It can be used to connect an external modem, plotter or serial printer, but nowadays there are few devices that need this kind of interface, basically it is USB interface.
Serial cables are used for serial communication between devices and are widely available. Serial cable with high compatibility, the most conventional use: desktop computer connected to various types of serial devices, such as computers and tax control machines, digital machine tools, PDA, Modem, set-top boxes, bar code machines, industrial instruments and other serial devices for communication functions.
Nine-pin serial port usually RS-232 interface to 9 pins (DB-9) or 25 pins (DB-25) of the type of appearance, the general personal computer will have two sets of RS-232 interface, respectively, known as COM1 and COM2.
Testing the instrumentation and microcomputer communication is mainly based on the RS232C standard interfaces, and the wiring of the serial port is also a certain standard. There are certain standards.
Extended information
On the nine-pin serial port male and female:
Serial cable is also divided into straight-through and crossover, straight-through is generally used to extend the PC and the device will be connected to the 2, 3, 5 2,3,5, respectively, because the PC for the general public, while the device for the female, so it happens to be common to both for extension and connection. For extension can also be used to connect; cross generally used for PC to PC docking, will be 2 to 3, 3 to 2, 5 to 5, generally both ends are female.
Before the emergence of computers, in order to connect the serial devices, EIA developed the RS232 standard. after the emergence of the PC, the existing serial devices become PC peripherals, naturally using the RS232 standard. Currently, the serial communication interface of PC adopts the EIA-RS-232C standard, with C representing the latest revision in 1969. The EIA-RS-232C standard specifies the electrical characteristics, logic levels, and functions of various signal lines.
The EIA-RS-232C standard uses positive and negative voltages to indicate the logic state, in the data signal line if the voltage is between -3V and -15V indicates logic 1, if the voltage is between +3V and +15V indicates logic 0; in the control signal line if the voltage is between -3V and -15V indicates the disconnect state, if the voltage is between +3V and +15V indicates the on state.
Between -3V and +3V and below -15V, above +15V voltage is meaningless.