Due to the 15MHz maximum sampling rate, streaming applications or large finite acquisitions should consider the required throughput if they are using older PXI hardware.
In order to continuously transfer large amounts of data, the entire PXI Express system must be designed with sufficient data bandwidth. The PXIe-6386 and PXIe-6396 use a x4 PXI Express connection to the PXI Express chassis backplane. These modules will operate with a x1 connection, but the amount of data that can be sent to/from the host computer will be limited. Tasks using multiple channels at higher sample rates may experience buffer errors.
Additionally, the bandwidth between the PXI Express chassis and host computer can limit data throughput. When selecting a PXI Express embedded controller, make sure the controller specifies a sufficient per-slot bandwidth (for example, at least 500 MB/s slot bandwidth) and a sufficient system bandwidth to operate all devices installed in the system. If a PXI Express remote controller is selected, it should use a x4 or higher connection. A x1 PXI Express remote controller does not have sufficient bandwidth to allow all channels to acquire/generate at maximum rates.
Maximum streaming rates:
PXIe-6396: 15MS/s x 8 ch x 32-bits = 480MB/s
PXIe-6386: 15MS/s x 8 ch x 16-bits = 240MB/s
For example, a NI PXIe-1078 chassis, the backplane architecture of which is pictured below, only has Gen-1 x1 PCI Express links from each slot. Slots 5-9 have Gen-1 x1 links back through a PCIe switch which has a Gen-1 x4 link to the controller. In either case, the actual link bandwidth of a slot is limited to 250MB/s, with possible further limitations on slots 5-9 depending on other modules sharing that PCIe switch.
Figure 1: PXIe-1078 Backplane Architecture
With the PXIe link’s theoretical bandwidth of 250MB/s, applications will not be able to get the throughput needed if streaming 8 channels from the PXIe-6396. Even when using the PXIe-6386, this will push the limitations of the chassis and may cause issues.
In contrast, the NI PXIe-1085 (24GB/s variant) has Gen-3 x8 PCI Express links from each slot to the Gen-3 PCIe Switches, which have Gen-3 x8 and Gen-3 x16 connections back to the controller slot. Choosing a chassis with this high of throughput, will allow for plenty of headroom for other modules or multiple PXIe-6386's or PXIe 6396's running at maximum rates without concerns of buffer errors.
Figure 2: PXIe-1085 Backplane Architecture