An interface control document (ICD) describes the methods and structures of a system’s inputs and outputs. It provides developers with documentation for building software or hardware that transfers data to and from the system. The ICD does not describe the internal workings of the system, but rather focuses on the exact functions and hardware components needed. It allows for testing without a full interface and can be modified freely without affecting external development.
An interface control document (ICD) is a formalized description of the methods and structures involved in providing inputs and receiving outputs from a specific system. The system described by the interface control document can be a software library or a hardware component. The document does not have to follow a single format but can be a collection of paragraphs, graphs or even just technical drawings of the interface hardware. When referring specifically to software, a control interface document can resemble an abstract programming interface (API), which describes public methods or functions that can be used to enter information into the library, and also describes the resulting output. An interface control document usually describes how to integrate the system into a larger system or how to connect it to a parallel system; it does not describe any of the internal workings of the system, which could be stated in a separate type of document.
The purpose of an interface control document is to provide hardware or software developers with documentation that can be used when building a system or software that will transfer data to and from the system described by the ICD. This usually means defining exact functions or hardware components so that their signatures are known and parameter tolerances are provided for use. In software engineering, this may mean knowing the name of a particular function, what kind of variables are accepted as parameters, and possibly what functional limits are placed on the values being passed. For a hardware component, this information may include the functions of the pins of a serial connector control, any hardware interrupts used, and the working speed of the device.
One thing that an interface control document doesn’t specifically describe is how the system translates input into output, or how output is produced, in general. This allows developers to have a narrowly focused view of the system when building an interface, but also requires system developers that the details of the ICD strictly adhere to the guidelines spelled out in the document itself. A convenience for authors of an interface control document and for developers of the system is that the internal implementation of the system is not described in the document and, therefore, can be modified freely without affecting the external development of interfaces based on the ICD extension.
In some situations, an interface control document can allow systems to be tested without actually having to use a full interface. This can be done by simulating the various types of output a system can generate as described in the ICD, and then passing that output through the externally developed interface. Systems that are only interested in handling one side of the system, such as output, in the case of hardware such as a display device, can ensure that the interface works within specification without requiring real-world input.
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