Direct Connection and crossover

Question 1: When to use crossover and when to use direct connection?

The Standard Serial Port Pin 2 is RX, and Pin 3 is TX (that is, the 9-pin Serial Port 2 is connected to the rxd pin of the master chip, pin 3 is connected to the txd pin of the main control chip). If the serial port of the Single Chip Microcomputer Development Board and the serial port of the computer are both standard serial ports, the two serial ports should be connected using a crossover line. However, in real life, there are many direct connections (it may be that there is no cross-line trouble in the production of direct connections). In order to cooperate with the use of direct connections, in the drawing board, connect the rxd pin of the main control chip to the three-pin serial port, and the txd pin of the main control chip to the two-pin serial port, so that the serial port on this development board is not a standard serial port, that is to say, 2 is changed to TX, 3 is changed to RX, and the direct connection should be used when connecting to the standard serial port. Check whether the serial port on a development board is a standard serial port or a non-standard serial port. It depends on how the schematic diagram is connected. ^-^

Question 2: How to identify cross-string and direct-line connections?

Test with A multimeter. If the General Principles of 2, 2, 3, and 3 are directly connected to the serial line; if the two and 3 connections, the general principles of 3 and 2 are crosstab lines.

Some people are confused that the outside of the mother port is an insulating rubber. How can I test the plug-in of the multimeter... Cut a small piece of solder wire (I have also considered using wire, but the wire is too thin, we cannot ensure it can be in good contact with the metal part of the mother port, the solder wire is a little thick, it can be very good contact with the metal in the mother's mouth, I have checked ), and then the multimeter probe is in contact with the solder wire to test.