CNR Slots on Motherboards

If you’ve seen one of the newer motherboards or read the motherboard shootout in our February 2001 issue, you must have noticed a new type of ‘slot’. This is a CNR (Communications and Network Riser) ‘slot’ that serves as an extension of the motherboard. Intel, the father of this technology, defines it as “A hardware scalable Original Equipment Manufacturer (OEM) motherboard riser and interface that supports the audio, modem, and LAN interfaces of core logic chipsets.” To put it in simpler terms, a CNR slot integrates some basic functionality on the motherboard needed to implement audio, modem, and network functions. This empowers hardware manufacturers to provide low-cost cards containing these three functions.

Until now, integrating these three functions had not been possible for a number of reasons. These include FCC and international telecom certification processes, motherboard space, and other manufacturer specific requirements. Moreover, integrating these three major sub-systems causes a potential increase in noise, and thereby decreases their performance. However, with CNR the noise-sensitive audio and network components get separated from the motherboard. So this is no longer an issue. CNR also reduces costs by eliminating the cost of a PCI bridge inherent in all PCI designs. 

• The CNR specification supports the following interfaces:

• Audio Codec ’97 (AC ’97) Interface: Used to support audio and/or modem functions.

• LAN interfaces: Provides one of two LAN interfaces for networking functions–an eight-pin interface for Platform LAN Connection
  (PLC) based devices or a seventeen-pin interface for Media Independent Interface
  (MII) based devices. 

• USB interface: Used to support technologies or functions that are being implemented on a USB interface. Also to be used for technologies such as broadband,
  DSL, and wireless.

• System Management Bus (SMBus) interface: Used specifically to provide plug-n-play functionality.

• Power: Signals that are required to support power management, as well as the main power supplies to operate the CNR board silicon and support circuitry.

• Reserved: Reserved for future use.
  
  One thing you must note is that this specification is not an upgrade option. A CNR card acts as a motherboard extension, so it needs to be pre-fitted and fully configured by manufacturers before the system is shipped to users. For upgrades, PCI will continue to be around, though the number of options that you could upgrade to will reduce, as they would’ve been taken care of by
  CNR. 

Before CNR, there was another technology known as AMR (Audio Modem Riser). However, it hasn’t done very well because with motherboard space at a premium, adding an AMR slot meant sacrificing a precious PCI slot. This is because an AMR slot cannot be used for PCI devices and vice-versa.
CNR, however, shares a slot with PCI. So the manufacturer has the option of either putting in a PCI or a CNR card. Finally, CNR has support for plug-n-play, unlike AMR. 

The good news for developers and manufacturers is that there is no royalty associated with
CNR. You are free to design your own CNR cards. Intel already has its Intel PRO/100 VE Desktop Adapter. Other manufacturers like Asus and Motorola are also developing cards using the CNR specification. More information on CNR can be obtained from
http://developer.intel.com/technology/cnr/. 

Kunal Dua