INTERFACING TO A MICROPROCESSOR CONTROLLER – The Serial Interface
In a serial interface, the data are sent 1 bit after the other on a single wire. There are a number of good reasons for doing this. First, the cabling is simpler because only two wires are needed (at a minimum), those being “data” and “return.” Second, shielding a small group of wires, which is often necessary in an electrically noisy industrial environment, is easier. Third, serial data can make use of existing single-channel data lines such as the telephone system (which may require using a modem). For these reasons,
serial data transfer is usually recommended for distances greater than 10-30 ft.
Because data always exist in a parallel form inside the computer, it must be converted to serial data before coming out the serial port. This is accomplished with a special
parallel-to-serial converter IC called a universal asynchronous receiver transmitter (UART). On the other end of the line, a receiver must convert the serial data back into parallel data, which is done with another UART. Figure 10 shows the basic serial data circuit.
Serial data are classified as being either synchronous or asynchronous. Synchronous data require that the data bytes be sent as a group in a “package.” It is used in sophisticated communication systems that move a lot of data and will not be further discussed here. Asynchronous data transfer is the more common (but slower) type of serial transfer and allows for individual bytes to be sent when needed.