By Chris Delvizis, Product Marketing Manager in DAQ at National Instruments, [email protected]
With plenty of options for the data acquisition device and bus system, selecting the right combination is a challenge. Here’s a look at the most common PC bus options and the technical considerations to keep in mind when choosing the right bus for your measurement application.
When you have hundreds of different data acquisition (DAQ) devices compatible with a variety of buses, selecting the right bus for your application needs may be challenging. Each bus has different advantages and optimizations. To help you decide, ask yourself these five questions.
1. How much data needs to stream across this bus?
All PC buses have a limit to the amount of data that you can transfer in a certain period of time. This amount is the bus bandwidth, which is often specified in megabytes per second (MB/s). The PCI bus, for example, features a theoretical bandwidth of 132 MB/s that is shared among all PCI boards in the computer. Gigabit Ethernet offers 125 MB/s of shared bandwidth across devices on a subnet or network. PCI Express and PXI Express offer dedicated data links that are capable of up to 1 GB/s per device.
When you take waveform measurements, your bus bandwidth must support the speed at which you acquire data. To calculate the minimum required bandwidth, take the number of bytes per sample (rounded up to the next byte), multiply by the sampling speed, and then multiply by the number of channels.
For example, a 16-bit (2-byte) device sampling at 4 MS/s on four channels requires the following bandwidth:
(2 Bytes/s) x (4 MS/s) x 4 channels = 32 MB/s
Your bus bandwidth needs to support the speed at which data is being acquired. Also, note that the actual observed system bandwidth will be lower than the theoretical bus limits. It depends on the number of devices in a system and any additional bus traffic from overhead. If you need to stream a lot of data on a large number of channels, bandwidth may be the most important consideration when choosing your DAQ bus.
2. What are the single-point I/O requirements?
Applications that require single-point reads and writes often depend on I/O values that are updated immediately and consistently. Bus latency is the time delay between when a driver software function is called and the actual hardware value of the I/O is updated. Depending on the bus you choose, this delay can range from less than a microsecond to a few milliseconds. Another important factor in single-point I/O applications is determinism, which is a measure of how consistently I/O can execute on time.
Buses that always have the same latency when communicating with I/O are more deterministic than buses with varied responsiveness.
Latency and determinism are important for control applications because they directly impact the reliability of the control loop. Therefore, when implementing closed-loop control applications, you should avoid buses such as wireless, Ethernet, or USB that are high in latency with poor determinism.
The software side of how a communications bus is implemented plays a big part in bus latency and determinism too. Buses and software drivers that have support for real-time OSs provide the best determinism and therefore give you the highest performance. In general, internal buses such as PCI Express and PXI Express are better for low-latency single-point I/O applications.
3. Do I need to synchronize multiple devices?
Many measurement systems have complex synchronization needs, whether you’re synchronizing hundreds of input channels or multiple types of instruments. The simplest way to synchronize measurements across multiple devices is to share a sample clock and a trigger. Many DAQ devices offer programmable digital lines for importing and exporting both clocks and triggers.
Certain buses, such as PCI and PCI Express, work with the real-time system integration (RTSI) bus, on which you can cable multiple boards in a desktop system directly together inside the case to make synchronization as easy as possible. This removes the need for additional wiring through the front connector and simplifies I/O connectivity.
The best bus option for synchronizing multiple devices is the PXI platform, including PXI and PXI Express. This open standard was designed specifically for high-performance synchronization and triggering, with several different options for synchronizing I/O modules within the same chassis or multiple chassis.
4. How portable must this system be?
Portability is important for many applications. External buses like USB and Ethernet are particularly effective for portable DAQ systems because of their quick hardware installation and compatibility with laptop computers. Bus-powered USB devices offer additional convenience because they are powered from the USB port and do not require a separate power supply. Wireless data transfer buses are another good option because the measurement hardware itself becomes portable while the computer can remain stationary.
5. How far will my measurements be from my computer?
The distance between the measurements you need and the computer’s location can drastically vary from application to application. To achieve the best signal integrity and measurement accuracy, place your DAQ hardware as close to the signal source as possible. Running cables across long distances is costly and can result in noisy signals. A solution to this problem is to use a portable computing platform to move the entire system closer to the signal source. Wireless technology can altogether remove the physical connection between the computer and the measurement, so you can take distributed measurements and send the data back to a central location.
