I/O Interface

The word “Interface” refers to the boundary between two circuits or devices. A standard for any interface comprises a set of specification for the functional, electrical and mechanical characteristics of all signal and power lines that cross the boundary. It defines a connector and the signal lines used. Computer system designer has to design a circuit that meets the requirement of a given interface standard.

Different interface is needed for every combination of I/O device and computer, we have to standardize interface signals and protocols. The structure of main bus is closely tied to the architecture of the computer, it is expected to charge from on computer to another. Since, all computer peripherals vary with wide range of transfer speeds and other requirements, it is equally difficult to develop a standard that covers all. An alternative solution is to define standards for certain classes of interconnection that are suitable for both computers and peripherals.

Some commonly used bus standards are shown in figure below. The CPU is connected through its processor bus to the memory and I/O interfaces. The properties of processors bus are determined by the structure of the computer and the CPU signal lines, different computer models made by the same manufacturer often have different bus structures. For example: The unibus in PDP-II computers where as SBI bus in VAX computers. A number of bus standards have evolved, for example, VME bus and multibus, which implement a full complement of the functions normally supported by a computer bus. They serve to provide a standard interface for connecting additional main memory modulus, secondary storage units or I/O devices.

The function of the bus mapping circuit is to translate one set of bus signals into the other and to provide any temporary buffering that may be needed because of differences in the timing of data transfers.

In figure, three different approaches are the multibus, IEEE 488 standards for interconnection of several devices via a bus and RS-232-C is the most commonly used interface standard used for connecting a single serial device. The IEEE-488 bus standard is intended for connecting laboratory instruments, plotters, floppy disks, and other law medium-speed peripherals to a controller. The interface circuit contains data and control registers that must be accessed explicity by the software.

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