Synchronization

Synchronization of data acquisition operations between multiple channels or devices has two aspects:

Start trigger: The signal to initiate all operations
Scan clock: The timing for repeated generation or acquisition of signals at a clocked rate

Synchronization can involve the coordination of triggering, clocking, or both. To synchronize the start of operations on multiple channels or devices, they must use a shared start trigger. To synchronize the clocked scanning operations on multiple channels or devices, they must use a shared scan clock.

The following definitions summarize some concepts of synchronization:

Type of Synchronization	Description
Start trigger synchronization	Channels or devices are configured to simultaneously start their operations from a shared start trigger.
Scan clock synchronization	Channels or devices use a shared scan clock to generate or measure signals.
Perfect synchronization	Channels or devices use both a shared start trigger and a shared scan clock. This does not imply a specific skew or latency performance between devices or between channels on a device.
Automatic synchronization	The default start trigger synchronization and scan clock synchronization supported by a DataAcquisition, the driver, and the hardware. This is the extent of synchronization provided by a DataAcquisition without any explicit synchronization configuration. When a DataAcquisition starts, it sends a start trigger signal to all connected channels in the DataAcquisition. The driver and device might support synchronization from that moment forward. For example, in some devices all channels use the same internal scan clock and a shared start trigger, so they are automatically synchronized without further configuration of the DataAcquisition.

Shared Triggers and Shared Scan Clocks

Typical data acquisition devices provide synchronization between their channels of the same subsystem. For example, all the analog input channels on one card use a shared scan clock. A DataAcquisition can configure start trigger and scan clock connections for wider synchronization needs. Use shared start triggers and shared scan clocks to synchronize data between:

Multiple subsystems in a device (analog input, analog output, counter input, etc.)
Multiple devices
Multiple CompactDAQ or PXI chassis

Note

Counter output channels run independently and are unaffected by synchronization connections.

Source and Destination Devices

You can share start triggers and scan clock connections to synchronize operations within a DataAcquisition. Synchronization connections can be:

Devices in a DataAcquisition connected to a start trigger or scan clock source on another device in the DataAcquisition
Devices and chassis in a DataAcquisition connected to a start trigger or scan clock source on another device in the DataAcquisition

A source device and terminal pair generates the synchronization signal and is connected to the destination device and terminal pairs. You must physically connect the source and destination terminals, unless they are internally connected. Check your device specifications for more information. Synchronization connections are added from the source device to one or more destination devices.

The source device provides the start trigger or scan clock signals.
The destination device receives a start trigger or scan clock signal.

For example, if you determine that a terminal on Dev1 will provide a start trigger and a terminal on Dev2 will receive that trigger, then Dev1 becomes your source device and Dev2 your destination device. You can have multiple destinations for your trigger and clock connections, but only one source.

Use addtrigger to add start trigger connections, and addclock to add a scan clock connection to your DataAcquisition.

Automatic Synchronization

In most cases, a DataAcquisition automatically starts all its devices at the same time when you start an operation. You must configure them to start synchronously when devices are not on a single chassis and do not share a clock. If you have not configured synchronization on such devices, the start operation reduces the latency between devices, running them very close together to achieve near-simultaneous signals. However, devices are automatically and perfectly synchronized in the DataAcquisition if they are:

Subsystems on a single device in the DataAcquisition. This synchronizes your analog input, analog output, and counter input channels.
Note
Counter output channels run independently and are unaffected by synchronization connections.
Modules on a single CompactDAQ chassis in the DataAcquisition.
PXI modules synchronized with a reference clock on a PXI chassis. For perfect synchronization, you must share a trigger as well. See Acquire Synchronized Data Using PXI Devices for more information.

Synchronization Scenarios

You must employ different techniques for synchronization, depending on the configurations of your channels, devices, and chassis. The following sections describe these different scenarios.

Multiple Channels on the Same Device or Module

In this topic, hardware that performs the signal conversion when not plugged into a chassis is referred to as a device; this includes USB devices. When the conversion hardware is a card plugged into a chassis, it is usually referred to as a module.

Data Acquisition Toolbox™ DataAcquisition software is based on the assumption that all channels of the same acquisition device or module use the same internal scan clock and start trigger. As such, these channels meet the requirements for perfect synchronization. For most vendors, this includes digital channels, analog channels, and counter input channels, but does not include counter output channels.

The following topics illustrate this scenario, providing automatic synchronization between multiple channels.

Exceptions: Some devices do not support setting the source of the start trigger or do not internally route start trigger signals between subsystems. These include National Instruments™ myDAQ and USB-6002. In such devices, only channels of the same subsystem support start trigger synchronization by default.

Multiple Modules in the Same CompactDAQ Chassis

Modules in the same CompactDAQ chassis use the chassis scan clock and start trigger. The Data Acquisition Toolbox DataAcquisition interface configures the chassis scan clock rate and issues the start trigger signal. The chassis in turn provides synchronized signals to its modules.

The following examples illustrate this scenario, providing synchronization between multiple modules in the same chassis without external connections or extra programming.

Exceptions: Some CompactDAQ modules have their own onboard clocks, for example, DSA modules.

Multiple Modules in the Same PXI Chassis

Modules in a PXI chassis share a common scan clock, but a Data Acquisition Toolbox DataAcquisition does not synchronize the start trigger for multiple modules in the chassis by default. The start triggers of multiple DSA modules can be synchronized using the AutoSyncDSA property, while other PXI modules require an external trigger connection for start trigger synchronization.

The following topics illustrate these scenarios, showing how to synchronize start triggers on multiple modules.

Multiple Devices Without Chassis or in Different Chassis

This scenario represents multiple devices or modules in their most independent configuration. The configuration could be multiple USB devices, for example, or modules in separate chassis. Neither the start triggers nor the scan clocks of these devices are synchronized by default.

The following topics illustrates these scenarios, showing how to synchronize start triggers and scan clocks on multiple devices without chassis or in different chassis, by way of an external connection.