Overview of DAQ Buses
While there are many different buses and form factors to choose from, this section focuses on the seven most common buses.
The Peripheral Component Interconnect (PCI) bus is one of the most commonly used internal computer buses today. With a shared bandwidth of 132 MB/s, PCI offers high-speed data streaming and deterministic data transfer for single-point control applications. There are many different DAQ hardware options for PCI, with multifunction I/O boards up to 10 MS/s and up to 18-bit resolution.
PCI Express is an evolution of PCI. The single biggest benefit of PCI Express architecture is the dedicated bus bandwidth provided by independent data transfer lines. Unlike PCI, in which 132 MB/s of bandwidth is shared among all devices, PCI Express uses independent data lanes that are each capable of data transfer up to 250 MB/s.
The PCI Express bus is also scalable from a single x1 (pronounced “by one”) data lane to x16 data lanes 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 and data throughput rates, and multiple devices do not have to compete for time on the bus.
The Universal Serial Bus (USB) was originally designed to connect peripheral devices, such as keyboards and mice, with PCs. However, it has proven useful for other applications, including measurement and automation.
USB delivers an inexpensive and easy-to-use connection between DAQ devices and PCs. USB 2.0 has a maximum theoretical bandwidth of 60 MB/s, which is shared among all devices connected to one USB controller. USB devices are inherently latent and nondeterministic. This means that single-point data transfers may not happen exactly when expected, and therefore USB is not recommend for closed-loop control applications, such as PID.
On the other hand, the USB bus has several characteristics that make it easier to use than some traditional internal PC buses. Devices that connect using USB are hot-pluggable. The bus also has automatic device detection, meaning that users do not have to manually configure their devices after plugging them in. Once the software drivers have been installed, the OS should detect and install the device on its own.
PCI eXtensions for Instrumentation (PXI) were 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 this open standard, and in 2006, passed the PXI Express specification to deliver PCI Express data transfer technology to the PXI platform.
Based on CompactPCI, PXI incorporates instrumentation extensions and more rigid system-level specifications to ensure an open yet high performance specification for measurement and automation. The benefits of PXI-based DAQ systems include rugged packaging that can withstand the harsh conditions that often exist in industrial applications. PXI systems also offer a modular architecture, which means that you can fit several devices in the same space as a single stand-alone instrument, and you have the ability to expand your system beyond the capacity of a desktop computer with a PCI bus. One of the most important benefits PXI offers is its integrated timing and triggering features. Without any external connections, multiple devices can be synchronized by using the internal buses resident on the backplane of a PXI chassis.
Ethernet is widely available. As a bus for DAQ, Ethernet is ideal for taking portable or distributed measurements at distances beyond the 5 m length of a USB cable. A single Ethernet cable can extend 100 m before needing a hub, switch, or repeater. This distance in combination with a large install base of networks in labs, offices, and manufacturing facilities makes it a good choice for distributing measurements to remote locations.
Though available network bandwidth is dependent on the number of networked devices, 100BASE-T (100 Mbit/s) Ethernet can accommodate multiple Ethernet DAQ devices running at full speed. In addition, Gigabit Ethernet (1000BASE-T) can aggregate data from multiple 100BASE-T networks or higher speed devices for larger systems.
Wireless technology extends the flexibility and portability of PC-based data acquisition to measurement applications where cables are inconvenient or impractical, such as wind farms or civil structures. While wireless reduces costs by eliminating cables and installation time, it also has the highest latency of any other DAQ bus, so applications requiring high-speed control or determinism are not recommended. There are many different wireless technologies available. The most popular is IEEE 802.11 (Wi-Fi).
Wi-Fi is among the easiest of wireless technologies to set up. Connecting to a Wi-Fi “hotspot” is as familiar to most as plugging in a USB cable. After 10 years of use in the IT sector, Wi-Fi is also secure. IEEE 802.11i (WPA2) is the highest commercially available wireless security standard to date with 128-bit AES encryption and IEEE 802.1x authentication. For streaming dynamic waveform signals, Wi-Fi provides more bandwidth than other wireless technologies, making it ideal for machine condition monitoring and other high-speed applications.
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