Selecting the Right Bus for Your Measurement Application

NI LabVIEW 2009 continues the intuitive graphical development environment and seamless integration of data acquisition hardware and PC buses. With more than 200 different hardware devices on multiple buses, how do you choose a bus to meet your application needs? This white paper discusses the available buses and outlines the various factors you need to consider when choosing the best bus for your measurement application.

Table of contents

Five questions you need to answer when choosing the best bus

1. How much data will I be transferring over this bus?

2. What are my single-point I/O requirements?

3. Do I need to sync multiple devices?

4. How portable should the system be?

5. How far are my measurements from my computer?

How much data will I be transferring over this bus?

All PC buses have a certain limit on the amount of data that can be transferred in a certain period of time. This is called the bus bandwidth and is usually expressed in megabytes per second (MB/s). If dynamic waveform measurements are important in your application, you must consider a bus with sufficient bandwidth.

The total bandwidth can be shared among multiple devices or dedicated to a single device, depending on the bus you choose. For example, the PCI bus provides a theoretical bandwidth of 132 MB/s shared among all PCI devices in the computer. Buses that provide dedicated bandwidth, such as PCI Express and fast PXI Express, achieve maximum data throughput for each device.

When making waveform measurements, you need a certain sampling rate and accuracy, which is determined based on your signal frequency. You can calculate the minimum bandwidth required by multiplying the number of bytes per sample point (if less than one byte, round it up to one byte) by the sampling rate and the number of channels.

For example, a 16-bit device (two bytes) with four channels sampled at 4 MS/s has:

Your bus bandwidth needs to be able to keep up with the rate of the signals being acquired, and it is important to note that the actual system bandwidth will be less than the theoretical bandwidth of the bus. The actual bandwidth depends on the number of devices in the system and any additional bus traffic introduced by overhead. If you need to transfer a lot of data over many channels, bandwidth may be the most important consideration in choosing your data acquisition bus.

What are my single-point I/O requirements?

Applications that require single-point reads and writes often require consistency and timeliness of I/O values. Based on how the bus architecture is implemented in hardware and software, the single-point I/O requirement may become the determining factor in your bus selection.

Bus latency is the responsiveness characteristic of an I/O. It is the time delay between the time a driver software function is called and the time the actual hardware value of that I/O is updated. Depending on the bus you choose, this latency can range from less than a microsecond to several milliseconds.

For example, in a PID control system, this bus delay may directly affect the maximum rate of the control loop.

Another important factor in single-point I/O applications is determinism, which is a measure of how consistently the I/O performs. A bus that always has the same latency when communicating with the I/O has a higher degree of determinism than a bus whose response characteristics can change. Determinism is important in control applications because it directly affects the reliability of the control loop, and many control algorithms are designed based on the assumption that the control loop will always execute at a constant rate. Any deviation from the expected rate will make the entire control system less efficient.

Discussing how the communication bus is implemented from a software perspective plays an important role in bus latency and determinism. Buses and software drivers that support real-time operating systems will provide the best determinism, which in turn gives you the highest performance. In general, internal buses, such as PCI Express and PXI Express, are better suited for low-latency single-point I/O applications than external buses (such as USB or wireless).

Do I need to sync multiple devices?

Many measurement systems have complex synchronization requirements, whether it is synchronizing hundreds of channels or synchronizing multiple types of instruments. For example, a stimulus-response system may require that output channels share the same sample clock and use triggers as input channels to implement I/O related operations and better analyze their results. Data acquisition devices on different NI buses provide this capability. Almost all NI data acquisition (DAQ) devices provide access to programmable multifunction input (PFI) ports, which can be used to route clock and trigger signals between different devices, and conveniently configure these PFI ports through software in NI-DAQmx. However, some buses have additional, built-in timing and trigger lines to make synchronization of multiple devices as convenient as possible. PCI and PCI Express devices provide a real-time system integration (RTSI) bus, through which multiple cards in a desktop system can be connected directly with wires inside the chassis. This eliminates the need for additional wiring from the front connector and simplifies I/O connections.

For synchronization of multiple devices, the best bus choice is the PXI platform, including PXI and PXI Express. This open standard is particularly suitable for high-performance synchronization and triggering, and has a large number of different options for synchronizing I/O modules within the same chassis and synchronizing multiple chassis.

How portable should the system be?

The introduction of portable computing has given engineers and scientists new and innovative ways to utilize PC-based data acquisition. Portability is an important factor for many applications and can easily become the primary reason for choosing one bus over another. For example, vehicle-mounted data acquisition applications benefit from compact and easily transportable hardware. External buses, such as USB and Ethernet, are particularly well suited for portable data acquisition systems because of their quick hardware installation and compatibility with portable computers. Bus-powered USB devices offer additional convenience because they do not require a discrete power supply. Using a wireless data transmission bus is another good option for achieving portability because the measurement hardware itself becomes portable while the computer remains in its original location.

How far are the measurements from my computer?

The distance between where you need to make measurements and where the computer is located can vary depending on the application. For the best signal integrity and measurement accuracy, you should place your data acquisition hardware as close to the signal source as possible. This can be a challenge for large-scale distributed measurements, such as those for structural health monitoring or environmental monitoring. Running long cables across bridges or factory floors is not only expensive, but also introduces noise. The solution to this problem is to move the acquisition system closer to the signal source using a portable computing platform. With wireless technology, the wired connection between the computer and the measurement hardware is removed, allowing you to make distributed measurements and transmit that data back to a central location. [page]

Selection guide for the most common buses

Based on the five questions outlined previously, Table 1 presents a selection guide for the most common data acquisition buses available.

Table 1. This table shows a bus selection guide based on application requirements and NI example products.

