Analysis of DB9 serial line definition

The cable of the DB9 port is commonly used in serial communication. This article will focus on the definition, testing, products and connection methods of the DB9 port, and introduce the DB9 port in detail.

1>DB9 port definition

Serial communication mainly uses 2/3/5 pins.

2>How to test the serial line

1. Multimeter

For male-to-male, directly use a multimeter to test whether the two ends are connected to determine whether the 2/3/5 pins can be measured. To measure the female head, it is necessary to put the multimeter into the pin air. Because the measuring end of the multimeter is relatively thick, it is necessary to solder two relatively thin metal pins, or pins of a resistor, etc. on the multimeter pins, so as to facilitate the measurement of the female head, using Turn on the multimeter and measure 2/3/5 of the serial port cable. When measuring, the straight line, 2/3/5 corresponds to 2/3/5, and the cross line is 2 to 3, 3 to 2, and 5 to 5 for measurement (that is, 2 at one end of the cross serial port measures 3 at the other end).

2. Use serial port assistant.

When the serial port realizes sending and receiving, RXD is connected to TXD, TXD is connected to RXD, and the serial port assistant is used to short the 2/3 pins of DB9, and the serial port assistant is used to send data.

3>Serial cable with crossover or direct connection

1. Serial line interface and connection method

There are three types of male and female heads: male to female, male to male, and female to female. The above three connection lines have crossover lines and straight lines, so there are a total of 6 connection methods.

2. What is straight line and cross line?

Direct serial cable: 2 to 2, 3 to 3, 5 to 5. Cross serial cable: 2 to 3, 3 to 2, 5 to 5.

3. Why do cross and direct serial lines appear?

The standard definition of DB9 is that pin 2 is RXD and pin 3 is TXD. When designing the circuit, some people connect pin 2 to TXD and pin 3 to RXD in order to use a direct connection.

In circuit design, if the transceiver and standard pin definitions are not reversed, a crossover wire must be used. Under normal circumstances: two female headers are crossed wires, one male and one female are straight connections. Communication conditions: RXD is connected to TXD, TXD is connected to RXD, and whether it is a cross-wire or a straight-line connection is selected according to the circuit design. Making a serial port cable: When there is only a straight cable at hand, it can be cut off. 2 pairs of 3, 3 pairs of 2, and 5 pairs of 5 are welded to form a crossover cable.