IEEE 1394 for Your Test and Measurement PC

What Is IEEE 1394?
IEEE 1394 (also known as FireWire) is a new high-speed serial bus that has been in development for more than a decade. Because of its high bandwidth, IEEE 1394 is gaining in popularity for a broader range of applications. Sony is standardizing on IEEE 1394 for appliances such as digital cameras, digital video disks (DVD), digital VCRs, and music systems. The PC industry has been trying to adopt IEEE 1394 for interfacing to mass storage devices (hard drives, CD drives, and DVD drives), printers and scanners for the past 3 years. Apple recently incorporated FireWire ports on the motherboard of their professional machines. Now, National Instruments has plugged the measurement world into 1394. Our 1394 products are available now for Windows (but not yet for Mac OS).

When To Use IEEE 1394
1394 is not the magic cure-all bus. It is a great bus for some applications, but you should always evaluate a bus based on your individual need. If you need a bus with the strengths of IEEE 1394, then use it. If you need a bus with the strengths of PCI, don't use IEEE 1394; use PCI.

• Up to 63 devices (currently NI-DAQ limits to 16)
• Long Distance (up to 500 m with extender)
• Hot Plug-and-Play
• Easy Configuration
• Switch between multiple PCs easily
• No need to open the PC
• High throughput (up to six DAQPad-6070Es at full rate)
• Better signal connectivity
• Isolation (with fiber-optic extender)
• Battery powered
• More measurements than PCMCIA can handle

• More expensive then PCI products
• Slower single-point speed.

• VXI-1394 - VXI Controller (with optional GPIB)
• GPIB-1394 - IEEE 488 Controller (HS488)
• DAQPad-6070E - 1.25 MS/s MIO - RTSI/SCXI Expansion
• Motion - FW-7344 - 4-Axis Stepper or Servo

IEEE 1394 Host Adapters
To use IEEE 1394 you must have some type of IEEE 1394 host controller on your PC. Some PCs have built-in 1394 ports, but for now most users will have to buy some type of plug-in host adapter. For desktop PCs, there are a wide variety of PCI-based host adapter boards; for laptop computers there are Cardbus host adapter cards. Make sure the computer has Cardbus, not just PCMCIA.

When looking for host adapters, there are three main things to look for to guarantee the best performance.

First is the maximum transfer rate. The current standard of 1394 offers 100 Mb/s, 200 Mb/s, and 400 Mb/s. All speeds are interoperable, but the bus will be able to transfer only as fast as its slowest link. All National Instruments 1394 products use the fastest available speed, 400 Mb/s.

Second, it is important to have a large asynchronous transfer FIFO. 1394 uses two methods of data transfer - asynchronous and isochronous. Asynchronous guarantees delivery and not timing, and isochronous guarantees timing but not delivery. Because loss of data is unacceptable in measurement applications, all currently available measurement products rely on asynchronous transfers. To get the best asynchronous performance, your asynchronous transfer FIFO should be at least as large as the maximum packet size you can send at a given speed (see Table 1). If you have a 400 Mb/s adapter but have only the FIFO depth to get the same amount of data across as a 200 Mb/s adapter, the extra speed won't help your measurement application. So for a 400 Mb/s product it is highly desirable to have a 2 KB FIFO. If a smaller FIFO is used, the bandwidth is drastically decreased. Unfortunately this is not always an easy-to-find spec for adapter devices. To find the maximum FIFO size, it may be necessary to call the manufacturer of the host adapter or look through the specification of their link-layer chip.

The third thing to look for, is OS compatibility. There are two types of host adapters supported native under Windows 98, Adaptec and Open Host Controller Interface (OHCI). Adaptec earned a special place in the host adapter market by providing an early, reliable implementation. However, OHCI is the future of 1394 host adapters, and is most likely to be used in future operating systems.