What is the difference between a direct serial cable and a cross serial cable? and How to quickly distinguish?

1. The difference between straight-through serial port and cross-serial port

The serial line is divided into straight-through and cross-connected. Straight-through is generally used to extend the PC and the device, and connect 2, 3, and 5 to 2, 3, and 5 respectively. Because the PC is generally male, and the device is mostly female, so they happen to be Universal, can be used for both extension and connection.

Crossover is generally used for PC and PC docking, 2 to 3, 3 to 2, 5 to 5, generally both ends are female.

2. How to distinguish between a crossover serial line and a direct serial line?

You can use the multimeter directly. In the detection, if 2 and 2 are connected, 3 and 3 are connected directly to the serial port; if 2 and 3 are connected, 3 and 2 are connected to the cross serial line.

Generally, the outside of the serial port cable is a piece of insulating rubber, and it is difficult to insert the probe of the multimeter. It is recommended to cut a small section of solder wire, and then touch the probe of the multimeter to the solder wire to test. Of course, every time you get a serial cable, you have to use a multimeter to test whether it is straight or crossed. The serial lines that are all male ports are rarely used in reality).

Note:

It should be noted that during operation, do not plug and unplug the serial port with power on. At least one end of the serial port is powered off when plugging and unplugging, otherwise the serial port will be easily damaged.

3. When to use crossover and when to use straight-through?

Standard serial port pin 2 is RX, pin 3 is TX, (that is, pin 2 of the nine-pin serial port is connected to the RXD pin of the main control chip, and pin 3 is connected to the TXD pin of the main control chip) , If the serial port of the microcontroller development board and the serial port of the computer are both standard serial ports, the two serial ports should be connected with a crossover cable.

However, in reality, most of the straight-through lines are used. In order to use the straight-through lines, when drawing the board, connect the RXD pin of the main control chip to the 3-pin of the serial port, and the TXD pin of the main control chip to the 2-pin of the serial port, so that this The serial port on the development board is not a standard serial port, that is, 2 becomes TX, and 3 becomes RX. Of course, a direct connection should be used when connecting to a standard serial port. To see whether the serial port on a development board is a standard serial port or a non-standard serial port, the most important thing is to see how the schematic diagram is connected.

Let's take RS232 Serial D-sub 9Pin Male to Female DB 9pin Molded Cable and RS232 Serial D-sub 9Pin Male to Male DB 9pin Molded Cable as an example. There are three types of connecting lines: male to female, male to male, and female to female. These three connection lines have crossover lines and straight lines respectively, so there are 6 connection methods in total.

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