When bits must be moved about within the computer itself, they are transmitted along wires. If the data to be transmitted is in 8-bits format bytes, then eight separate, discrete wires must simultaneously carry the eight representative electrical voltages between the two points. This simultaneous transmission of the eight bit-voltages that constitute a byte is referred to as "parallel transfer". Parallel transfer, then,is done byte-by-byte.Since all eight bits arrive at their destination at the same instant, parallel data transfer can be accomplished at extremely high speeds. These qualities make it the preferred method of data transfer whenever possible.
Data transfer, especially high-speed data transfer,demands a tightly controlled environment. The internal temperature of the computer must be regulated and the electrical properties of resistance, capacitance, and inductance carefully pre-calculated. As long as data is being moved about inside a computer, this environment is stable and predictable. But a great deal of computer data must be transported to the outside world. Microcomputers communicate with peripheral devices such as printers, terminals, modems, print buffers,etc. These processes are known collectively as input/output, or simply I/O.
The primary objective of any interface is to provide a medium for the transfer of data. Further more, self-protection and usability are also important goals for any interface. Once such an interface has been established , the transfer of data to external environments is possible.
When considering parallel transfer for the interface, two major problems arise. The first is the wire itself. At least nine wires - eight for the data bits, one for circuit common ("ground")- are needed. Still more wires are usually required to control the flow of data across the interface. Another problem lies in the very nature of the bits/voltages themselves. When a bit/voltage changes state from a one to a zero, or vice versa, it does so very rapidly -in the order of nanoseconds (one billionth of a second).This abruptness is itself an essential part of the process of data transfer. Slow changes between zero and one are not even recognized as data. As a cable gets longer,its electrical properties (capacitance & inductance) restrict the abruptness with which a bit can change between zero and one, and data corruption or loss becomes likely. Because of this,the speed inherent in parallel data transfers makes transmission over long cables problematic.
Therefor, its use is restricted to a few peripheral devices (such as printers) that are likely to be used in close proximity to the computer, or that must be operate at very high speeds.
The obvious alternative to sending all bits simultaneously on multiple wires is to send them singly, one after the other. At the receiving end, the process is reversed and the individual bits are reassembled into the original byte. With just one bit to transmit at a time,data can be transferred with a simple electrical circuit consisting of only two wires. This scheme - known as "Serial Transfer" - reduces the bulk and much of the expense of the parallel technique.
This saving is offset by a decrease in efficiency: it takes at least eight times longer to transmit eight individual bits one after the other than to transmit them all simultaneously in parallel. This speed limit is insignificant for many typical applications. serial peripheral devices are slow, at least in comparison to the internal speed of microprocessors. Each involves some time-consuming, sometimes mechanical process that greatly limits its speed: printers are limited by the speed of their print-heads, modems by the frequency restrictions of the telephone lines, and disk drives by their slow rotational speed.So the speed inherent in the process of parallel data transfer is largely wasted on such peripheral devices. The serial method, therefore ,can afford to sacrifice some speed while still adequately servicing the peripheral devices. In such cases,the sacrifice in speed is inconsequential in comparison to the increased reliability and transmission range.
In the late 1960's a need surfaced for remote access to mainframe computers. It become desirable for the end-users to access computers from remote locations. Short distances- a few hundred feet, perhaps within the same building- could be spanned by the addition of extra wires. For truly distant remote access, telephone lines were considered. For many reasons computer data cannot be injected directly into the telephone network. A translating device - the Modem - is required.
When computerized telecommunications was in its infancy, the Bell System supplied most of the data equipment to its lines. Bell naturally exercised strict control over the modem interface. But as activity in the telecommunications field increased, and more and different kinds of equipment began to appear, Bell surveyed the hodgepodge of equipment that the computer industry was threatening to connect to its lines. It saw little that it liked and much that it felt would compromise and complicate the delivery of communications service to the public. The telephone companies predictably prohibited the connection of most of these devices.
A typical DTE device is an ordinary video terminal with a keyboard and a video display. Data on pin 2 of the DTE is transmitted, while the same data on pin 2 of a DCE (modem) is received data.
An important distinction between the kinds of signals of the interface is between data signals and control signals. Data signals are simply the pins which actually transmit and receive the characters, while control signals are everything else. If a modem can automatically answer the telephone, for example, it must be able to report an incoming call to the computer and not start transferring data to the computer without first receiving a "OK, I'm ready to receive now" confirmation from the computer.
There are generally two or three such inquire-confirm pairs on an interface that allow one device to "talk" to the other. There is in practice no guarantee that a modem and/or terminal will implement any or all of these handshaking features. The manufacturers of equipment may arbitrarily decide to apply some of the standard handshaking, no handshaking at all, or to invent a scheme of their own.
The interface standard document consist of four parts:"Interface Between Data Terminal Equipment and Data Communications Equipment Employing Serial Binary Data Interchange."