When you are faced with a variety of instruments connected to the bus, it may be difficult to make the most appropriate choice for your application. It can be said that each bus has its own advantages and corresponding optimization techniques. Therefore, please ask yourself the following four questions, compare the function options of the most common PC bus, you can make a decision.
â— What bus can be used on instruments and computers? â— What kind of bus performance do I need? â— What environment will the instrument be used in? â— How easy is it to set up and configure the bus?More information about the instrument control bus
â— Common Bus Selection Guide â— Instrument Control Hardware Bus Overview 1. What bus can be used on instruments and computers?An instrument typically provides one or more bus options for instrument control; the PC also typically provides multiple bus options for instrument control. If you don't have a bus connected to an instrument on your PC, you can add a bus through a board or an external converter. There are many types of buses used for instrument control, which can be roughly divided into the following categories:
• Independent bus for connection to rack-mounted instruments, including test and measurement-specific buses such as the GPIB bus, as well as other PC standard buses such as serial bus (RS232), Ethernet bus and USB bus. You can also use some independent buses as a medium for other independent bus transfers, such as USB to GPIB converters. • Interface buses embedded in modular instruments include PCI, PCI Express, VXI, and PXI. You can also use these buses as a medium to add separate buses to PCs that do not have separate buses, such as using NI PCI-GPIB controller boards. 2. What kind of bus performance do I need?The three main factors that affect bus performance are bandwidth, latency, and instrument implementation.
• Bandwidth is the rate at which data is transmitted, which is typically measured in units of megabits per second. ◠Delay is the time of data transmission, usually in seconds. For example, when transmitting over Ethernet, large blocks of data are broken down into small segments and then sent as multiple packets. The delay is the transmission time of one of the packets. • Instrument implementation of bus software, firmware, and hardware will affect bus performance. Not all instruments are born consistently. Whether it is a user-defined virtual instrument or a traditional instrument designed by the manufacturer, the compromises used in the specific implementation of the instrument will affect the performance of the instrument. One of the benefits of virtual instruments is that the end user, as the designer of the instrument, can make the best compromise decision in the process of instrument implementation.
Figure 1. Comparing the theoretical bandwidth and latency of the mainstream test and measurement bus.
3. What environment will the instrument be used in?When developing an instrument control application, it is important to fully consider its deployment environment. The main factors you need to consider are the distance between the instrument and the PC, and the robustness of the interface and cable. These two factors are critical when selecting a bus for an instrument control system.
Distance from instrument to PC
If your instrument is close to the PC (less than 5 meters), you have the flexibility to choose any bus type. If your instrument is away from the PC, for example, in another room or in another building, you should consider the architecture of the distributed instrument control system. Distributed instrument control systems may include expanders, repeaters, LAN/LXI, or LAN converters (eg, Ethernet to GPIB converters).
Ruggedness of interfaces and cables
If your instrument is in a noisy environment, such as an industrial environment, consider using an interface bus that provides protection to isolate environmental interference. For example, in a production hall, GPIB or USB would be a more appropriate option because its cable is securely locked and has a rugged shielding rating.
4. How easy is it to set up and configure the bus?When you are selecting the bus interface, please pay attention to its settings and installation methods. Some instruments are deployed in places where there is a lot of user interaction, such as in a lab. This is why you should consider the SUB bus interface, which is very convenient to use and consistent with user habits. For instrument control systems that need to be considered for safety, you should be aware that the information technology department may prohibit the use of buses such as Ethernet/LAN/LXI. If you are certain that Ethernet/LAN/LXI is the best bus interface for your instrument control system, then when you deploy it in an environment where security is a concern, you should be involved in information technology throughout the design implementation process. The departments work together.
5. Instrument Control Hardware Bus Overview GPIBThe General Purpose Interface Bus (GPIB) is one of the most common I/O interfaces in standalone instruments. GPIB is an 8-bit parallel digital communication interface with data transfer rates up to 8 Mb/s. A GPIB controller bus can connect up to 14 instruments with a routing distance of less than 20 meters. However, you can overcome these limitations by using GPIB expanders and extenders. GPIB cables and connectors are available in a wide range of industrial grades and can be used in any environment.
GPIB is not a PC industrial bus and is rarely used on PCs. However, you can add GPIB instrument control functions to your PC using a single board, such as PCI-GPIB, or an external converter such as NI GPIB-USB.
Serial busThe serial bus is a device communication protocol primarily used on older desktops and laptops. Please don't confuse it with USB. Among many devices, the serial bus is the most common instrument communication protocol, and many GPIB-compatible devices also have an EIA232 port. EIA232 and EIA485/EIA422 can also be referred to as RS232 and RS485/RS422.
The concept of serial communication is simple. The serial port sends and receives one bit of information at a time. Although it is slower than parallel communication for each entire byte transfer, the serial bus is simpler and uses a longer distance.
