Modular instruments use a frame, into which different types, or a varying number, of functional cards can be plugged. This is so that the instrument can accommodate a range of input/output channels or tailor its measurement capability according to the specific application being addressed.
Modular instruments generally use a computer user interface instead of displays and controls embedded in the instrument’s frame or package. Because they do not have their own user interface, modular instruments are often called faceless instruments. By sharing a computer display and keyboard/mouse (the “face” of the instrument), modular instruments can save the expense of multiple front-panel interfaces.
Without the traditional front panel, the most common approach to using modular instruments involves writing a test program, which configures the instruments, conducts the measurements, and reports results. For this reason, modular instruments typically are supplied with programmatic software interfaces, called drivers, to ease the job of communicating with an instrument module from a programming language.
Modular instrument types:
The modular instruments used in industries holds different standards, which include:
Personal computer plug-ins
VME is the oldest of the modular standards. For applications supported by VME, it offers the widest range of vendor support. CompactPCI is relatively new and emerging.
Applications that are dominated by computing or analysis, with a small amount of instrumentation, will be best served by VME or CompactPCI. These modular standards offer a broader range of CPUs, more computer peripherals, and more choice in operating systems
All these standards are used for instrument systems, only VXI and a derivative of CompactPCI (called PXI) were developed expressly for instrumentation.
Two of the modular instrument standards were designed specifically to meet the needs of higher-performance instrumentation and complex system building. These are the VXI and PXI modular instruments.
If the application involves general-purpose instrumentation and measurement switching, these modular instrument standards are the best choices. At this point, VXI has the largest number of vendors and the widest range of products.
Standards-based modular instruments can accept products from many different vendors, as well as user-defined and constructed modules. PC plug-ins are not part of a formal standard. The ubiquity of the personal computer, however, has made the PC motherboard I/O bus a defacto standard for instruments.
Advantages of modular instruments:
High channel count:
High-channel-count measurements, such as data acquisition, really benefit from the standards-based modular form factor. Module size is relatively big, so a large number of channels can be included.Data acquisition usually requires many types of channels. Not everything can be purchased from one vendor. The mix and match capability of modular instruments allows the system to be easily configured from multiple vendors.
Automated test systems:
Modular instruments address the needs of automated test systems very well. Automated test systems typically involve many types of instruments. The instruments are driven by software, not directly manipulated by a user. Packaging density is a concern, and, in fact, the logistics of racking and cabling are as important to system design as making measurements. With modular instruments, modules can be acquired from multiple vendors.
For a given volume and multiple instruments, modular instruments usually create the smallest, most compact solution.
In a system where signal sources, signal routing via switches, and measurements must be carefully coordinated, modular instruments are a very good choice.
Disadvantages of modular Instruments:
Modular, standards-based instruments are not appropriate for all applications. They are bulky so transportable, not portable.
They must be programmed, direct manipulation of instrument controls is difficult when performing experiments or troubleshooting.