How to distinguish direct serial line and cross serial line?

The serial line also points through and crosses, pass-through is generally used to extend the PC and equipment, will be 2, 3, 5 respectively connected 2, 3, 5, because the PC is generally male, and the device is mostly female, so just they are universal, can be used for extension can also be used for connection; crossover is generally used for PC docking with PC, 2 to 3, 3 to 2, 5 to 5, usually both ends are female!

Before the computer appeared, the EIA established the RS232 standard for connecting serial devices. After the emergence of PC, the existing serial devices become PC peripherals, naturally adopted RS232 standard. At present, the serial communication interface of PC adopts eia-rs-232c standard, and C represents the latest change in 1969. The EIA-RS-232C standard provides for electrical characteristics, logic and various signal line functions. The EIA-RS-232C standard uses the positive and negative voltage to represent the logic state, on the data signal line if the voltage between the -3v~-15v represents the logic 1, if the voltage between the +3v~+15v represents the logic 0; on the control signal line if the voltage between -3v ~-15v indicates the disconnection state, if the voltage in +3v ~+ 15V indicates the on state. Voltages between -3v ~+3v and below -15v, above +15v, are meaningless.
The CPU and the terminal are used TTL level and the positive logic, TTL level with the 1,0v to express the logic of logic 0, which is incompatible with the level and negative logic used in EIA, need to be converted in the interface circuit. The EIA-RS-232C standard does not define the physical characteristics of the connector. As a result, DB-25, DB-15 and DB-9 connectors of various types are present,PC COM1 and COM2 serial interface with DB-9 connectors。 The ELA-RS-232C standard stipulates that when the BER is less than 4%, the wire length 15m is agreed. In practical applications, when the use of 9600b/s, ordinary twisted shield line, the transmission distance of up to 30m ~35m.The PC's COM1 and COM2 two serial interface DB-9 connectors are male (PIN) headers that provide 9 pin functions for asynchronous communication。 respectively: ① foot (DCD) data carrier detection, ② foot (RXD) receive data, ③ foot (TXD) send data, ④ foot (DTR) data terminal preparation, ⑤ foot (SG) signal ground, ⑥ foot (DSR) data device ready, ⑦ foot (RaS) request sent, ⑧ foot (CTS) clear send, ⑨ foot (R I) ringing indication.DB-9 male (PIN) head arrangement order for example with left, DB-9 female (hole) head in order such as right, pay special attention to the male pin number from sitting to right, the female head is from right to left . EIA-RS-232C Standard, there are three send signals: TXD, RTS and DTR, the typical output current of each line is ±8ma/±12v. It is usually powered by RTS and DTR to provide approximately 192mW of power.


The PC side is generally male,

In combination with the above instructions, the modem is not involved. The serial port data transmission only must have the receiving data end ② foot and sends the data end ③ the foot to be possible. The receiving data end of a device should be connected to the sending data end of a device. That is, the reception and transmission intersect each other, the signal to the corresponding phase.

Question 1: When to use crossover, when to use direct connection?
Standard serial Port pin 2 is RX, pin 3 is TX, (that is, the nine-pin serial port 2nd pin is connected to the RXD pin of the master chip, 3rd PIN is connected to the main control chip TXD PIN), assuming that 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!

In general: The connection line of the RS232 DB-9 interface contains three kinds: male to female, male to male, mother to mother. Each of the three connectors has crossover lines and straight lines, so there is a total of 6 connections in the same way.

Question 2: How to identify the cross-serial line and direct-connect serial line?
Measured with a multimeter, assuming that the 2 and 2 pass, 3 and 3 are straight serial lines; Assuming that 2 and 3 pass, 3 and 2 are cross-serial lines.
Someone confused the outside of the mother is an insulating rubber, multimeter probe can not be inserted how to measure? 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 it with the metal part of the mother's mouth to perfect contact, solder wire coarse, it can be very good contact with the metal in the mother's mouth), and then the multimeter probe contact solder wire to test it can.
Of course, assuming that every time you get a serial cable to take a multimeter to measure whether it is straight or cross is not very troublesome, teach you a easy way to distinguish the good method, that is: Two mother is cross, a male and female is a direct connection . (The serial line of the male port at both ends seems very small)

Note: It is strongly recommended not to live plug-in serial port, at least one end of the plug-in is a power outage, or the serial port easily damaged.

How to distinguish direct serial line and cross serial line?