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Programming with the PicoScope 4000 Series8

4.6

Sampling modes

PicoScope 4000 Series PC Oscilloscopes can run in various sampling modes.

Block mode. In this mode, the scope stores data in internal RAM and then

transfers it to the PC. When the data has been collected it is possible to examine

the data, with an optional aggregation factor. The data is lost when a new run is

started in the same segment, the settings are changed, or the scope is powered

down.

Rapid block mode. This is a variant of block mode that allows you to capture

more than one waveform at a time with a minimum of delay between captures.

You can use aggregation in this mode if you wish.

Streaming mode. In this mode, data is passed directly to the PC without being

stored in the scope's internal RAM. This enables long periods of slow data collection

for chart recorder and data-logging applications. Streaming mode provides fast

streaming at up to 6.6 MS/s (150 ns per sample). Aggregation and triggering are

supported in this mode.

In all sampling modes, the driver returns data asynchronously using a

callback.

This

is a call to one of the functions in your own application. When you request data from

the scope, you pass to the driver a pointer to your callback function. When the driver

has written the data to your buffer, it makes a

callback

(calls your function) to signal

that the data is ready. The callback function then signals to the application that the

data is available.

Because the callback is called asynchronously from the rest of your application, in a

separate thread, you must ensure that it does not corrupt any global variables while it

runs.

In block mode, you can also poll the driver instead of using a callback.

4.6.1

Block mode

In block mode, the computer prompts a PicoScope 4000 series PC Oscilloscope to

collect a block of data into its internal memory. When the oscilloscope has collected

the whole block, it signals that it is ready and then transfers the whole block to the

computer's memory through the USB port.

Block size. The maximum number of values depends upon the size of the

oscilloscope's memory. The memory buffer is shared between the enabled

channels, so if two channels are enabled, each receives half the memory. These

features are handled transparently by the driver. The block size also depends on

the number of memory segments in use (see ps4000MemorySegments).

Sampling rate. The PicoScope 4000 Series PC Oscilloscopes can sample at a

number of different rates according to their model, selected timebase and the

combination of channels that are enabled. The maximum sampling rate can be

achieved with a single channel enabled, or with these two-channel combinations:

AC, AD, BC and BD. All other combinations limit the maximum sampling rate of

scope, as specified in its Data Sheet.

Setup time. The driver normally performs a number of setup operations, which

can take up to 50 milliseconds, before collecting each block of data. If you need to

collect data with the minimum time interval between blocks, use rapid block mode

and avoid calling setup functions between calls to ps4000RunBlock, ps4000Stop

and ps4000GetValues.