Instrument Control Bus Comparison (GPIB, USB, PCI, PCI Expr

Instrument Control Bus Comparison (GPIB, USB, PCI, PCI Express, and Ethernet/LAN/LXI)

GPIB The first bus we examine is the IEEE 488 bus, more familiarly known as GPIB (General Purpose Interface Bus). GPIB is an industry-proven bus designed for instrument control applications. GPIB has been a robust, reliable communications bus for the past 30 years and remains the most common choice for instrument control today due to its low latency and acceptable bandwidth. The advantage of GPIB is that it is widely adopted by the industry, with over 10,000 instrument models featuring a GPIB interface.

With a maximum bandwidth of 1.8 MB/s, GPIB is best suited for communicating with and controlling discrete instruments. The latest high-speed version, HS488, increases bandwidth to 8 MB/s. Data transfer in GPIB uses a message-based communication model, and ASCII characters are most commonly used. Multiple GPIB instruments can be connected by cables with a total distance of 20 meters, and the bandwidth is shared by all instruments on the bus. Although GPIB has relatively low bandwidth, its latency is much lower than USB and especially Ethernet (i.e., better performance). Although GPIB has the best software available and stable cables and connectors that can withstand the harshest physical environments, GPIB instruments are not automatically detected or automatically configured when connected to a system. GPIB is ideal for automation of existing instruments or systems that require highly specialized instruments.
USB In recent years, USB (Universal Serial Bus) has become increasingly popular for connecting computer peripherals. This popularity has spread to the test and measurement field, and more and more instrument manufacturers are adding USB device controller capabilities to their instruments. Hi-Speed ​​USB has a maximum transfer rate of 60 MB/s, making it an attractive option for connecting and controlling instruments (both discrete instruments and virtual instruments with data rates below 1 MS/s). Although most laptops, desktops, and servers may have multiple USB ports, those ports are usually connected to the same host controller, so the USB bandwidth is shared among the ports. USB latency is intermediate (between Ethernet, which has the highest latency, and PCI and PCI Express, which have the lowest latency). The upper limit for cable length is 5 meters. The advantage of USB devices is that they are automatically detected. Unlike other LAN or GPIB technologies, USB devices are instantly recognized and configured by the PC when they are connected. Of all the buses studied here, the USB connector is the least robust and least secure. External cable covers are required to keep it properly secured. USB devices are well suited for applications including portable measurements, data logging for laptops or desktops, and in-vehicle data acquisition. Because of its popularity on PCs, especially its plug-and-play ease of use, the bus has become a popular communication method for discrete instruments. The USB Test and Measurement Class (USBTMC) specification describes the communication requirements for a wide range of test and measurement equipment.
PCI Of all the buses examined here, PCI and PCI Express have the best bandwidth and latency specifications. PCI has a bandwidth of 132 MB/s, which is shared by all devices on the bus. PCI has a baseline latency performance of 700 ns, which is excellent compared to Ethernet, which has a latency of 1 ms. PCI uses register-based communication. Unlike the other buses mentioned here, PCI does not connect to external instruments via cables. Instead, PCI is an internal PC bus for PC plug-in cards and modular instrumentation systems such as PXI, so distance measurements are not directly applicable. However, when connecting to a PXI system, the PCI bus can be "extended" up to 200 meters using the NI fiber-optic MXI interface. Because PCI connections are used internally in computers, it stands to reason that the robustness of the PCI connector may be limited by the stability and robustness of the PC in which it is connected. The PXI modular instrument system is built around PCI signaling and is connected through high-performance backplane connectors and multiple screw terminals to enhance connectivity. If the PCI or PXI module is installed properly, Windows will automatically detect and install drivers for the module after the system starts. The common advantage of PCI (and PCI Express) with Ethernet and USB is that they are ubiquitous on PCs. PCI is one of the most widely adopted standards in the history of PCs. Today, every desktop computer can provide PCI slots or PCI Express slots. Generally speaking, PCI instruments require lower costs because these instruments rely on the power, processor, display and memory of the host computer where they are located, and no longer need to configure these hardware separately in the instrument.

