Should be understandable. Open the device, get the data,
print it out. This is basically the guts of demo/inp.c,
-without error checking or fancy options. Including all
-the appropriate headers is sometimes a little tricky.
+without error checking or fancy options.
Compile it using
@@ -61,7 +60,6 @@ Compile it using
cc tut1.c -lcomedi -o tut1
Hopefully it works.
A few notes: The range variable tells comedi which gain to use when measuring an analog voltage. Since we don't know (yet) which numbers are valid, or what each means, @@ -85,18 +83,23 @@ you probably prefer to have this number translated to a voltage. Naturally, as a good programmer, your first question is: "How do I do this in a device-independent manner?" -
For each subdevice, the comedi kernel module keeps a -'range_type' variable. This variable contains the number -of available ranges (i.e., gains) that you can select, -along with an offset in a list of range information -structures. If you know the range_type variable, you -can use these macros: -
RANGE_OFFSET(range_type) -RANGE_LENGTH(range_type) -
to extract such information. However, you want the -actual voltage information, not some integer offset -in a table. Rather than messing with the library -internals, use the function +
Most devices give you a choice of gain and unipolar/bipolar +input, and Comedi allows you to select which of these to +use. This parameter is called the "range parameter", since +it specifies the "input range" for analog input (or "output range" +analog output.) The range parameter represents both the gain +and the unipolar/bipolar aspects. +
Comedi keeps the number of available ranges and the largest +sample value for each subdevice/channel combination. (Some +devices allow different input/output ranges for different +channels in a subdevice.) +
The largest sample value can be found using the function: +
comedi_get_maxdata() +
The number of available ranges can be found using the function: +
comedi_get_n_ranges() +
For each value of the range parameter for a particular +subdevice/channel, you can get range information using the +function:
ptr=comedi_get_range(comedi_file,subdevice,channel, range)
which returns a pointer to a comedi_range structure. @@ -112,8 +115,7 @@ typedef struct{ }comedi_range; -
As you might expect, ptr[range] is for range 'range', -which you provided to comedi_data_read() above. 'min' represents +
The structure element 'min' represents the voltage corresponding to comedi_data_read() returning 0, and 'max' represents comedi_data_read() returning 'maxdata', (i.e., 4095 for 12 bit A/C converters, 65535 for 16 bit, @@ -136,12 +138,6 @@ volts=comedi_to_phys(it,data,range,maxdata); data=comedi_from_phys(it,volts,range,maxdata); -
You probably noticed (and were worried) that we haven't -discussed how to determine maxdata and range_type. Well, -you could ask the kernel this information each time you need -it, but since there are other variables, special cases, -and several subdevices to worry about, it would be nice -if the library could take care of this... (read on...)
@@ -167,7 +163,7 @@ file=comedi_open("/dev/comedi0");
calls open(), like we did explicitly in a previous
section, but also fills the comedi_t structure with
lots of goodies -- information that we will need to use
-soon.
+soon.
Specifically, we needed to know maxdata for a specific subdevice/channel. How about: