[Design Application]
Take Advantage Of Rider-Card Standards To Shrink PC Connectivity Costs
Riser cards provide new ways to deploy modem functions, helping conserve PCI slots, reduce part count, and speed time to market.
What's the fastest, easiest, and least expensive way to implement connectivity with the outside world on a PC? Just take advantage of recent riser-card standards that let manufacturers decouple and redeploy the various functions that make up modems and other communications subsystems. These standards continue a trend that began when external modem boxes turned into modem add-in cards, which then became modem software running on the PC's host CPU.
Specifically, riser cards let users move the most complex functions off the communications card so it can be shrunk and installed in a slot on the motherboard, rather than in a PCI slot of its own. Modems no longer need DSPs, modem control, and call-handling logic, leaving behind only the bare minimum hardware for a physical and electrical connection—the codec and telephone interface circuits.
A similar cost-savings strategy can be applied to other connectivity subsystems, such as DSL and Ethernet. Given the riser card's now-simplified interface requirements, it's possible to replace the card's PCI parallel interface with a less-expensive serial interface. To realize all these economies, there need to be standards for implementing the riser cards and the serial interface. These cards have arrived, and OEMs and their customers can begin reaping the benefits.
As with most PC standards, the initiatives surrounding riser cards have taken shape as a series of company- and industry-sponsored recommendations. Some of these overlap. On the surface, some even appear to compete with each other. The three primary standards OEMs need to be concerned with when implementing PC connectivity based on riser cards are Audio Mobile Riser and Mobile Daughter Card (AMR/MDC), Communication Network Riser (CNR), and Advanced Communication Riser (ACR). These standards attempt to make PC connectivity less expensive and more efficient to implement by reducing the amount of communications-specific hardware required and by simplifying the communications channel interface.
The key is to recognize that each type of communication subsystem is comprised of a consistent set of functional blocks, that there are optimum ways to deploy these blocks, and that these "rules of deployment" are similar whether designers are working on modems, audio, Ethernet, DSL, or HomePNA (which implements local area networks in the home using telephone jacks). One major benefit is that riser cards reduce the competition for PCI slots as the proliferation of add-on options grows, the size of PCs shrinks, and the number of available PCI slots is reduced.
Modems, a major focus of the standards, illustrate the various issues surrounding riser-card implementations. All modems, for example, must somehow implement the functions provided by the following parts:
Direct access arrangement (DAA): The DAA circuit provides the Public Switched Telephone Network (PSTN) isolation circuitry and two- to four-wire RJ-11 interface necessary for the PC to physically and electrically link to the telephone network.
Codec: This provides the digital-to-analog and analog-to-digital conversion that turn voltages on the phone line into discrete binary values (sines and cosines) processors can handle.
DSP: The DSP demodulates the sines and cosines from the codec into bytes of information the CPU can understand. It then modulates data from the CPU into sines and cosines. These real-time, computationally intensive operations not only modulate and demodulate the data stream, they also perform error checking and data compres-sion/decompression.
Modem controller: This device handles PCI and modem control.
EEPROM: The programmable memory is used to upgrade modem features. It's also important for assigning vendor identifications required for getting the Microsoft Windows Hardware Quality Labs (WQHL) "stamp of approval" for each function.
UART: The UART provides parallel-to-serial and serial-to-parallel conversion between the modem's data path and the CPU or system bus.
Over the past decade or so, the cost of implementing these functions has dropped dramatically. This is largely due to their migration out of the hardware-based modem which, as previously stated, has transitioned from a box to a PCI card to a riser card (Fig. 1). Such a migration path, where functions are implemented in software, results in the most dramatic cost reductions.
At the "less than $20" tier, all functions are implemented within the modem as hardware, including DSP and modem control. At "less than $15," those two functions are moved into software running on the CPU. This takes advantage of the "free" unused CPU cycles available on today's powerful Pentium-class machines while making the modem much easier to upgrade. Instead of reprogramming the EEPROM chip or buying another modem, designers can upgrade simply by downloading software over the Internet. At "less than $10," even greater cost savings are available by getting rid of the PCI control and migrating the entire modem controller to the motherboard chip set.
This strategy works for modems as well as for HomePNA, DSL, Ethernet, and audio. As with modems, both HomePNA and DSL have traditionally been implemented as PCI add-in cards, with each of these functions comprised of a digital controller and an analog interface. And just like modems, the opportunity exists to move the digital controller to the motherboard chip set. In fact, that's what the new class of Southbridge chips—commonly known as Super Southbridge—provide.
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