USB communications device class (orUSB CDC) is a compositeUniversal Serial Busdevice class. It is most commonly used forcomputer networking devices akin to anetwork card, providing an interface for transmittingEthernet orATM frames onto some physical media. It is also used formodems,serial ports,ISDN,fax machines, andtelephony devices that perform regular voice calls.
Devices of this class are also implemented inembedded systems such asmobile phones so that a phone may be used as a modem, fax ornetwork port. The data interfaces are generally used to perform bulk data transfer.
Microsoft Windows versions prior toWindows Vista do not work with the networking parts of the USB CDC, instead using Microsoft's own derivative namedMicrosoft RNDIS, a serialized version of theMicrosoft NDIS (Network Driver Interface Specification). With a vendor-suppliedINF file, Windows Vista works with USB CDC and USB WMCDC devices.[1]
TheAbstract Control Model (ACM) protocol defined in the CDC specification can be used to implement a general-purpose serialUART. For example, an adapter cable which is USB on one end andRS-232 on the other may present the USB end to the operating system as a CDC-ACM device which uses the operating system's standard CDC-ACM driver. Such an adapter provides a way to upgrade industrial equipment that uses RS-232 such asCNC machinery without changing the equipment's hardware or software. Most USB serial adapters do not use USB's CDC-ACM protocol but rather their own vendor-defined drivers and protocols which support more features, but there are a few adapters such as the Prolific PL2503 that do use standard CDC-ACM.[2] Furthermore, CDC-ACM is commonly implemented in software onmicrocontrollers such as theESP32 to provide aserial console for configuring and debuggingembedded systems.[3]
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