Other motherboard manufacturers may choose a completely different implementation, opting instead for a high-end PCI audio card. In this case, the software modem in the riser card will be the primary codec, a condition that must be accommodated by the software modem drivers. This can be done at runtime by detecting the system configuration (primary/secondary) in the Southbridge and reverting to the proper codec to support it.
These strategies for optimizing PC communications subsystems in riser-card implementations make a difference in cutting cost, reducing time-to-market, and enhancing end-user convenience. They illustrate how, even in a market increasingly defined by standards, OEMs should carefully consider the products they choose.
The standards themselves also have implications regarding design flexibility, functional richness, and breadth of industry support. Not all types of communications categories, such as DSL, are supported by all standards. OEMs should be aware of these differences, just as they should be alert to differences between component providers. They also should realize these standards do not support an aftermarket I/O expansion slot, which should be implemented by system manufacturers or system integrators at the factory. Standard I/O expansion slots, like those supported by the PCI bus architecture, are still intended to continue serving the retail channel upgrade market.
As stated earlier, there are three key standards for implementing riser-card-based connectivity solutions in PCs—AMR/MDC, CNR, and ACR (see "10 Key Facts About The Standards," p. 116).
The first riser-card form-factor connectivity standard, AMR/MDC, dealt with audio and modems. It defined the architecture, electrical characteristics, and mechanical requirements of the riser/daughter card. Proposed by Intel in 1997, it has been accepted by the PC industry.
AMR/MDC uses a serial interface, AC link, described in the AC '97 standard. It supports multiple codecs connected in series. One of the codecs, defined as the primary codec, provides the bit clock for the remaining codecs. These codecs are known as secondary codecs. This architecture facilitates the modem architecture in which the modem front end (codec and DAA) is connected via the serial AC-link bus directly to the Super Southbridge chip set (Fig. 3, again). The chip set also contains the audio and modem control required to drive the AC-link bus. Incorporating AC link directly in the core-logic chip set lets users directly connect audio codecs (AC '97) and modem codecs (MC '97) to the host CPU's core-logic chip set.
As a follow up to AMR/MDC, Intel recently announced the CNR standard. It supports AC link, so audio and modems can be implemented in the same manner as in AMR. Yet CNR adds new buses for networking devices such as HomePNA and Ethernet.
CNR-compliant Super Southbridge chips with integrated network controllers, in addition to modem and au-dio codecs, are now available. These network controllers, known as media access controllers (MACs), interface to PHY network analog interfaces. The PHY is the only component required to interface the LAN or HPNA directly to the Super Southbridge chip. It can be implemented directly on the motherboard or the riser card (Fig. 5).
In addition to the AC '97 and LAN interface, CNR supports two other interfaces, USB and SMBus. The SMBus is used to access EEPROM for the riser card. This makes it easier to assign the vendor IDs required for Microsoft WHQL certification.
An alternative to the Intel-sponsored CNR is the ACR, put forth by the ACR Special Interest Group (Fig. 6). ACR is similar to CNR, except its ACR connectors are backwards-compatible with AMR. Also, ACR supports a high-speed packet bus for integrating DSL modems. And, the ACR specification was designed by a large consortium of companies.
Like CNR, ACR supports AC link, Ethernet/HPNA interfaces, and an EE-PROM bus. But it also supports an integrated packet bus (IPB) for DSL. ACR (but not CNR), then, would support the integrated DSL/modem/HPNA riser.
Another significant difference is the PCI interface. According to the ACR Special Interest Group web site, www.acrsig.com/doc01.htm, the ACR standard is meant to "preserve choice in selection of discrete silicon components to promote industry innovation and product differentiation." ACR doesn't require interfacing to an Intel chip set. The PCI connector, as opposed to CNR's entirely new connector, also preserves AMR compatibility. ACR uses a low-cost PCI connector in a reverse offset fashion so existing AMR cards can be used in new ACR motherboards.
The standard selection—AMR/MDC, CNR, or ACR—depends on factors such as cost, parts availability, compatibility with existing designs, and future direction. For example, AMR/MDC specifies support for multiple channel audio, such as what's required for DVD audio. But today's Super Southbridge chips only support two-channel (stereo) audio. Likewise, the AMR spec calls for two-modem support, but the current Super Southbridge chips only support one modem.
OEMs need to consider how they will partition the audio, modem, and other subsystems to address various user models, and whether there are robust riser chip sets and driver software solutions available to support them. Fortunately, these are choices OEMs have now that they didn't have before. By migrating functionality away from the add-in card and onto the motherboard, the riser-card standards open new possibilities for reducing PC connectivity costs while tailoring designs to user environments. Depending on which choices OEMs make, exploiting these possibilities can be profitable.