Control and I/O cabling

Connections between the measurement hardware and the PC controller have a major impact on the overall capability and cost of the complete system. It is impractical and unwise, however, to assume that the advertised theoretical data-handling speeds of any given interface will be reached during any practical application. Although the user must evaluate alternatives that provide the necessary speed (i.e., information bandwidth) for any particular application, shown in Table 4.4 are benchmarks run on actual interfaces using realistic data-acquisition hardware. The numbers shown are intended only as examples to illustrate the wide differences in performance under directly comparable conditions. The performance comparisons are for interfaces connected to a VXI mainframe containing a “typical” digitizer and a 64-channel data-acquisition card. The interfaces compared are a GPIB (IEEE-488), FireWire^® (IEEE-1366), MXI-2 (unique to VXI), and an “embedded” controller (i.e., a PC/controller placed directly on the computer backplane of the measurement system).

Data loggers. In the case of data loggers or scanning voltmeters, focus is generally on the cost and ease of use for a low-channel-count application. Many data-logger applications have no need for connection to a PC, so the friendly standalone user interface provides a very high value to the user. If a PC connection is required, low-cost, low-data-rate connections (such as RS-232) are very popular and easy to implement because virtually all PCs are equipped with such a serial interface. For short distances, higher data rates can be handled with IEEE-488 connections, albeit at a somewhat higher price. As more PCs have included USB and FireWire interfaces as standard components, more measurement devices that use such links have appeared on the market. In all cases, the information bandwidth from a data logger is unlikely to be further hampered by the performance of the PC I/O used because the digital transmission speeds are typically far greater than the bandwidths of the signals being measured.

Both control information and data flows across the same digital connection without adversely impacting overall performance.

Computer backplane. VME/VXI/PXI have a broad spectrum of connection possibilities, each with advantages and disadvantages. For instance, it is possible to use an “embedded” controller, which means that the PC physically plugs into one of the slots in the mainframe. This allows for direct data transfer over the VME/VXI/PCI backplane between the measurement modules and the PC, and eliminates any need for a special I/O connection between the PC and the mainframe. However, the downside to this is that embedded controllers often cost two or three times more than their equivalent-performance counterparts that are not designed to plug into the mainframe. The rapid evolution of PC technology makes the desktop or laptop PC much less expensive and often far

TABLE 4.4 The Numbers Shown Are Intended Only as Examples to lllustrate the Wide Differences in Performance Under Directly Comparable Conditions

more powerful than embedded machines of the same vintage. So, users often choose to use an external (to the measurement mainframe) controller, which requires the use of specialized I/O. For VXI, these connections are available in a variety of prices and performances, including IEEE-488, MXI, and FireWire interfaces.

Interfaces must be chosen carefully, with insight into the continuous data rate required and the maximum latency that can be tolerated. For example, if a system is intended to be set up once, then run continuously for long periods of data collection, the user would be interested primarily in the efficiency and speed of transferring large blocks of data from the mainframe to the PC. On the other hand, if a system is designed to be set up repeatedly, armed, and triggered on short-duration repetitive events, the user might be more interested in the efficiency and speed of transferring very small blocks of data and/or control information. The “latency” of responding to single-byte transfer might actually make one particular type of interface unusable, and the total speed for large block transfers might make another interface inadequate to sustain continuous data collection. To some extent, this highlights an inherent difference between

“control” and “data” transfers. Control tends to occur intermittently, while data is more of a continuous requirement. For that reason, VME/VXI designers provide alternate paths for data transmission, often at speeds far greater than might be required for mere control and set-up purposes. These alternate paths are typically used to move data between measurement modules or from measurement to recording modules, rather than from modules to the PC.

In VXI, the “local bus” was defined to allow such flexibility, and commercial implementations demonstrate data rates up to 100 Mb/sec between VXI modules.

In data-acquisition applications, this is particularly valuable in moving very high-channel-count data from the measurement modules to digital recording modules for post-measurement analysis. In VME, the “front-panel data port”

can provide similar capability. Such alternate data paths allow continuous data movement without using any of the bandwidth of the VME/ VXI backplane, which is thereby available for more “mundane” data or control tasks, or for moving summary data to the PC. For systems with very high channel count and information bandwidths, such alternate paths are an absolute must. Figure 4.4 is a block diagram of an example system.

PC plug-in cards. In the case of PC plug-in cards, the need for special I/O is gone because the boards already reside in the PC. That is the greatest fundamental strength of the PC plug-in architecture. Again, both control and data are moved across the internal PC backplane, which provides high data rates up to the limit of the number of cards that can be plugged into the PC physically. For proprietary modular systems, both control information and data typically occur over the same low-cost digital links mentioned earlier (RS-232, USB, and FireWire), so the system design must accommodate the combined information bandwidth.

Network-based systems. Clearly, a LAN-based architecture uses the same data path for control and for data as the computer-based architecture. The fact that it is a serial link allows for maximum flexibility. The connection works as well over long distances as it does over short ones, allowing the measurement modules to be used in benchtop or in remote measurement applications. Advances in LAN technology can be “inherited” by the data-acquisition system, permitting connections to existing LAN structures, or even wireless connections. However, the system designer must give more thought to the total information bandwidth required because all data flows across the same serial connection.

If the measurement modules are connected to an existing LAN, the user must realize that LAN availability will affect system performance, and that other LAN transactions can drastically reduce the information bandwidth of the data-acquisition system. Therefore, the concept of performing data compression, averaging, scaling, and units conversion in the measurement module is an important issue to address as the total potential channel count of the system increases. As more “smart sensors” are created, providing their own calibrated digitized output data, the flexibility and utility of LAN architectures will be further enhanced (for more detailed information on smart sensors, see Chapter 47).