Design of virtual instrument system under linux platform

The development of computers and their interface technologies and the deficiencies exposed by traditional test and measurement instrument systems have made computer-based virtual instrumentation increasingly the dominant test and measurement instrument. Virtual instrumentation systems are used in a wide range of industrial applications for their platform versatility, scalability, ease of upgrade, and high levels of intelligence. Inserting PC-based (ISA, PCI)-based digital acquisition boards into PCs and industrial control computers to form hardware systems, writing Windows system platform drivers and soft panels to implement software functions has become the industry's main solution.

However, in the practice of testing tasks in the field and harsh environments, we found that virtual instruments based on PC or industrial computer exposed many problems, such as: large size, inconvenient to carry; plug-in structure, easy to loose contact, not fastened With mechanical hard disk as the main storage medium, poor seismic performance and so on.

The embedded computing platform featuring 32-bit embedded microprocessors and embedded operating systems has enabled computing to enter the post-PC era. The small size and high reliability of the embedded system can meet the needs of portable virtual instruments in field and harsh environments. Based on the embedded computing platform, designing a virtual instrument system becomes a new idea for building a test system.

By building an embedded computing platform based on the PC104 bus, adding instrument cards and their functional programs, we have implemented a variety of test instruments for radar electronic equipment. The technical problems that need to be solved in building virtual instruments based on embedded systems focus on the construction of system platforms, the design of interfaces and drivers, and the design of soft panels.

The hardware system includes an embedded motherboard, an instrument function board, a Flash storage medium (DOC or CF card), a liquid crystal display, a touch screen, and a signal interface. As shown in Figure 1. The liquid crystal display and the touch screen realize human-computer interaction, the signal interface is used for coupling test signals, the embedded main board is used as the control and calculation unit, and the instrument function board realizes the function of the specific instrument.

Figure 1. System hardware composition diagram

In Figure 1, the components are in the order of stacking, which are touch screen, liquid crystal display, PC104 motherboard, oscilloscope card, multimeter card.

The functional board and the embedded motherboard are mechanically and electrically interconnected by stacking on the PC104 bus. This approach has the following benefits:

1. Electrical contact is highly tight. The boards are connected in depth through multiple rows of pins, which are much tighter than the ISA and PCI slots.

2. The mechanical structure is firm. The four studs are tightly connected between the boards, so that the mechanical connection between the boards is very strong and there is no sloshing.

3. The electrical characteristics of the PC104 pin are fully compatible with ISA. The electrical characteristics of the PC104 Plus pin are fully compatible with PCI, so that the function board based on ISA or PCI bus design can be reused from the electrical principle, which is conducive to the smooth process of the system transformation. transition.

Discarding the hard disk and using DOC or CF card as the external storage medium can greatly improve the system's ability to withstand vibration and shock.

The hardware system described above can provide hardware guarantee for small and reliable virtual instrument systems, but the problems caused by small system storage capacity and limited resources bring difficulties to the design of software systems. An embedded operating system must be used, and software programming must consider small size and high efficiency.

We use embedded linux as the operating system and write the driver for the instrument under the linux platform. Instrument soft panels are implemented using TIny X and GTK+ as graphical interface solutions. The software structure of the system is shown in Figure 2:

Figure 2. System software component composition diagram