Since the beginning of PC-based data acquisition and control in the 1980s, one question has remained a constant consideration for all who would specify a new DAQ system. Is this application better served by an external I/O "box" connected to the PC via some communications link, or an internal "board" system plugged into a slot within the computer? If anything, this question has become more complicated as technology has progressed.
Today, you have a wider — and better — choice of interfaces for your DAQ system than ever before. External box systems based on 100Base-T and Gigabit Ethernet (including LXI), USB, GPIB, CAN, RS-232/485, as well as a variety of proprietary interfaces, are available. On the plug-in board side of the coil are interfaces for PCI, PXI, PCI Express, Cardbus and ExpressCard. Even boards for the original IBM PC's ISA bus are still available. To further complicate matters, hybrid systems like UEI's popular RACKTangle series offer the advantages of an external box with the flexibility and reconfigurability of a board system.
Choosing what's best for a given application is not an easy task. Some of the parameters used to compare interfaces are:
- system accuracy,
- I/O count,
- distance between DAQ system and sensors,
- system portability,
- overall cost,
- bandwidth (DAQ speed),
- expected product life.
In the early days of PC-based DAQ, the general rule of thumb was: If you want high accuracy, choose a box. If you want high speed measurements, you need a board solution. Of course, there always was a "gray" area in between that could be addressed by either type of system. Today, however, the gray area is much wider than before. Both pure board level products as well as hybrid devices (external chassis that allow the installation of I/O boards) are now available with very high accuracies. This has been made possible by a variety of changes in technology, much of which has little to do with the measurement technology itself. While a detailed description of these changes is beyond the scope of this article, a few notes might be both informative as well as interesting from a historical perspective.
Perhaps the three most significant technology shifts that allow boards to be used in high accuracy applications are:
- Multi-layer printed circuit board technology with integral shielding was quite expensive in the 1980s. Today, 4 and 6 layer boards are the norm.
- All the electronics in a PC have migrated to MOS technology. For those too young to have ever played with the LSTTL logic in the original PC, consider yourself lucky. LSTTL generated enormous switching transients. A scope connected to +5 V in a TTL system looked more like the output of a cardiac ward's EKG machine. Compared to TTL, today's systems look like the patient has "flat-lined".
- In the 1980s, the DC/DC converters often operated at 30 to 50 kHz...right in the frequency band of interest in many systems. Today most DC/DCs operate at 500 kHz or more, well above the bandwidth of most DAQ systems.
Of course, measurement technology has changed as well, and the availability of high accuracy delta sigma converters make it possible to "integrate out" much of the high frequency noise produced inside a PC.
Sample and/or Update Rate
As mentioned previously, in the early days of DAQ, high-speed applications were typically handled by board-level products. The internal buses, even though slow by today's standards, were faster than the data transfer rates provided by any external communications link. Today, the story is somewhat different as only the highest speed applications are beyond the capability of USB, Gigabit Ethernet, or Firewire, which theoretically support data transfers of 62.5, 50 and 30 million 16-bit sample transfers per second respectively (although the theoretical limits are rarely achievable).
The new internal board-level bus, PCIExpress, is based upon multiple very high speed ( ≈2 Gbps) serial paths. Though 2 Gbps is quite fast, the PCI Express spec does not stop there. PCI Express allows up to 16 of these serial links in each direction. The total possible data transfer rate of a full PCI Express implementation is 32 Gbps in each direction. This is fast enough for all but the highest digital scope "type" applications.
Interestingly, the very highest sample rates are once again provided by external box systems, but rather than transferring the data directly into a host computer, these systems transfer the data directly into high speed memory contained within the DAQ box itself. After the acquisition is complete, this data can then be downloaded to the host PC for subsequent display and analysis.
I/O Channel Count
Most people assume that external boxt-ype systems inherently allow for more expandability and may be a better choice for a large system than a plug-in board system. That is often true, especially considering most of today's desktop and even tower PCs typically include only a few I/O slots. Also, more and more users are switching to laptop computers, which have extremely limited I/O support.
Distance Between Host Computer and Sensor
This is an important consideration in many applications, more important than many people realize. It is important for two major reasons:
First, running long multi-conductor cables between your test system and sensors can be very expensive, especially with a large system. Conversely, running a single, twisted-pair CAT5 communication cable, is relatively "cheap and easy".
Second, each foot of wire connecting your sensor or output to a remote host computer increases your susceptibility to noise. Quiet measurements of 18-bits and greater are almost impossible to achieve when you have long connection wires. Mounting the DAQ system close to the signal source, however, significantly reduces this noise potential.
Portability of DAQ System
Some systems need to be portable, of course. A wide variety of small, external box devices are available today that meet this need, which is a far better approach than dragging a desktop or tower PC around a test site. Don't overlook PXI when portability is important, however, because several compact 4- and 6-slot PXI chassis are now available that can also meet this need.
Preferred Host Computer
Laptop computers are becoming more popular and their capabilities have expanded to the point where they are now suitable for almost any type of user. Your options for developing a plug-in board-based DAQ system around your laptop, however, are rather limited. Although many PCMCIA/PC-Card options, as well as a number of CompactFlash-based devices, are now available, their capabilities and expandability are severely limited. Most new laptops now come with Ethernet and USB ports, and many include Firewire as well.
Portability Among Different Host Platforms
External box systems certainly have the edge here. Even if your next PC is functionally identical to your existing computer, do you really want to remove all your I/O boards and install them in your new computer? Also, as technology changes, the number and types of slots inside computers change. If your current system has 4 PCI boards in it, are you sure your next PC will have homes for them? Of course, there is no guarantee your next computer will have the same external connections as your current PC, but the probability is almost certainly higher.
The "old" rule of thumb was that, all else being equal, a plug-in board-based system was likely to carry a smaller price tag. This is no longer the case today, however, as some of the lowest cost DAQ interfaces ever released now offer USB and/or Ethernet interfaces.
Expected Product Life
With so many interfaces and system options available today, some of them will not survive for long. It is important, therefore, that you be aware of trends in the marketplace and not select a system type that is likely to become obsolete during the lifetime of your DAQ system. It is also smart to select an interface and system type that is growing in popularity rather than a time-worn design that may be good today but declining in market acceptance and, quite possibly, "end-of-lifed"and unavailable tomorrow.
Based upon the considerations we have described above, it is usually pretty clear which technology provides the best solution for a given application. Should this not be the case, the decision of which system to purchase may revert to the age-old standard of head to head competition among DAQ vendors. In such cases, the decision is often based upon which vendor will offer the best support. One final thing to consider is how much a particular vendor charges for support. Though many DAQ vendors offer their customers free phone/email based technical support, there are a few who do not. Often the price of support is enough to sway a decision one way or another.