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, 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 and other features of interest to system designers. AMR and CNR are incompatible, at both the physical and electrical level. Although we have seen a few CNR cards, mostly modems and sound adapters, CNR cards are not much easier to find than AMR cards.

ACR slot

AMR and CNR are both Intel technologies. AMD, VIA and the rest of the everyone-who-is-not-Intel camp came up with an alternative called the ACR slot, which is found on some Intel-free motherboards. The ACR slot is physically a standard PCI slot connector, which you can recognize because it's turned 90 degrees to the other PCI connectors on the motherboard. In theory, the ACR slot offers several advantages over the AMR/CNR slot, including its use of standard connectors and its additional flexibility because of the greater number of available pins. In practice, we've never seen or even heard of a card designed to fit that slot, so it is effectively a wasted connector.

Intel warns that the AMR and CNR interfaces are not rigidly defined, so it is quite possible that any given AMR or CNR card simply will not work in a particular AMR or CNR slot. If your motherboard has an AMR, CNR, or ACR slot, we suggest you pretend it's not there.