User`s guide

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Continuous Data Transfer in PCIS-DASK

The PCIS-DASK double buffer mode functions were designed according to the

principle described above. If you use

AI_AsyncDblBufferMode/DI_AsyncDblBufferMode to enable double

buffer mode, the following continuous AI/DI function will perform double-

buffered continuous AI/DI. You can call

AI_AsyncDblBufferHalfReady/DI_AsyncDblBufferHalfReady to

check if data in the circular buffer is half full and ready for copying to the

transfer buffer. Then you can call

AI_AsyncDblBufferTransfer/DI_AsyncDblBufferTransfer to copy

data from the ready half buffer to the transfer buffer.

6.2.2 Single-Buffered Versus Double-Buffered Data Transfer

Single-buffered data transfer is the most common method for continuous data

transfer. In single-buffered input operations, a fixed number of samples are

acquired at a specified rate and transferred into user’s buffer. After the user’s

buffer stores the data, the application can analyze, display, or store the data to

the hard disk for later processing. Single-buffered operations are relatively

simple to implement and can usually take advantage of the full hardware

speed of the device. However, the major disadvantage of single-buffered

operation is that the maximum amount of data that can be input at any one

time is limited to the amount of initially allocated memory allocated in driver

and the amount of free memory available in the computer.

In double-buffered operations, as mentioned above, the data buffer is

configured as a circular buffer. Therefore, unlike single-buffered operations,

double-buffered operations reuse the same buffer and are able to input or

output an infinite number of data points without requiring an infinite amount of

memory. However, there exits the undesired result of data overwritten for

double-buffered data transfer. The device might overwrite data before PCIS-

DASK has copied it to the transfer buffer. Another data overwritten problem

occurs when an input device overwrites data that PCIS-DASK is

simultaneously copying to the transfer buffer. Therefore, the data must be

processed by the application at least as fast as the rate at which the device is

reading data. For most of the applications, this requirement depends on the

speed and efficiency of the computer system and programming language.

Hence, double buffering might not be practical for high-speed input

applications.