The concept of serial communication and interface circuit

With the development of computer systems and the development of computer networks, communication functions are becoming more and more important. The communication mentioned here is the exchange of information between only the computer and the outside world. Therefore, communication includes both information exchange between a computer and an external device, as well as between a computer and a computer. Since serial communication transmits information one bit at a time on a transmission line, the transmission line is small, and information can be transmitted by means of an off-the-shelf telephone network, and therefore, it is particularly suitable for long-distance transmission. It is also common to exchange data serially for human-machine-switched devices and serial-storage external devices such as terminals, printers, logic analyzers, and magnetic disks that are not far from the computer. In real-time control and management, multiple microcomputer processors are used to form a hierarchical distribution control system, and communication between CPUs is generally serial. Therefore, the serial interface is a commonly used interface for microcomputer application systems.

Many peripherals and computers communicate in a serial manner. The serial mode mentioned here refers to the way information is transmitted between peripherals and interface circuits. In fact, the CPU and the interface still operate in parallel.

1 concept of serial communication

Picture 1-1

The term "serial communication" refers to the use of a data signal line between a peripheral device and a computer (in addition, a ground line is required, and a control line may also be required), and data is transmitted one bit at a time on a data signal line, each Bit data occupies a fixed length of time. As shown in Figure 1-1. This communication method uses less data lines and can save communication costs in long-distance communication. Of course, its transmission speed is slower than parallel transmission.

Since the CPU and the interface are transmitted in parallel, the interface and the peripheral are transmitted serially. Therefore, in the serial interface, there must be a "receive shift register" (string → parallel) and "transmission shift". Register" (and → string). The structure of a typical serial interface is shown in Figure 1-2.

Figure 1-2

In the data input process, the data enters the "receive shift register" of the interface from the peripheral device by 1 bit, and the data is received from the "receive shift" after the bits of 1 character have been received in the "receive shift register". The register "enters" the "data input register". The CPU reads the received characters from the "Data Input Register". (Parallel read, ie D7~D0 are simultaneously read into the accumulator). The shift speed of the "receive shift register" is determined by the "reception clock".

During the data output process, the CPU sends the characters to be output (in parallel) to the "data output register", and the contents of the "data output register" are transferred to the "transmission shift register", which is then shifted by the "transmission shift register". Send data 1 bit to 1 bit to the peripheral. The shift speed of the "transmission shift register" is determined by the "transmission clock".

The "control register" in the interface is used to accommodate various control information that the CPU sends to this interface. These control information determine how the interface works.

Each bit of the "Status Register" is called a "status bit" and each status bit can be used to indicate a status or some error during data transfer. For example, using the D5 bit of the status register to "1" indicates that the "data output register" is empty, the D0 bit indicates "data input register full", and the D2 bit indicates "parity error".

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