PCI Express
PC IExpress is similar to PCI. It is the latest evolution of the PCI standard, similar to the relationship between Hi-Speed ​​USB and USB. Therefore, much of what has been said above about PCI also applies to PCI Express. The main performance difference between PCI Express and PCI is that the PCI Express bus has a higher bandwidth and can allocate dedicated bandwidth to each device. Of all the buses discussed in this article, only PCI Express can provide dedicated bandwidth to each peripheral bus. GPIB, USB, and LAN all share bandwidth among all connected peripherals. In PCI Express, data is transferred at a speed of 250 MB/s in one direction over point-to-point connections called "narrowbands." Each PCI Express connection can be made up of multiple narrowbands, so the bandwidth of the PCI Express bus depends on how it is implemented in the slots and devices. A x1 (1 narrowband) connection provides 250 MB/s of bandwidth, a x4 (4 narrowbands) connection provides 1 GB/s of bandwidth, and a x16 (16 narrowbands) connection provides 4 GB/s of dedicated bandwidth. It is worth noting that PCI Express provides backward software compatibility, meaning that users who migrate to the PCI Express standard can retain their PCI software investment. PCI Express is also expandable via external cables. The high-speed, internal PC bus was designed for fast communications. Therefore, PCI and PCI Express are ideal bus choices for high-performance, data-intensive systems that require large bandwidth and for systems that integrate and synchronize multiple types of instruments.
Ethernet/LAN/LXI Ethernet has long been a choice for instrument control. It is a mature bus technology that has been widely used in many applications outside of test and measurement. 100BaseT Ethernet technology has a maximum theoretical bandwidth of 12.5 MB/s. Gigabit Ethernet or 1000BaseT increases the maximum bandwidth to 125 MB/s. In all cases, Ethernet bandwidth is shared by the entire network. Gigabit Ethernet has a theoretical bandwidth of 125 MB/s, which is faster than high-speed USB, but its performance degrades dramatically when multiple instruments and other devices share the network bandwidth. The bus uses message-based communication, and some header information added to the communication packet significantly increases the data transmission overhead. Given this, Ethernet has the worst latency of all the bus technologies in this article. Nevertheless, Ethernet is still a strong choice for creating distributed system networks. Ethernet has a maximum operating distance of 85 to 100 meters without repeaters, and there is no distance limit if repeaters are used. No other bus can support such long separation distances from the controlling PC to the platform. Like GPIB, Ethernet/LAN does not support automatic configuration. Users must manually assign IP addresses and subnet configuration to their instruments. Similar to USB and PCI, Ethernet/LAN connections are ubiquitous in modern PCs. This makes Ethernet an ideal choice for distributed systems and remote monitoring. Ethernet technology is often used in conjunction with other bus and platform technologies to connect measurement system nodes. These local nodes themselves may consist of measurement systems connected via GPIB, USB, and PCI. Ethernet physical connections are much more stable than USB connections, but less robust than GPIB or PXI.
LXI (LAN eXtensions for Instrumentation) is an upcoming LAN-based standard. The LXI standard defines specifications for discrete instruments with Ethernet connections, adding triggering and synchronization features.
Summary: Instrument Bus Performance
Although it is conceptually convenient to designate a single bus or communication standard as the "final" or "ideal" technology, history tells us that several alternative standards are likely to continue to coexist as each bus technology has its own unique advantages and disadvantages.
Test system developers can create hybrid systems that take advantage of the strengths of multiple buses and platforms. Hybrid test and measurement systems combine components from modular instrumentation platforms (such as PXI and VXI) and discrete instruments connected via GPIB, USB, and Ethernet/LAN. The key to creating and maintaining a hybrid system is to implement a system architecture that transparently recognizes multiple bus technologies and leverages an open, multi-vendor computing platform (such as PXI) for I/O connectivity. Another key to successfully developing a hybrid system is to ensure that the software you choose at the driver, application, and test system management levels is modular. Although some vendors offer vertically integrated software solutions for specific instruments, the most useful system architectures break down software functionality into interchangeable modular layers that do not lock your system into a specific piece of hardware or vendor. This layered approach provides the best code reuse, modularity, and lifecycle. For example, VISA (Virtual Instrument Software Architecture) is a vendor-neutral software standard for configuring, programming, and troubleshooting instrument systems consisting of interfaces such as GPIB, VXI, serial (RS232/485), Ethernet, USB, and/or IEEE 1394. VISA is a very useful tool because its API for programming VISA functions is similar to the APIs for multiple communication interfaces.
With a hybrid system, you can combine the advantages of multiple types of instruments, including legacy and specialized devices. Although it is very attractive to find a big unified solution for instrumentation, engineering practice requires test engineers to use instruments and related bus technologies that meet their specific application requirements.
Related NI Products
Software:
• NI TestStand test management framework
• LabVIEW graphical programming environment
• Signal Express interactive measurement software
Hardware:
• Modular instruments (oscilloscopes, multimeters, RF, switches, and others)
• Multifunction data acquisition
• PXI system components (chassis and controllers)
• Instrument control (GPIB, USB, and LAN)