel modules). You can, of course, also run comedi_config
+at a command prompt.
</para>
<para>
This tutorial goes through the process of configuring &comedi;
for two devices, a
-<literal>National Instruments AT-MIO-16E-10</literal> (which has the
-driver mentioned above), and a
+<literal>National Instruments AT-MIO-16E-10</literal>, and a
<literal>Data Translation DT2821-F-8DI</literal>.
</para>
<para>
-The NI board is plug-and-play, and the <command>man</command> page
-tells you that you need to configure the PnP part of the board with
-<command>isapnptools</command>. The <command>isapnptools</command>
-package is a little cryptic, but the concepts are simple. Once you've
-learned how to use it, you can settle on a
-<filename>/etc/isapnp.conf</filename> file such as this:
-</para>
-
+The NI board is plug-and-play. The current ni_atmio driver
+has kernel-level ISAPNP support, which is used by default
+if you do not specify a base address. So you could simply
+run comedi_config as
<screen>
-# ANSI string -->National Instruments, AT-MIO-16E-10<--
-(CONFIGURE NIC2400/10725401 (LD 0
- (IO 0 (BASE 0x0260))
- (INT 0 (IRQ 3 (MODE +E)))
-# (DMA 0 (CHANNEL 5))
-# (DMA 1 (CHANNEL 6))
- (ACT Y)
-))
+comedi_config /dev/comedi0 ni_atmio
</screen>
-
-<para>
-(This file also contains a few lines about overall configuration and
-about the sound card that happens to be in the same computer.)
-Currently, the driver doesn't use DMA, but it may in the future, so
-the DMA lines are commented out. It has been reported that the
-National Instruments board does not always work with interrupts other
-than IRQ 3, and that the device ignores the IRQ and DMA information
-given here. However, keep the information here to remind yourself that
-the numbers aren't arbitrary.
-</para>
-
<para>
-The <command>man</command> page
-explains that the option list is supposed to be
-“<literal>(I/O base),(IRQ)</literal>”, so use the same
-numbers as in <filename>/etc/isapnp.conf</filename>, i.e.,
-<literal>0x260,3</literal>.
-</para>
-<para>
For the <literal>Data Translation</literal> board, you need to have a
list of the jumper settings; these are given in the &comedi; manual
section about this card. (Check first to see whether they are still
-correct!)
+correct!)
The card discussed her is a <literal>DT2821-f-8di</literal>. The
<command>man</command> page of <command>comedi_config</command> tells
you that you need to know the I/O base, IRQ, DMA 1, DMA 2. However,
</screen>
and the full configuration command is:
<screen>
-/usr/sbin/comedi_config /dev/comedi1 dt2821-f-8di 0x200,4,,1,1,1
+comedi_config /dev/comedi1 dt2821-f-8di 0x200,4,,1,1,1
</screen>
The differential/single-ended number is left blank, since the
driver already knowns (from the board name), that it is
</section>
-
+<section>
+<title>
+Anti-aliasing
+</title>
+<para>
+If you wish to aquire accurate waveforms, it is vital that you use an
+anti-alias filter. An anti-alias filter is a low-pass filter used to
+remove all frequencies higher than the Nyquist frequency (half your sampling rate)
+from your analog input signal
+before you convert it to digital. If you fail to filter your input signal,
+any high frequency components in the original analog signal will create
+artifacts in your recorded digital waveform that cannot be corrected.
+</para>
+<para>
+For example, suppose you are sampling an analog input channel at a rate of
+1000 Hz. If you were to apply a 900 Hz sine wave to the input, you
+would find that your
+sampling rate is not high enough to faithfully record the 900 Hz input,
+since it is above your Nyquist frequency of 500 Hz. Instead, what you
+will see in your recorded digital waveform is a 100 Hz sine wave! If you
+don't use an anti-alias filter, it is impossible to tell whether the 100
+Hz sine wave you see in your digital signal was really produced by a
+100 Hz input signal, or a 900 Hz signal aliased to 100 Hz, or a 1100 Hz
+signal, etc.
+</para>
+<para>
+In practice, the cutoff frequency for the anti-alias filter is usually
+set 10% to 20% below the Nyquist frequency due to fact that real filters
+do not have infinitely sharp cutoffs.
+</para>
+</section>
</section>
projects / comedilib.git / commitdiff