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A riser board looks like an expansion board and works like an expansion board. Using the quack-standard—if it looks like a duck and quacks like a duck—a riser board is an expansion board. But really it's not, at least according to the people who have created the standards that make it possible to consider riser boards as being expansion boards. In its various riser board standards, Intel claims that its riser boards are not an expansion board standard.
Rest assured you're not the only one confused by riser boards. The concept underlying them is the same as an expansion board—put extra circuitry on a special card so that it can be easily removed and changed. The boards look like expansion boards, pegged at the same height and using similar expansion connectors. Risers slide into a motherboard slot placed adjacent to ordinary expansion boards so that, at least from the outside, they look like simply another expansion board.
Intel doesn't want you to think of riser boards as expansion boards because it doesn't want to confuse you with more than one expansion board standard. It doesn't want third-party manufacturers putting riser boards in boxes and selling them in retail stores. Likely the company fears someone will buy a riser and discover they can't slide it into a PCI slot, and the lonely worker handling support will get a barrage of telephone calls asking for help.
Functionally, a riser board is simply a bend in a motherboard. It rises perpendicularly from the motherboard (much like an expansion board) to hold circuitry that otherwise would not fit on the motherboard (much like an expansion board). Riser boards can even use a slot position, much like an expansion board, but they follow their own standards, which have absolutely nothing to do with PCI. This independence, in fact, is one of the graces of the riser board. It allows you to change or customize your system using a low-cost product that avoids the complications (and cost) of a full PCI interface. Moreover, the Audio Modem Riser (AMR) slot doesn't count against the tiny maximum number of PCI boards that can be connected to your computer's PCI bridge circuitry.
Put another way, riser boards offer a system designer three big benefits: space, timeliness, and isolation. The physical size of the riser board amounts to additional real estate on which to locate electronic circuitry, as well as bracket space at the back of the computer for extra connectors. Using a riser board, a motherboard can be more compact. In fact, Intel created the original standard riser, the Audio Modem Riser, as an adjunct to its small-footprint microATX motherboard.
Today, communication and networking standards change faster than most computer standards. Keeping up with these changes may require motherboard-makers to revise their designs more often than they would otherwise like (they would otherwise like never to change their designs, because each change racks up design expenses). By restricting the changes to a small riser board, the manufacturer trims the cost of keeping its motherboards timely.
There's another reason for putting audio circuits on a separate board. Audio circuits are the most prone to picking up interference from the rest of your computer. Analog audio uses signals that vary in a wide range, over more than three orders of magnitude. Interference typically produces tiny signals that are ignored by logic circuits but are amplified into bothersome noise by analog circuits. A signal one-thousandth the strength of a normal logic signal would have zero effect on the operation of your computer but would be audible in high-quality speakers attached to your computer. By putting the interference-prone circuits on a separate board, designers can physically isolate them from the other signals on the motherboard, which adds a degree of electrical isolation as well.
With these ideas in mind, Intel and other companies have created three standards for riser boards meant to work in modern computers. These include the original Audio Modem Riser standard, the Communications and Networking Riser standard, and the Advanced Communications Riser standard.
Audio Modem Riser
The first of the riser-board standards independent of a specific motherboard design was Intel's original Audio Modem Riser (AMR). The AMR specification was originally published on June 30, 1998, and updated to its final form (version 1.01) on September 10, 1998. It envisions a board capable of holding up to four audio codecs on a single board that's at most 6.875 inches long and 4.2 inches high. It uses a single 42-contact edge connector located between the AGP port and other port connectors on the motherboard. It takes the space of an ordinary expansion board but not the connector or the circuitry, so it doesn't count against the maximum number of PCI boards that can be inside a single computer.
The interface was designed to allow a great deal of flexibility. Not only does it allow for four codecs where two would typically be used—one to implement a modem, one for the equivalent of a soundboard—a special set of split I/O signals allows, for example, a sound system implementation on the motherboard to communicate with a modem on the riser board. These signals take the form of four separate serial data channels from the chipset (typically) to the audio circuitry on the riser board. The interface also provides the power and clock signals needed to make the audio circuits work, as well as signals for future applications (including the USB bus).
Although you'll still find some references to the AMR design, Intel has replaced it with the Communications Network Riser standard (discussed next) and has removed the AMR specification from its Web site.
Communications Network Riser
On February 7, 2000, Intel released its replacement for the AMR design, which it called the Communications and Networking Riser (CNR). The "networking" part of the name marks the major difference. CNR incorporates provisions for networking and USB interfaces as well as the audio and modem technologies of the earlier design. The full specification for the CNR board and its interfaces is available at the Intel Web site at www.intel.com.
