Comparison of serial port and parallel port

Serial Interface The serial interface is usually referred to as the serial port. The English logo is Serial or COM. The current PC usually has two serial ports COM 1 and COM 2. These two interfaces are the standard interfaces that were first introduced in computers, namely the RS232 standard. Before the standard was produced, the peripherals of the computers produced by each manufacturer were also produced together with the computer, which was incompatible with the peripherals of other manufacturers. . The RS232 standard is too slow, and now the remaining two interfaces are used for the mouse and the external "cat". The serial port differs from the parallel port in that its data and control information are transmitted one by one. Although this speed will be slower, the transmission distance is longer than the parallel port. Usually COM 1 uses a 9-pin D-connector, while COM 2 uses an older DB25-pin connector. The interface is divided into an asynchronous (Asynchronous) interface and an asynchronous (Synchronous) interface. In order for the receiving end to know that new data has appeared on the data line, it is most convenient to use a synchronization signal. The advantage of synchronous communication is that you can use any frequency allowed by the line, but it is not convenient in actual use, so the serial communication now uses asynchronous communication. The earliest serial interface RS232 is an asynchronous communication protocol. Asynchronous communication requires coordination of the frequency of both parties, called a handshake. Whether we use 28, 36 or 56K "cats" can be connected to the same access address, because the series of "嘀嘀" after the dialing is a handshake to coordinate the communication frequency of both parties.

Parallel interface parallel interface is also referred to as parallel port, the English logo is Parallel or LPT, PRN. Currently, the parallel interface in the computer is mainly used as a printer port, instead of a 36-pin connector but a 25-pin D-connector. The so-called "parallel" means that 8-bit data is simultaneously transmitted through parallel lines, so that the data transfer speed is greatly improved, but the length of the line for parallel transmission is limited, because the length is increased, the interference is increased, and the data is prone to error. There are now five common parallel ports: 4-bit, 8-bit, half-eight, EPP and ECP. Most PCs are equipped with 4-bit or 8-bit parallel ports. Computers that support all IEEE1284 parallel port specifications are basically equipped with ECP parallel ports. . Standard parallel port refers to 4-bit, 8-bit, and half 8-bit parallel ports. The 4-bit port can only input 4 bits of data at a time, but can output 8 bits of data; the 8-bit port can input and output 8 bits of data at a time. Developed by companies such as Intel, the EPP port (enhanced parallel port) allows 8-bit bidirectional data transfer and can be connected to a variety of non-printer devices such as scanners, LAN adapters, disk drives and CD-ROM drives. Developed by Microsoft and HP, the ECP port (Extended Parallel Port) supports command cycles, data cycles, and multiple logical device addressing. MA (Direct Memory Access) can be used in a multitasking environment. At present, almost all Pentium-class motherboards have integrated parallel ports and are labeled Par-allel 1 or LPT 1, which is a 25-pin dual-row socket.

USB interface USB is “Universal Serial Bus”, Chinese name is general serial bus. This is a new interface technology that has been widely applied in the PC field in the past two years. In theory, USB technology consists of three parts: a PC system with a USB interface, a device that can support USB system software, and a device that uses a USB interface. USB devices come in two different connectors, called the A series and the B series. A Series connectors are designed primarily for those that require a permanent connection to the cable, such as a hub, keyboard, and mouse. The USB connectors on most motherboards are A-Series connectors. The B-Series connectors are designed for devices that require a separate cable. Such as printers, scanners, modems, etc. The physical USB plug is a small rectangular shape. In theory, USB can connect 127 devices in series, but in actual application testing, it may not be possible to connect 3 to 4 devices in series. Moreover, as the USB product itself, only the keyboard has an input and output dual-head design, and other products have only one input interface, so it is impossible to connect another USB device. In this case, if you need to connect multiple USB devices, you need a connected bridge - USB HUB, but the USB HUB can not be more than three consecutive serial connections, that is, it cannot be on the third USB interface that is connected in series. Connect the USB HUB in series. Current ATX motherboards typically have only two built-in USB ports. Today, USB has been used in a variety of peripherals on PCs. Output devices, including scanners, digital cameras, digital camcorders, audio systems, displays, etc., are mainly based on USB's high-speed data transmission capabilities. On the input device side, the USB keyboard, mouse, and joystick are extremely stable and have few problems. There are also USB "cats" for DSL, USB ZIP drives for IOMEGA, and USB PC cards for eTek.