An 18-slot chassis will allow you to synchronize the acquisition clocks on up to 14 PXI-4461/4462 or PXI-447x devices
1, corresponding to up to 112 DSA channels. You will need to use multiple PXI chassis to synchronize more than 14 PXI-4461/4462 or PXI-447x devices. PXI-449x cards support only reference clock synchronization, and cannot share a master timebase in the method described below. As a result, 449x cards cannot be directly synchronized with PXI-447x cards, which only support master timebase clock synchronization.
When using multiple chassis, it is necessary to use NI timing and synchronization modules, such as the PXI-6653, PXI-6652, and PXI-6651 to share the clock and trigger signals between chassis. The PXI-6653 and PXI-6652 modules are able to generate clock and trigger signals and route internally or externally generated signals
2. The PXI-6651 cannot generate signals internally. It is only used to route externally generated signals.
The PXI-665x modules use front-panel SMB connectors (like those on the PXI-4472) to send and receive clock and trigger signals and distribute these to the PXI backplane. For most multi-channel DSA systems, a PXI-665x resides in Slot 2 of each chassis. You will need one module per chassis, plus an additional module every three chassis after the first three chassis. The table below shows the relationship between chassis and modules.
| Number of PXI Chassis | Number of PXI-665x Required |
| 4 | 5 |
| 5 | 6 |
| 6 | 7 |
| 7 | 9 |
| 10 | 13 |
If two or more DSA devices are synchronized, they must share an oversample clock signal greater than the sample rate. In the PXI form factor, the oversample clock can only be driven from slot 2 on the PXI Star trigger line. Therefore, install a PXI-6653 or PXI-6652 module in Slot 2 of the master chassis. Install additional PXI-6651, PXI-6652, or PXI-6653 modules in Slot 2 of all the slave chassis. If you are synchronizing 4 or more chassis, install the remaining PXI-6653 or PXI-6652 modules in the leftmost available slots in the master chassis (Slot 3, Slot 4, and so on).
When synchronizing multiple chassis, the PXI-665X devices must be programmed to share the Sync Pulse, the Start Trigger and either the Sample Clock Timebase or Reference Clock between the chassis. In order to use the 665X devices, you must have the NI-SYNC installed in addition to NI-DAQmx. There are several examples included in the NI-SYNC download, including an example called "Share Clock Between Chassis." This example demonstrates how to share a clock source that you select between a master chassis and a slave chassis using the NI-SYNC API.
The PXI-665x ships with a VISA-based instrument driver. It can be programmed from any environment supporting DLL calls including LabVIEW, LabWindows/CVI, and Microsoft C++. In addition, the software includes a variety of excellent examples to help you quickly get started with programming a multi-chassis DSA system.
More than a dozen examples are provided for programming complex multi-chassis hardware systems in LabVIEW, LabWINDOWS/CVI, and Microsoft Visual Studio. Furthermore, LabVIEW examples are included to illustrate "Ethernet software synchronization" to use a separate PXI controller or desktop CPU for each chassis. This sort of "distributed processing" can be very helpful in managing processing or streaming bandwidth for high channel systems running at fast sampling rates.
Additional Information
1You cannot install the NI 447x devices in Slots 16, 17, or 18 of the chassis because you cannot route the oversample clock to those slots.2The difference between the NI 6652 and NI 6653 is the accuracy of the internal oscillator. The NI 6653 has an Oven Controlled Crystal Oscillator (OCXO) and can generate a 10 MHz signal with +/- 3.2 ppb accuracy. The NI 6652 has a Temperature Controlled Crystal Oscillator (TCXO) and can generate a 10 MHz signal with +/- 2.5 ppm accuracy. Thus, the NI 6653 is used when the tightest synchronization is required.