PC Hardware in a Nutshell, 3rd Edition

The Strange Case of the AMR/CNR/ACR Slot

Nearly everything inside a PC is designed to be user-installable. The Audio Modem Riser (AMR), Communications and Networking Riser (CNR), and Advanced Communication Riser (ACR) slots are exceptions. Although their presence on many recent motherboards intrigues some upgraders, these slots were never intended as general-purpose expansion slots. All of them were designed to be used by OEM system builders, not by backyard mechanics. Here’s what you need to know about AMR, CNR, and ACR slots:

AMR slot

Intel developed the AMR slot to provide an easy, standardized way to integrate modem and audio functions into finished systems at minimal cost, but OEM system builders ignored it in droves. Why? Mainly because the AMR slot took the place of a standard PCI slot, and most motherboard designers and system builders rightly preferred having an extra PCI slot to having an AMR slot of dubious utility. The AMR slot also had limited functionality and no support for Plug and Play. The result was that, although some motherboards included an AMR slot, very few AMR-compatible cards were ever developed and those that were achieved only limited distribution. We’ve seen exactly one AMR card.

CNR slot

Intel’s answer to the problems of AMR was to redesign the AMR slot. The CNR slot, shown in Figure 1-2, can coexist with a standard PCI slot, allowing either a CNR card or a standard PCI card to use the slot position interchangeably. CNR also adds Plug and Play support ...