Timing Considerations for C Series Devices

Note  C Series devices do not support hardware-timed single-point sample mode or Wait for Next Sample Clock.

Analog Input

You can use multiple analog input devices of different types in the same task, and NI-DAQmx automatically synchronizes them as long as they are in the same chassis. NI-DAQmx supports only one analog input task at a time per cDAQ chassis.

AI Convert Clock Considerations

The NI 9201, NI 9203, NI 9205, NI 9206, NI 9211, NI 9217, and NI 9221 use multiplexed sampling controlled by a per-slot AI Convert Clock. If you have multiple devices in one task, their AI Convert Clocks run in parallel, which may cause channels on multiple devices to be sampled at the same time. You can set the AI Convert Rate and the Delay From Sample Clock differently on each device. When setting AI Convert timing attributes/properties, you must use the ActiveDevs attribute/property to specify the device to which you are referring. External clocking of the AI Convert Clock is not supported.

The default AI Convert Clock rate for the NI 9201, NI 9203, NI 9205, NI 9206, NI 9211, NI 9217, and NI 9221 uses 10 µs of additional settling time between channels, compared to the fastest AI Convert Clock rate for the device. When the Sample Clock rate is too high to allow for 10 µs of additional settling time, the default AI Convert Clock rate uses as much settling time as is allowed by the Sample Clock rate. If there are multiple NI 9201, NI 9203, NI 9205, NI 9206, NI 9211, NI 9217, and NI 9221 devices in the same task, the same amount of additional settling time is used for all devices in the task, even if their maximum AI Convert Clock rates differ.

Sampling Rate Considerations

With the NI 9211, NI 9217, and NI 9219, if the sampling rate of a hardware-timed acquisition exceeds the maximum sampling rate of the module, the most recently acquired sample may be read multiple times and no warning or error is generated. Exceeding the maximum sampling rate of other devices in the same task generates warnings or errors. The first sample of a hardware-timed acquisition with the NI 9211, NI 9217, and NI 9219 is sampled when the task is committed. Software-timed acquisitions with the NI 9211, NI 9217, and NI 9219 always wait for a new sample to be acquired.

The maximum sampling rate of the NI 9215 depends on which channel(s) you are acquiring from. For instance, a task acquiring from any combination of ai0, ai1, and ai2 can sample at faster rates than a task that includes ai3. The maximum sampling rates are attainable only when sampling from ai0. If you have multiple NI 9215 devices in the same task, they sample in parallel. For instance, multiple NI 9215 devices acquiring from ai0 may be able to achieve a faster sampling rate than a single NI 9215 acquiring from ai3.

The NI 9225, NI 9229, NI 9233, NI 9234, NI 9235, NI 9236, NI 9237, and NI 9239 have both a maximum and a minimum sampling rate. Refer to the specifications for your device to determine the sampling rate range.

When the NI 9234 is in a task with a C Series device that has a different sample clock timebase, NI-DAQmx always chooses the sample clock timebase with the highest frequency. To override this selection, you can set the sample clock timebase in the Sample Clock Timebase Source attribute/property.

Hardware and On-Demand Timing for the NI 9225, NI 9229, NI 9233, NI 9234, NI 9235, NI 9236, NI 9237, and NI 9239

The NI 9225, NI 9229, NI 9233, NI 9234, NI 9235, NI 9236, NI 9237, and NI 9239 do not support the on-demand timing type. All NI 9225, NI 9229, NI 9233, NI 9234, NI 9237, and NI 9239 acquisitions and generations require hardware timing from a steady clock. You cannot set the Sample Clock Source attribute/property to an external source when an NI 9225, NI 9229, NI 9233, NI 9234, NI 9235, NI 9236, NI 9237, or NI 9239 is in the task. With the NI 9225, NI 9229, NI 9233, NI 9234, NI 9235, NI 9236, NI 9237, and NI 9239, external clocking from arbitrary external signal sources such as encoders and tachometers is not supported.

Analog Output

Only one hardware-timed analog output task per chassis at a given time is supported, but the number of concurrent software-timed analog output tasks is limited only by the available channels. A single C Series analog output device cannot be used for hardware-timed and software-timed tasks at the same time.

Digital Input/Output

Only one hardware-timed digital input task and one hardware-timed digital output task per chassis at a given time is supported, but the number of concurrent software-timed digital I/O tasks is limited only by the available lines.

Hardware-timed digital input/output is only supported on correlated digital I/O modules. Refer to Digital I/O Considerations for C Series for more information.