The CNR design uses a new 60-pin connector with a preferred location at the edge of the motherboard: outboard of its normal PCI expansion slots, a position formerly used by a legacy ISA slot on many motherboards. Significantly, the new connector is incompatible with AMR boards.
Despite the physical incompatibility, the CNR standard starts with some of the basic functional compatibilities as with boards made for the AMR specification. At heart, CNR incorporates the Audio Codec (AC) '97 interface. Unlike AMR, which permits four AC '97 channels, the CNR specification originally allowed only two, although it was revised (to version 1.2) on November 8, 2001, to permit three.
The networking side of the CNR design defines two different board implementations, each using the same connector but with different pinouts. The "A" variation uses eight contacts for a platform LAN connection interface, meaning that the hard work of generating the network signals is on the motherboard. The riser only packages them, for example, putting them on the same RJ-11 jack as the modem signals. The "B" variation uses 17 contacts for a Media Independent Interface (MII, as defined under the IEEE 802.3u specification) bus, allowing the riser board circuitry to set up any kind of physical connection (which makes the riser board circuitry more complex). In other words, the "A" variation allows for less complex riser boards, whereas the "B" variation allows for greater versatility (for example, permitting the manufacturer to install different network interfaces by changing the riser board).
The CNR design also allows for the integration of a USB interface on the riser board. With the CNR version 1.1 revision of October 18, 2000, the specification added support for USB version 2.0 at 480Mbps.
So that the CNR board will be properly identified by the host computer, each board has five signal lines for a System Management Bus (SMBus) interface. The host computer can use the bus to interrogate ROM memory on the board to identify its function and its resource needs each time the host boots up. The CNR design also includes several lines to provide power to the circuitry on the riser board at 12, 5, and 3.3 volts (DC) as well as sleep-mode voltage so the circuitry of the CNR board may remain active while the host is in sleep mode. (This voltage allows the CNR circuitry to wake the host computer upon modem ring or network request.)
The physical specifications for the CNR board are similar to those of the AMR board. Both boards are limited to the same maximum height, 4.2 inches (106.68 millimeters), dictated by host height restrictions within the PCI standard. CNR boards have a slightly shorter maximum length than AMR boards, at 6.579 inches (167.11 millimeters). The connector is, of course, different, unique to the CNR board, although the pin spacing on its edge connector matches that of the standard PCI connector.
Advanced Communications Riser
This need for yet another new connector and other shortcomings of the CNR design led an industry group to design its own alternative to CNR on February 11, 2000. That organization, which called itself the Advanced Communications Riser Special Interest Group (ACR SIG), has grown to 55 members at the time of this writing, including, significantly, Advanced Micro Devices, but not Intel.
Working together, they created the Advanced Communications Riser (ACR) board to add network and Digital Subscriber Line (DSL) capabilities to simple add-on boards without losing compatibility with AMR boards or adding the need for a new connector.
Physically, the ACR slot uses a 120-pin PCI connector (for its ready availability), but the connector is reversed and offset in its position in its slot. This change in orientation and position makes it compatible with AMR boards. The first 42 contact positions on the ACR board have the same functions and signals as the AMR board. The ACR design envisions its slot position on the outside edge of the motherboard, replacing the legacy ISA slot in many motherboard designs.
An AMR board can slide directly into an ACR slot. An ACR board will, too, although it won't slide into an AMR slot because of the longer connector on the ACR board.
The electrical functions of the ACR board start with the four Audio Codec '97 channels of the AMR board and add two more AC '97 channels, for a total of six.
In addition, the ACR design provides two new pins as a serial data channel for board identification. Each ACR board includes an onboard ROM chip to identify the board and its capabilities to the host computer as part of the Plug-and-Play setup process.
One of the new sets of 14 signals is the Integrated Packet Bus, developed in conjunction with the ACR design to directly link the host microprocessor with the communications subsystem and control high-speed Internet connections. Two sets of 18 signals each provide a pair of channels that separately link your chipset to phone-line networking and ordinary twisted-pair networks following the 10Base-T or 100Base-T standards. The ACR specification reserves an additional eight contacts for future applications, such as wireless networking adapters.
The ACR design specifically recognizes home phone-line networking (HomePNA). It envisions modem-makers offering a single multifunction board that includes a V.92 modem, a HomePNA network adapter, and a DSL adapter. A single ACR card will link to your home phone-line network, your telephone line as a regular modem (for data as well as dialing and Internet phone service), and your DSL line, without the need for external adapters. All phone-line communications functions can then use a single RJ-11 jack on the ACR card.
Although the ACR SIG claims that its standard is open, the specification is available only to members of the group. Its Web site is located at www.acrsig.org.
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