Direct Connect serial line, cross serial line

Question 1: When to use crossover and when to use Direct connect.

Standard serial Port pin 2 is RX, pin 3 is TX, (that is, the nine-pin serial port 2nd PIN connection is the main control chip RXD pin, 3rd pin is connected to the main control chip TXD pin), if the SCM Development Board serial port and the computer serial port are standard serial port, the two serial ports should be connected with a crossover line. But in real life straight line most (may be straight wire production up without cross-wire trouble it), in order to cooperate with the use of direct connection, in the artboard, the main control chip RXD pin connected to the serial port of 3 feet, the main control chip TXD pin connected to the serial port of 2, so this development Board is not a standard serial port, That is, 2 becomes tx,3 to Rx, and the standard serial connection should of course use straight line. Look at a development board on the serial port is the standard serial port is also a non-standard serial port, it is essential to see how the schematic is connected ^-^

Question 2: How to identify the cross-serial line and the direct-connect serial line.

Measured with a multimeter, if 2 and 2, 3 and 3 are straight serial lines, if 2 and 3 pass, 3 and 2 are the cross-serial line of the general.

Someone confused mother mouth outside is a piece of insulating rubber, multimeter probe can not go in how to test it ... Then cut a small section of welding solder (I have also considered the use of wire, but the wire is too thin, we can not guarantee that it can be in good contact with the metal parts of the mother mouth, solder wire coarse, it can be in contact with the metal in the mother's mouth, I have tested the hey), and then the multimeter probe contact solder wire to test