hil_task_read :: Quanser C Functions

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hil_task_read

Reads the specified number of samples from the task buffer of a reader task.

Description

The hil_task_read function reads the specified number of samples from the task buffer of a task created using hil_task_create_reader. If there's not enough samples in the task buffer, then this function will block until the requested number of samples becomes available or the task stops. Since the task reads the hardware at the sampling rate specified in the call to hil_task_start, and stores the data in the task buffer, this function will never block for longer than the requested number of samples times the sampling period.

Because this function blocks until enough data is available and the task buffer is filled at a given sampling rate, calling this function synchronizes the caller to that sampling rate (provided the task buffer is not being filled faster than we can read the data). Thus, the hil_task_read function may be used to implement control systems, system identification, synchronous data streaming and other operations requiring a fixed sampling rate. For control systems, the num_samples parameter is typically 1, since control calculations need to be performed on each sample. For data streaming, the num_samples parameter is typically half the number of samples in the task buffer to implement double-buffering.

Warning Many cards allow the digital I/O lines to be programmed as inputs or outputs. The digital I/O lines are configured as inputs or outputs using the hil_set_digital_directions function. All the channels which will be used as digital inputs or outputs must be configured accordingly using this function. Failure to configure the digital I/O may result in the hil_task_read function failing to read or write the digital I/O as expected.

Prototype

t_error 
hil_task_read(t_task    task,
              t_uint32  num_samples,
              t_double  analog_buffer[],
              t_int32   encoder_buffer[],
              t_boolean digital_buffer[],
              t_double  other_buffer[]);

Parameters

t_task task

A handle to the task, as returned by one of the task creation functions.

t_uint32 num_samples

The number of samples to read from the task buffer. Each "sample" consists of all the input channels specified when the task was created using hil_task_create_reader. For example, if num_samples is 5 and the task is configured to read 3 analog channels and 2 encoder channels, then the analog output buffer will contain 15 elements and the encoder output buffer will contain 10 elements.

t_double [] analog_buffer

An array for receiving the voltage values read from the analog inputs. The array must contain num_analog_channels * num_samples elements, where num_analog_channels is the number of analog channels specified when the task was created. The array is organized as a linear array of samples, with each sample consisting of a group of channels. For example, if analog input channels 0, 1 and 3 are being read, than the data appears in the array as follows, where the numbers correspond to channel numbers:

...

This ordering is equivalent to defining the buffer as:

                t_double analog_buffer[num_samples][num_channels];

If the buffer is defined this way then pass the buffer as the analog_buffer argument using the syntax: &analog_buffer[0][0].

If no analog channels were specified in the call to hil_task_create_reader then this parameter may be set to NULL.

t_int32 [] encoder_buffer

An array for receiving the count values read from the encoder inputs. The array must contain num_encoder_channels * num_samples elements, where num_encoder_channels is the number of encoder channels specified when the task was created. The array is organized as a linear array of samples, with each sample consisting of a group of channels. For example, if encoder input channels 0, 1 and 3 are being read, than the data appears in the array as follows, where the numbers correspond to channel numbers:

...

This ordering is equivalent to defining the buffer as:

                t_int32 encoder_buffer[num_samples][num_channels];

If the buffer is defined this way then pass the buffer as the encoder_buffer argument using the syntax: &encoder_buffer[0][0].

If no encoder channels were specified in the call to hil_task_create_reader then this parameter may be set to NULL.

t_boolean [] digital_buffer

An array for receiving the values read from the digital inputs. The array must contain num_digital_channels * num_samples elements, where num_digital_channels is the number of digital channels specified when the task was created. The array is organized as a linear array of samples, with each sample consisting of a group of channels. For example, if digital input channels 0, 1 and 3 are being read, than the data appears in the array as follows, where the numbers correspond to channel numbers:

...

This ordering is equivalent to defining the buffer as:

                t_boolean digital_buffer[num_samples][num_channels];

If the buffer is defined this way then pass the buffer as the digital_buffer argument using the syntax: &digital_buffer[0][0].

If no digital channels were specified in the call to hil_task_create_reader then this parameter may be set to NULL.

t_double [] other_buffer

An array for receiving the values read from the other inputs. The array must contain num_other_channels * num_samples elements, where num_other_channels is the number of other channels specified when the task was created. The array is organized as a linear array of samples, with each sample consisting of a group of channels. For example, if other input channels 0, 1 and 3 are being read, than the data appears in the array as follows, where the numbers correspond to channel numbers:

...

