Learn more about Data Acquisition Toolbox

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 Clock	Sample Clock
Sample Clock	Convert 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.