NI-TClk Overview

NI-TClk Synchronization Help

Edition Date: August 2016

Part Number: 370956M-01

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Use TClk synchronization to allow multiple instruments to simultaneously respond to triggers, to align Sample Clocks on multiple instruments, and/or to simultaneously start multiple instruments.

The NI-TClk library allows you to synchronize NI modular instruments in the following situations:

  • In a single PXI chassis or in multiple PXI chassis
  • In a single PC
  • When an individual instrument uses triggers for any purpose. For example, when you want two different modular instruments to share a Start Trigger or when you want to use the Start Trigger from one modular instrument as the Reference Trigger on another modular instrument.
  • When Sample Clock rates are equal, and/or when the Sample Clock rates are different.
Note  Refer to NI-TClk External Sample Clock and External Sample Clock Timebase Considerations and NI-TClk Sample Clock Delay for more information about using the Sample Clocks.

NI-TClk does not limit the number of modules or chassis that you can synchronize.

The TClk Signal

The Trigger Clock (TClk) is a periodic signal used to synchronize triggers. The devices do not share the TClk signal. Each device generates the TClk signal from its Sample Clock or Sample Clock timebase. Use the Sync Pulse and Sync Pulse Clock signals to synchronize TClk signals on different devices.

When using an internal Sample Clock, devices must have a common Reference Clock. When using an external Sample Clock, devices must have a common Sample Clock or Sample Clock timebase.

Note   Common Sample Clock timebases apply only to devices that support decimation, such as digitizers.

Example

The following figure shows an example of synchronizing two devices that have internal Sample Clocks and a common 10 MHz Reference Clock.

Device 1 is the Start Trigger master, with a Sample Clock rate of 50 MHz. Device 2 is the Start Trigger slave, with a Sample Clock rate of 20 MHz. The TClk frequency is 5 MHz.

The Effect of TClk on Trigger Response Time

Since devices react to TClk-synchronized triggers with delays and uncertainties that are comparable to the TClk period (200 ns or more), the trigger reaction time for synchronized devices is higher than that for individual devices. However, this condition does not impact time stamping of the Reference Triggers for acquisitions, so the Reference Trigger positions are correctly reported for TClk-synchronized devices.

Instrument Drivers and NI-TClk

An instrument driver is a library of VIs or functions that control individual devices. For example, NI-SCOPE, NI-FGEN, NI-RFSA, NI-RFSG, and NI-HSDIO are instrument drivers. NI-TClk is a library designed for use with instrument drivers.

High-level NI-TClk VIs or functions are different from typical instrument driver VIs or functions because the TClk VIs or functions accept arrays of sessions instead of individual sessions. A session is a software concept for communicating and interacting with a particular physical instrument or a subset of a physical instrument. For example, you can use an NI-SCOPE session to communicate with a digitizer, and an NI-HSDIO acquisition session to communicate with the acquisition subset of a digital waveform generator/analyzer.

In LabVIEW, obtain these sessions from your instrument driver, by using the <driver name> Get Session Reference VI. In C, NI-TClk accepts regular instrument driver sessions.

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