When implementing a PXI system, choosing the right system architecture is very important. The system architecture will affect not only the actual performance of a system, but also the ability to expand the system in future. There are various architectures to choose from based on your application needs. One of the key benefits of using the PC-based, modular PXI platform is that there are almost no software changes required if you want to move from one system architecture type to another due to changes in your applications or future expansion.
This white paper discusses the various system architectures, advantages and constraints of each, and how you can combine different system architectures to create hybrid systems.
2. PXI Embedded Controller and Chassis
In this architecture, there is a single PXI chassis with a PXI embedded controller. This is the simplest and most common architecture. The PXI embedded controller dictates features like processing speed and streaming to disk, while the size of the chassis determines how many modules can physically fit in the system. For example, a 4-slot chassis can fit up to 3 peripheral modules compared to an 18-slot chassis which can fit up to 17 peripheral modules.
Figure 1: 18-slot PXI chassis with a PXI embedded controller in the leftmost slot (slot 1)
- Simplest hardware and software implementation
- Industrial form factor and small foot print
- Windows, Linux and Real-Time OS such as LabVIEW Real-Time
- Less vendors than commercially available PCs
3. PC Control of PXI
One way to achieve lower cost and high performance is to move to an architecture using an external PC. While PXI embedded controllers may be high performing, they don't always offer all of the configuration flexibility that PCs do. With MXI-Express and MXI-4 interface kits, users can control PXI systems directly from desktop, workstation, or server computers. During boot-up, the computer will recognize all peripheral modules in the PXI system as PCI/PCI Express devices since the links are transparent to software application and drivers. The fiber-optic cabling of MXI-4 provides electrical isolation between the PC and PXI chassis, as well as cabling up to 200 m per link. Using MXI-Express you can control your PXI system with a sustained throughput of up to 832 MByte/s. This enables you to take advantage of the PXI platform as well as all the benefits and flexibility of the desktop PC.
Figure 2: PC Control of PXI with MXI-Express
- Lower cost
- PC/workstations/servers offer cutting edge technology
- Cabling up to 200m with electrical isolation
- Since an external PC is part of the system it is not an industrial form factor
4. Laptop Control of PXI
Using the ExpressCard MXI or PCMCIA CardBus you can directly control a PXI system from a laptop computer. During boot-up, the computer will recognize all peripheral modules in the PXI system as PCI/PCI Express devices since the links are transparent to software application and drivers. This architecture is ideal for mobile and portable applications and you can pair it with DC-powered chassis to provide mobile solutions for applications such as field tests, in-vehicle data logging, NVH, NDT and RF testing. Using ExpressCard MXI you can control your PXI system with a sustained throughput of up to 214 MByte/s. With both PC and Laptop control of PXI, you can maximize processor performance with minimized cost by using a desktop or laptop computer since they are software transparent and requires no additional programming.
ExpressCard MXI interface kit
PCMCIA CardBus interface kit
Figure 3: Laptop Control of PXI
· Mobile and portable
· Lower cost
· Since a laptop computer is part of the system it is not an industrial form factor
5. Multiple PXI Chassis with PC or PXI Embedded Controller
Embedded controller, PC and Laptop control of PXI architectures are all limited by the number of slots available in a single PXI chassis, but many high channel count applications require more than the 17 modules usable in the largest PXI chassis. One way to increase the channel count of the system is adding more PXI chassis to the system.
You can add additional chassis with MXI-4 in either a daisy-chain or star configuration. In the daisy-chain configuration, connect the PXI chassis in series with each other. Every time data crosses a PCI bridge, the expected total data throughput across that bridge is cut in half. This restriction in data throughput in the daisy-chain configuration is caused by unequal PCI bus arbitration between all of the slots and can be particularly important in high throughput applications. One way to get around this limitation is to use a star configuration or the 2 port MXI-Express. In the star configuration, the PXI chassis are all connected directly to the PC. This means that the PC has several MXI-4 PCI boards to it, each connected to a single chassis. In this case, the maximum throughput to each individual chassis is the same and limited only by the total throughput the PC can sustain. By using a PC with multiple independent buses, you can increase the aggregate throughput that the system can handle when in a star configuration. For more information on topologies for multichassis configurations, refer to the MXI-4 Series User Manual.