Typically, engineers use a serial interface to transfer ASCII data. They use three transmission lines to complete the communication: ground, transmit, and receive. Because serial communication is asynchronous, a port can transmit data on one line and receive data on another line. Other lines can be used for signal handshake, but are not required. Key metrics for serial communication are baud rate, data bits, stop bits, and parity bits. These parameters must match if two serial ports are to communicate.
USBUniversal Serial Bus (USB) is primarily used for peripheral devices connected to PCs such as keyboards, mice, scanners, and disk drives. In the past few years, the number of devices supporting USB connections has increased dramatically. USB is a plug-and-play technology. When a new device is added, the USB host automatically detects the device, sends an inquiry to identify the device, and configures the appropriate device driver for it.
USB 2.0 is fully compatible with low speed and full speed devices. Its high-speed mode has a data transfer rate of up to 480 Mbit/s (60 MB/s). The latest USB 3.0 specification features an ultra-fast mode with a theoretical data transfer rate of up to 5.0 Gbit/s.
Although the USB bus was originally designed for PC peripherals, its speed, wide applicability and ease of use make it attractive in instrument control applications. The USB bus also has some shortcomings in instrument control: First, the USB cable is not an industrial standard, and may cause data loss in a noisy environment; in addition, the USB cable has no locking device, and the cable can be easily The PC was pulled out; and even with a repeater, the USB cable has a maximum transmission distance of only 30 m.
EthernetEthernet is a mature technology that is widely used in measurement systems for general network connectivity and remote data storage. Currently, there are more than 100 million computers with external network interfaces configured worldwide. Moreover, Ethernet also provides functional options for instrument control. Ethernet is defined based on the IEEE 802.3 standard and theoretically supports data rates of 10 Mbits/s (10 BASE-T), 100 Mbit/s (100BASE-T), and 1 Gbit/s (1000BASE-T). Among them, the most common one is 100 Mbit/s (100BASE-T) Ethernet.
Ethernet-based instrument control applications take full advantage of the Ethernet bus features, including remote instrument control, easy instrument sharing, and easy-to-use data results publishing. In addition, users can take advantage of existing Ethernet networks in the company or lab. However, for some companies, this feature of Ethernet can cause some trouble: corporate network administrators may need to be involved in the development of instrumentation applications.
Instrumentation control based on Ethernet bus has other drawbacks, such as possible problems with actual transmission rate, transmission certainty, and security. Although the Ethernet bus can achieve a theoretical transmission rate of up to 1 Gbit/s, in actual use, since the network is also occupied by other applications, and there are problems such as data transmission failure, this theoretical transmission rate is rarely realized. In addition, due to the unstable transmission rate, it is difficult for Ethernet to guarantee the certainty of data transmission. Finally, for sensitive data, users need to take additional security measures to ensure data integrity and confidentiality.
PCIThe PCI bus is usually not used directly for instrument control, but as a peripheral bus that implements instrument control by connecting to a GPIB or serial communication bus. In addition, due to its high PCI bus bandwidth, it is often used in the backplane bus of modular instruments. At this time, its I/O bus is built into the measurement equipment.
PXIPXI (PCI extension for instrumentation systems) is based on the PCI platform and is a rugged bus for measurement and automation systems. PXI combines the electrical bus characteristics of PCI with the ruggedness, modularity and Eurocard mechanical package of CompactPCI, with the addition of a dedicated synchronous bus and important software features. These technologies make the PXI bus a high-performance, low-cost deployment platform for measurement and automation systems for applications such as line testing, military and aerospace, machine condition monitoring, automotive, and industrial testing. PXI was developed in 1997 and officially launched in 1998 as an open industry standard to meet the growing demand for complex instrumentation systems. Today, the PXI standard is managed by the PXI Systems Alliance (PXISA). The alliance is made up of more than 65 companies that promote PXI standards, ensure interchangeability of PXI, and maintain PXI specifications. PXI is used extensively on modular instrumentation platforms based on compact, high-performance measurement hardware with integrated timing and synchronization resources, making it an ideal replacement for traditional stand-alone instruments.
PCI ExpressSimilar to PCI, PCI Express is not directly used for instrument control, but as a PC peripheral bus for connecting GPIB devices for instrument control. However, due to the extremely high speed of the PCI Express bus, it can be used as a backplane bus for modular instruments.
VXIThe VXI (VME Expansion for Instrumentation) bus is the first attempt at a multi-vendor industrial instrument standard. VXI was first introduced in 1987 and was subsequently defined as the IEEE 1155 standard. Disadvantages of the VXI bus include the lack of software standards and the inability to significantly increase system throughput; and because VXI does not use standard commercial PC technology, system cost cannot be reduced.
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