added documentation

diff --git a/demo/cmd.c b/demo/cmd.c
index d007269b24e0e6b1a99eecd61763ad6809689a5e..269ac27fcb1931a003cbb130a74744162f66a456 100644 (file)
--- a/demo/cmd.c
+++ b/demo/cmd.c
@@ -1,6 +1,7 @@
 /*
-   A little input demo for commands
-
+   An example for directly using Comedi commands.  Comedi commands
+   are used for asynchronous acquisition, with the timing controlled
+   by on-board timers or external events.
  */
 
 #include <stdio.h>
@@ -133,18 +134,43 @@ static void do_cmd_1(comedi_t *dev)
           an external digital line 3 as a source, you would use
           src=TRIG_EXT and arg=3. */
 
+       /* In this case, we specify using TRIG_NOW to start
+        * acquisition immediately when the command is issued.  
+        * The argument of TRIG_NOW is "number of nsec after
+        * NOW", but no driver supports it yet.  Also, no driver
+        * currently supports using a start_src other than
+        * TRIG_NOW.  */
        cmd.start_src =         TRIG_NOW;
        cmd.start_arg =         0;
 
+       /* The timing of the beginning of each scan is controlled
+        * by scan_begin.  TRIG_TIMER specifies that scan_start
+        * events occur periodically at a rate of scan_begin_arg
+        * nanoseconds between scans. */
        cmd.scan_begin_src =    TRIG_TIMER;
        cmd.scan_begin_arg =    1000000;        /* in ns */
 
+       /* The timing between each sample in a scan is controlled
+        * by convert.  Like above, TRIG_TIMER specifies that
+        * convert events occur periodically at a rate of convert_arg
+        * nanoseconds between scans. */
        cmd.convert_src =       TRIG_TIMER;
        cmd.convert_arg =       100000;         /* in ns */
 
+       /* The end of each scan is almost always specified using
+        * TRIG_COUNT, with the argument being the same as the
+        * number of channels in the chanlist.  You could probably
+        * find a device that allows something else, but it would
+        * be strange. */
        cmd.scan_end_src =      TRIG_COUNT;
        cmd.scan_end_arg =      4;              /* number of channels */
 
+       /* The end of acquisition is controlled by stop_src and
+        * stop_arg.  The src will typically be TRIG_COUNT or
+        * TRIG_NONE.  Specifying TRIG_COUNT will stop acquisition
+        * after stop_arg number of scans, or TRIG_NONE will
+        * cause acquisition to continue until stopped using
+        * comedi_cancel(). */
 #if 1
        cmd.stop_src =          TRIG_COUNT;
        cmd.stop_arg =          100;