Figure 4: Daisy chain configurations using MXI-4 : (A) Daisy-Chain Configuration from a PC and (B) Daisy-Chain Configuration from an PXI Embedded Controller
IIn the MXI-Express 2-port interface kit, the PCI Express board (PCIe-8362/72) has two cabled PCI Express links, each of which you can cable to individual PXI chassis to provide a sustained throughput of up to 832 MByte/s (two chassis combined). With multiple PCI Express slots in the computer you can control several PXI systems with dedicated bandwidth to each PXI system. This enables you to take advantage of the high channel count instrumentation of PXI as well as all the benefits and flexibility of the desktop PC.
Figure 5: The 2-port MXI-Express provides simultaneous
control of two PXI chassis with combined sustained throughput of up to 832 MByte/s.
- Can achieve higher channel counts
- Lower per channel cost compared to multiple embedded controllers or PCs
- Can have chassis up to 200m apart with MXI-4 fiber optic cabling
- Increased channel count means increased demands on the PC and controller
6. Distributed Architecture
The distributed architecture breaks the system into nodes, where each node is a self-sufficient system in itself. A distributed architecture is very scalable when it comes to channel count. You can add another node to the overall system in order to increase the channel count.
The distributed architecture brings some added complexities. Each node can function independently; however in a large system, it is key that all the nodes function together to meet the overall system requirements. You must coordinate hardware and software synchronization through a designated “master” node. While all of the slave nodes might be exactly the same in both their hardware configuration and application software, the master will have any extra hardware needed as well as a different software implementation.
For systems with only a few nodes, the master might also double as a functional node. In larger systems, the master is a node dedicated solely to controlling and synchronizing the entire system.
- Highly flexible and scalable
- Can synchronize multiple chassis up to 200m apart
- Complex implementation of hardware and software
7. Hybrid Architecture
Hybrid architecture combines components from multiple ATE platforms such as PXI, PCI, GPIB, VXI, USB, or LAN/LXI into one system. By implementing the hybrid architecture, engineers can easily integrate new components into their existing system without a system redesign. Since PXI is based of the PC platform it has the standard I/O interfaces such as USB, serial, GPIB, LAN and other standard PC peripherals. This feature coupled with the high bandwidth-low latency PCI/PCI Express bus in the backplane, gives PXI the ability to seamlessly integrate with any ATE platform.
Figure 7: Hybrid Architecture
- Increases longevity of the test system
- Maximize return on investment
- Complex system implementation
Each system architecture has advantages and should be chosen based on specific needs. Any time you set up a PXI system it is important to think ahead about current application needs, cost constraints, performance, future expandability, and choose the architecture that best meets all your needs. If you are integrating the PXI system into an already existing ATE system it could be done seamlessly due to the modularity and flexibility of the PXI platform.
9. Relevant NI Products and Whitepapers
National Instruments, a leader in automated test, is committed to providing the hardware and software products engineers need to create these next generation test systems.
- NI TestStand Test Management Framework
- LabWindows/CVI ANSI C Development Environment
- LabVIEW Graphical Programming Environment
- Signal Express Interactive Measurement Software
- Modular Instruments (Oscilloscopes, Multimeters, RF, Switching, and more)
- Multi-function Data Acquisition
- PXI System Components (Chassis and Controllers)
- Instrument Control (GPIB, USB, and LAN)
Test System Development Resource Library
National Instruments has developed a rich collection of technical guides to assist you with all elements of your test system design. The content for these guides is based on best practices shared by industry-leading test engineering teams that participate in NI customer advisory boards and the expertise of the NI test engineering and product research and development teams. Ultimately, these resources teach you test engineering best practices in a practical and reusable manner. Download guides from the Test System Development Resource Library.