The first bus we will look at is the IEEE 488 bus, familiarly known as GPIB (general-purpose interface bus). GPIB is a proven bus designed specifically for instrument control applications. GPIB has been a robust, reliable communication bus for 30 years and is still the most popular choice for instrument control because of its low latency and acceptable bandwidth. It currently enjoys the widest industry adoption, with a base of more than 10,000 instrument models with GPIB connectivity.
With a maximum bandwidth of about 1.8 MB/s, it is best suited for communicating with and controlling stand-alone instruments. The more recent, high-speed revision, HS488, increased bandwidth up to 8 MB/s. Transfers are message-based, often in the form of ASCII characters. Multiple GPIB instruments can be cabled together to a total distance of 20 m, and bandwidth is shared among all instruments on the bus. Despite relatively lower bandwidth, GPIB latency is significantly lower (better) than that of USB and especially Ethernet. GPIB instruments do not autodetect or autoconfigure when connected to the system; though GPIB software is among the best available, and the rugged cable and connector are suitable for the most demanding physical environments. GPIB is ideal for automating existing equipment or for systems requiring highly specialized instruments.
USB (universal serial bus) has become popular in recent years for connecting computer peripherals. That popularity has spilled over into test and measurement, with an increasing number of instrument vendors adding USB device controller capabilities to their instruments.
Hi-Speed USB has a maximum transfer rate of 60 MB/s, making it an attractive alternative for instrument connectivity and control of stand-alone and some virtual instruments with data rates below 1 MS/s. Though most laptops, desktops, and servers may have several USB ports, those ports usually all connect to the same host controller, so the USB bandwidth is shared among all the ports. Latency for USB falls into the better category (between Ethernet at the slow end and PCI and PCI Express at the fast end), and cable length is limited to 5 m. USB devices benefit from autodetection, which means that unlike other technologies such as LAN or GPIB, USB devices are immediately recognized and configured by the PC when a user connects them. USB connectors are the least robust and least secure of the buses examined here. External cable ties may be needed to keep them in place.
USB devices are well-suited for applications with portable measurements, laptop or desktop data logging, and in-vehicle data acquisition. The bus has become a popular communication choice for stand-alone instruments due to its ubiquity on PCs and especially due to its plug-and-play ease of use. The USB Test and Measurement Class (USBTMC) specification addresses the communication requirements of a broad range of test and measurement devices.
PCI and PCI Express achieve the best bandwidth and latency specifications among all the instrumentation buses examined here. PCI bandwidth is 132 MB/s, with that bandwidth shared across all devices on the bus. PCI latency performance is outstanding; benchmarked at 700 ns, compared to 1 ms in Ethernet. PCI uses register-based communication. Unlike the other buses mentioned here, PCI does not cable to external instruments. Instead, it is an internal PC bus used for PC plug-in cards and in modular instrumentation systems such as PXI, so distance measures do not directly apply. Nonetheless, the PCI bus can be “extended” by up to 200 m by the use of NI fiber-optic MXI interfaces when connecting to a PXI system. Because the PCI connection is internal to the computer, it is probably fair to characterize the connector robustness as being constrained by the stability and ruggedness of the PC in which it resides. PXI modular instrumentation systems, which are built around PCI signaling, enhance this connectivity with a high-performance backplane connector and multiple screw terminals to keep connections in place. Once booted with PCI or PXI modules in place, Windows automatically detects and installs the drivers for modules.
An advantage that PCI (and PCI Express) share with Ethernet and USB is that they are universally available in PCs. PCI is one of the most widely adopted standards in the history of the PC industry. Today, every desktop PC has either PCI slots, PCI Express slots, or both. In general, PCI instruments can achieve lower costs, because these instruments rely on the power source, processor, display, and memory of the PC that hosts them, rather than incorporating that hardware in the instrument itself.
PCI Express is similar to PCI. It is the latest evolution of the PCI standard, much as Hi-Speed USB is to USB. Therefore, much of the above evaluation of PCI applies to PCI Express as well.
The main difference between PCI and PCI Express performance is that PCI Express is a higher bandwidth bus and gives dedicated bandwidth to each device. Of all the buses covered in this tutorial, only PCI express offers dedicated bandwidth to each peripheral on the bus. GPIB, USB, and LAN, divide bandwidth across the connected peripherals. Data is transmitted across point-to-point connections called lanes at 250 MB/s per direction. Each PCI Express link can be composed of a multiple lanes, so the bandwidth of the PCI Express bus depends on how it is implemented in the slot and device. A x1 (by 1) link provides 250 MB/s; a x4 link provides 1 GB/s; and a x16 link provides 4 GB/s dedicated bandwidth. It is important to note that PCI Express achieves software backward compatibility, meaning that users moving to the PCI Express standard can preserve their software investments in PCI. PCI Express also extensible by external cabling.
High-speed, internal PC buses were designed for rapid communication. Consequently PCI and PCI Express are ideal bus choices for high-performance, data-intensive systems where large bandwidth is required, and for integrating and synchronizing several types of instruments.
Ethernet has long been an instrument control option. It is a mature bus technology and has been widely used in many application areas outside of test and measurement. 100BaseT Ethernet has a theoretical max bandwidth of 12.5 MB/s. Gigabit Ethernet, or 1000BaseT, increases the max bandwidth to 125 MB/s. In all cases, Ethernet bandwidth is shared across the network. At 125 MB/s Gigabit Ethernet is theoretically faster than Hi-Speed USB, but this performance quickly declines when multiple instruments and other devices are sharing network bandwidth. Communication along the bus is message based, with communication packets adding significant overhead to data transmission. For this reason, Ethernet has the worst latency of the bus technologies featured in this tutorial.
Nonetheless, Ethernet remains a powerful option for creating a network of distributed systems. It can operate at distances up to 85 to 100 m without repeaters and with repeaters has no distance limits. No other bus has this range of separation from the controlling PC or platform. As with GPIB, autoconfiguration is not available on Ethernet/LAN. Users must manually assign an IP address and subnet configuration to their instrument. Like USB and PCI, Ethernet/LAN connections are ubiquitous in modern PCs. This makes Ethernet ideal for distributed systems and remote monitoring. It is often used in conjunction with other bus and platform technologies to connect measurement system nodes. These local nodes may themselves be composed of measurement systems relying on GPIB, USB, and PCI. Physical Ethernet connections are more robust than USB connections, but less so than GPIB or PXI.
LXI (LAN eXtenstions for Instrumentation) is an emerging LAN-based standard. The LXI standard defines a specification for stand-alone instruments with Ethernet connectivity that adds triggering and synchronization features.