ClockSource

Specify clock that governs hardware conversion rate

Description

For all supported hardware except Measurement Computing™ analog output subsystems, ClockSource can be set to Internal, which specifies that the acquisition rate is governed by the internal hardware clock.

Use this table to map to the National Instruments® terminology.

Data Acquisition Toolbox™NI_DAQmx
Scan ClockSample Clock
Sample ClockConvert Clock

For subsystems without a hardware clock, you must use software clocking to govern the sampling rate. Software clocking allows a maximum sampling rate of 500 Hz and a minimum sampling rate of 0.0002 Hz. An error is returned if more than 1 sample of jitter is detected. Note that you might not be able to attain rates over 100 Hz on all systems.

Characteristics

Usage

AI, AO, common to all channels

Access

Read/write

Data type

String

Read-only when running

Yes

Values

Advantech

{Internal}

The internal hardware clock is used (AI only).

External

Externally control the channel clock (AI only).

Software

The computer clock is used.

Measurement Computing

{Internal}

The internal hardware clock is used.

External

Externally control the channel clock.

Software

The computer clock is used.

National Instruments

{Internal}

The internal hardware clock is used.

External

Externally control the channel clock (AO only).

ExternalSampleCtrl

Externally control the channel clock. This value overrides the ChannelSkew property value (AI only). This value does not apply to cards with simultaneous sample and hold.

    Note:   If you set ClockSource to ExternalSampleCtrl then the value of ExternalSampleClockSource specifies the pin whose signal is used as the channel clock for conversions on each channel.

ExternalScanCtrl

Externally control the scan clock. This value overrides the SampleRate property value (AI only).

    Note:   If you set ClockSource to ExternalScanCtrl then the value of ExternalScanClockSource specifies the pin whose signal is used as the scan clock to initiate conversions across a group of channels.

ExternalSampleAndScanCtrl

Externally control the channel and scan clocks. This value overrides the ChannelSkew and SampleRate property values (AI only). This value does not apply to cards with simultaneous sample and hold.

    Note:   If you set ClockSource to ExternalSampleAndScanCtrl then the value of ExternalSampleClockSource specifies the pin whose signal is used as the channel clock for conversions on each channel, and the value of ExternalScanClockSource specifies the pin whose signal is used as the scan clock to initiate conversions across a group of channels.

    Note:   If you set the ClockSource property to one of the External options, you must also set the SampleRate property to a value close to the external clock rate. SampleRate does not directly affect the external device, and the device will not use SampleRate if you have set an external clock rate, but this ensures that the toolbox configures itself correctly for expected data rates.

Sound Cards

{Internal}

The internal hardware clock is used.

See Also

Properties

ChannelSkew, SampleRate

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