Specifications

Comedi
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int comedi_dio_bitfield2(comedi_t *device, unsigned int subdevice, unsigned int write_mask, unsigned int *bits, unsigned int
base_channel);
Each channel from base_channel to base_channel + 31 is assigned to a bit in the write_mask and bits bitfield with bit
0 assigned to channel base_channel, bit 1 assigned to channel base_channel + 1, etc. If a bit in write_mask is set, the
corresponding bit in
*
bits will be written to the digital output line corresponding to the channel given by base_channel plus
the bit number. Each digital line is then read and placed into
*
bits. The value of bits in
*
bits corresponding to digital output
lines is undefined and device-specific. Channel base_channel + 0 is the least significant bit in the bitfield. No more than 32
channels at once can be accessed using this method. Warning! Older versions of Comedi may ignore base_channel and treat
it as 0 unless the subdevice has more than 32 channels.
The digital acquisition functions seem to be very simple, but, behind the implementation screens of the Comedi kernel module,
they are executed as special cases of the general instruction command.
4.1.2 Single analog acquisition
Analog Comedi channels can produce data values that are samples from continuous analog signals. These samples are integers
with a significant content in the range of, typically, 8, 10, 12, or 16 bits.
Single samples can be read from an analog channel using the function comedi_data_read():
int comedi_data_read(comedi_t *device, unsigned int subdevice, unsigned int channel, unsigned int range, unsigned int aref,
lsampl_t *data);
This reads one such data value from a Comedi channel, and puts it in the user-specified data buffer.
The range parameter is the zero-based index of one of the gain ranges supported by the channel. This is a number from 0 to N-1
where N is the number of ranges supported by the channel. Use the function comedi_get_n_ranges() to get the number of
ranges supported by the channel, the function comedi_find_range() to search for a suitable range, or the function comed-
i_get_range() to get the details of a supported range.
The aref parameter specifies an analog reference to use: AREF_GROUND, AREF_COMMON, AREF_DIFF, or AREF_OTHER.
Use the function comedi_get_subdevice_flags() to see which analog references are supported by the subdevice.
In the opposite direction, single samples can be written to an analog output channel using the function comedi_data_writ-
e():
int comedi_data_write(comedi_t *device, unsigned int subdevice, unsigned int channel, unsigned int range, unsigned int aref,
lsampl_t data);
Raw data values read or written by the above functions are unsigned integers less than, or equal to, the maximum sample value
of the channel, which can be determined using the function comedi_get_maxdata():
lsampl_t comedi_get_maxdata(comedi_t *device, unsigned int subdevice, unsigned int channel);
Conversion between raw data values and uncalibrated physical units can be performed by the functions comedi_to_phys()
and comedi_from_phys():
double comedi_to_phys(lsampl_t data, comedi_range *range, lsampl_t maxdata);
lsampl_t comedi_from_phys(double data, comedi_range *range, lsampl_t maxdata);
There are some data structures in these commands that are not fully self-explanatory:
• comedi_t: this data structure contains all information that a user program has to know about an open Comedi device. The
programmer doesn’t have to fill in this data structure manually: it gets filled in by opening the device.
• lsampl_t: this ‘data structure’ represents one single sample. On most architectures, it’s nothing more than a 32 bits value.
Internally, Comedi does some conversion from raw sample data to ‘correct’ integers. This is called ‘data munging’.
• comedi_range: this holds the minimum and maximum physical values for a gain range supported by a channel of a subdevice,
and specifies the units. This can be used in combination with the channel’s ‘maxdata’ value to convert between unsigned
integer sample values (of type lsampl_t or sampl_t) and physical units in a nominal (uncalibrated) way using the comedi_t-
o_phys() and comedi_from_phys() functions. Use the comedi_get_maxdata() function to get the ‘maxdata’ value
for the channel.