1Maximum theoretical data rates based on the following bus specifications: PCI, PCI Express 1.0, PXI, PXI Express 1.0, USB 2.0, 100Mbps Ethernet, and Wi-Fi 802.11g.

Data Acquisition Bus Overview

While there are many different buses and form factors to choose from, this section focuses on the seven most common buses, including:

• PCI
• PCI Express
• USB
• PXI
• PXI Express
• Ethernet
• wireless

Figure 1 organizes these buses into the PC-bus architecture of NI data acquisition products, from plug-in to hot-swappable external buses.

[+] Enlarge image

Figure 1. A variety of buses are available to meet your data acquisition needs.

PCI

Figure 2. PCI M Series Multifunction DAQ

The Peripheral Component Interconnect (PCI) bus is one of the most commonly used internal computer networks today. With a shared bandwidth of 132 MB/s, PCI provides high-speed data transfer and deterministic data transfer performance for single-point control applications. PCI has many different data acquisition hardware options, including multifunction I/O boards with up to 10 MS/s and up to 18-bit accuracy.

PCI Express

Figure 3. PCI Express X-Series Multifunction DAQ

PCI Express is an evolution of PCI that brings a new level of innovation to the PC industry. One of the biggest advantages of the PCI Express architecture is the dedicated bus bandwidth provided by independent data transfer lines. Unlike PCI, where the 132 MB/s bandwidth is shared by all devices, PCI Express uses independent data paths, each of which can transfer data at a rate of up to 250 MB/s.

The PCI Express bus can also be expanded from a single x1 (pronounced "by 1") data lane to a x16 data lane for a maximum throughput of 4 GB/s, enough to fill a 200 GB hard drive in less than a minute. For measurement applications, this means higher, sustained sampling rates and data throughput rates so that multiple devices do not have to compete for the bus. [page]

Learn about the new X Series DAQ devices for PCI Express

USB

Figure 4. USB bus-powered M Series with direct BNC connection

The Universal Serial Bus (USB) was originally designed to connect peripheral devices (such as keyboards and mice) to PCs. However, it has proven very useful in many other applications including measurement and automation. USB provides a low-cost, easy-to-use connection between data acquisition devices and PCs. High-speed USB 2.0 has a maximum theoretical bandwidth of 60 MB/s, which is shared by all devices connected to a single USB controller. USB devices are inherently latency-prone and non-deterministic. This means that single-point data transfers will not occur exactly as expected, and therefore, USB is not suitable for high-performance control applications.

On the other hand, the USB bus has several features that make it easier to use than some traditional internal PC buses. USB devices are hot-swappable, so they eliminate the tedious step of shutting down the PC to add or remove devices. The bus also has automatic device detection, which means that users do not have to manually configure their devices after plugging them in. Once the software driver installation is complete, the operating system detects and installs the device itself.

View NI CompactDAQ Sensor Systems for USB 2.0

PXI Platform

Figure 5. The PXI platform consists of a backplane, controller, and I/O modules.

PCI eXtensions for Instrumentation (PXI) was developed to bridge the gap between desktop PC systems and high-end VXI and GPIB systems. The PXI Systems Alliance, with more than 200 members, maintains the open standard and in 2006, approved the PXI Express specification to implement PCI Express data transfer technology on the PXI platform.

Based on CompactPCI, PXI incorporates instrument system expansion and stricter system level specifications to ensure openness and higher performance for measurement and automation. The technical advantages of PXI's data acquisition system include the ability to withstand the harsh environments often found in industrial applications through rugged packaging. The PXI system also provides a modular architecture, which means that you can assemble multiple devices in the same chassis and use them as a single independent instrument, and you can expand your system to make its functions far beyond a PC with a PCI bus. Another of the most important technical advantages provided by PXI is its integrated timing and triggering functions. Without any external connections, multiple devices can be synchronized using the internal bus integrated on the backplane of the PXI chassis.

Ethernet

Figure 6. NI Ethernet DAQ for C Series Modules

Ethernet is the backbone of nearly every corporate network in the world, making it widely available. As a bus for data acquisition, Ethernet is well suited for portable or distributed measurements over distances beyond the 5-meter reach of a USB cable. A single Ethernet cable can extend up to 100 meters before a hub, switch, or repeater is required. This distance, combined with the large installed base of networks in labs, offices, and manufacturing facilities, makes Ethernet ideal for long-distance distributed measurements. While the available network bandwidth depends on the number of networked devices, 100BASE-T (100 MB/s) Ethernet can accommodate Ethernet data acquisition devices. In addition, Gigabit Ethernet (1000BASE-T) can accommodate more data from a 100BASE-T network to support larger systems.

wireless technology

Figure 7. NI Wi-Fi DAQ for C Series Modules

Wireless technology extends the flexibility and portability of PC-based data acquisition to measurement applications that were previously difficult to wire, such as wind farms or civil installations. Wireless technology significantly reduces costs by eliminating wiring and installation. However, wireless technology has higher latency than any other data acquisition bus and is therefore not recommended for applications that require high-speed control or determinism. There are many different wireless technologies available, the most popular of which is IEEE 802.11 (Wi-Fi).

Wi-Fi is among the easiest wireless technologies to set up. Connecting to a Wi-Fi "hotspot" is very similar to plugging in a USB cable. After 10 years of use in IT departments, Wi-Fi has also become secure. IEEE 802.11i (WPA2) is the most stringent wireless security standard commercially available today, with 128-bit AES encryption and IEEE 802.1X authentication. For data streaming of dynamic waveform signals, Wi-Fi offers higher bandwidth than other wireless technologies, making it ideal for machine condition monitoring and other high-speed applications.