Communications and Networking Riser

Communications and networking riser (CNR) is a slot found on certain PC motherboards and used for specialized networking, audio, and telephony equipment. A motherboard manufacturer can choose to provide audio, networking, or modem functionality in any combination on a CNR card. CNR slots were once commonly found on Pentium 4-class motherboards, but have since been phased out in favor of on-board or embedded components.

Physically, a CNR slot has two rows of 30 pins, with two possible pin configurations: Type A and Type B, each with different pin assignments. CNR Type A uses 8-pin PHY interface, while Type B uses 17-pin media-independent interface (MII) bus LAN interface. Both types carry USB and AC'97 signals.

As with AMR, CNR had the cost savings potential for manufacturers by removing analog I/O components from the motherboard. This allowed the manufacturer to only certify with the US Federal Communications Commission (FCC) for the CNR card, and not the entire motherboard. This resulted in a quicker production-to-market time for new motherboards, and allowed mass-production of CNR cards to be used on multiple motherboards.

The ACR slot was a competing specification developed by a group of third-party vendors. Its principal advantage over CNR was the backwards-compatible slot layout which allowed it to use both AMR and ACR cards. The same group also developed a physically smaller version, the MDC.

Intel developed the CNR slot to replace its own audio/modem riser (AMR) technology, drawing on two distinct advantages over the AMR slot it replaced; CNR was both capable of being either software based (CPU-controlled) or hardware accelerated (dedicated ASIC), and was plug-and-play compatible. On some motherboards, a CNR slot replaced the last PCI slot, but most motherboard manufacturers engineered boards which allow the CNR and last PCI slot to share the same space.

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