This ordering is equivalent to defining the buffer as:

                t_double other_buffer[num_samples][num_channels];

If the buffer is defined this way then pass the buffer as the other_buffer argument using the syntax: &other_buffer[0][0].

If no other channels were specified in the call to hil_task_create_reader then this parameter may be set to NULL.

Return value

The return value is the number of samples read from the task buffer. This value may be less than the requested number of samples (including 0) if the task is stopped or has finished processing the total number of samples indicated in the call to hil_task_start. Otherwise a negative error code is returned. Error codes are defined in quanser_errors.h. A suitable error message may be retrieved using msg_get_error_message.

Error codes

QERR_HIL_TASK_READ_NOT_SUPPORTED

This function is not supported by the board-specific HIL driver for this board type.

QERR_INVALID_TASK_HANDLE

An invalid task handle was passed as an argument. Once a task has been deleted using hil_task_delete the task handle is invalid.

QERR_INVALID_OPERATION_HANDLE

An invalid operation handle was passed as an argument to the board-specific HIL driver. Once a task has been deleted using hil_task_delete the operation handle is invalid.

QERR_TOO_MANY_SAMPLES_FOR_BUFFER

The number of samples requested in the read or write operation is more than the number of samples being buffered by the task. Increase the buffer size for the task or read or write fewer samples.

QERR_MISSING_ANALOG_INPUT_BUFFER

Analog input channels have been specified but no analog input buffer has been provided for the read operation.

QERR_MISSING_ENCODER_INPUT_BUFFER

Encoder input channels have been specified but no encoder input buffer has been provided for the read operation.

QERR_MISSING_DIGITAL_INPUT_BUFFER

Digital input channels have been specified but no digital input buffer has been provided for the read operation.

QERR_MISSING_OTHER_INPUT_BUFFER

Other input channels have been specified but no other input buffer has been provided for the read operation.

QERR_READING_FROM_WRITE_ONLY_TASK

An attempt was made to read from a write-only task.

QERR_BUFFER_OVERFLOW

For a read operation, the buffer has overflowed. For a write operation, there is no more data left in the buffer. The sampling frequency is too fast for the rate at which data is being read from or written to the buffer.

QERR_DRIVER_INCOMPATIBLE_WITH_BOARD_DLL

The board-specific HIL driver passed an invalid parameter to the operating system specific kernel-level driver for the board. The board-specific HIL driver is likely not compatible with the operating system specific kernel-level driver for the board. Make sure both are up-to-date and compatible versions.

QERR_INTERNAL_BUFFER_TOO_SMALL

The board-specific HIL driver used an internal buffer that was too small for the operating system specific kernel-level driver for the board. The board-specific HIL driver is likely not compatible with the operating system specific kernel-level driver for the board. Make sure both are up-to-date and compatible versions.

QERR_OUT_OF_REQUIRED_SYSTEM_RESOURCES

There are not enough system resources to perform the requested operation. Try rebooting, requesting fewer samples, or adding more memory to your machine.

QERR_OUT_OF_MEMORY

There is not enough memory to perform the operation.

Requirements

Include Files	Libraries
hil.h	hil.lib;quanser_runtime.lib;quanser_common.lib

Examples


/*
Reads 5000 samples at 1 kHz from the first four analog input channels, and first two
encoder input channels using SYSTEM_CLOCK_1. Return values are ignored for simplicity.
*/

t_uint32 analog_channels[]  = { 0, 1, 2, 3 };
t_uint32 encoder_channels[] = { 0, 1 };
t_double frequency          = 1000;
t_uint32 samples            = 5000;
t_uint32 samples_in_buffer  = frequency;
t_uint32 samples_to_read    = 1;

static t_double analog_buffer[4];
static t_int32  encoder_buffer[2];
t_task task;

hil_task_create_reader(board, samples_in_buffer, analog_channels, ARRAY_LENGTH(analog_channels),
                       encoder_channels, ARRAY_LENGTH(encoder_channels), NULL, 0, NULL, 0, &task);
hil_task_start(task, SYSTEM_CLOCK_1, frequency, samples);
for (int index = 0; index < samples; index += samples_to_read) {
    hil_task_read(task, samples_to_read, analog_buffer, encoder_buffer, NULL, NULL);
    ...
}
hil_task_stop(task);
hil_task_delete(task);

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