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Students who do embedded development or hardware development , You must hear it often RS232、RS485、TTL And so on , If you are not clear about these concepts , There will be problems in use , This paper introduces the meaning of these concepts （ See if the picture below is dizzy ）

1 A serial port

First of all, make it clear , A serial port 、COM Port refers to the form of hardware interface , and TTL、RS-232、RS-485 Refers to the level signal

1.1 UART Interface

Universal asynchronous transceiver （UniversalAsynchronous Receiver/Transmitter）,UART It is the logic circuit of serial port transceiver , This part can be made into a chip , It can also be embedded into other chips as a module , Single chip microcomputer 、SOC、PC There's going to be UART modular .

UART Yes 4 individual pin（VCC、GND、RX、TX）, It's using TTL level , The low level is 0（0V）, The high level is 1（3.3V Or more ）. 

1.2 COM mouth

COM Port is the serial communication port , Serial port for short , It's very common D type 9 Pin plug , There are only two protocols for this interface ：RS-232 and RS-485. Will not be TTL Level . Generally, only RXD、TXD, Plus GND.

1.3 USB mouth

Universal serial bus , And serial port are two concepts . Although it's also serial communication , But because of USB The communication timing and signal level are completely different from serial port , So it has nothing to do with the serial port .USB It's a high-speed communication interface , be used for PC Connect all kinds of peripherals ,U disc 、 Key mouse 、 mobile hard disk drive 、 And of course "USB To serial port " Module .（USB Serial port module , Namely USB Interface UART modular ）

2 TTL

TTL level ： full duplex （ Logic 1：2.4V ~ 5V, Logic 0：0V ~ 0.5V）

How to connect

Level indication

3 RS232

It is the asynchronous transmission standard interface formulated by the electronics industry association , At the same time corresponding to the level standard and communication protocol （ sequential ）, Its level standard ：+3V ~ +15V Corresponding 0,-3V ~ -15V Corresponding 1.RS232 The logic level and TTL Not the same, but the agreement is the same .RS-232 Suitable for communication between local devices , The transmission distance generally does not exceed 20m.RS-232 Only one to one communication is allowed .

RS-232 level ： full duplex （ Logic 1：-15V ~ -3V, Logic 0：+3V ~ +15V）

Connection diagram

Level indication

4 RS485

RS485 Is a serial interface standard , For long distance transmission, differential transmission is adopted , It's a differential signal , The anti-jamming ability is better than RS232 Much better . The pressure difference between the two lines is -(2~6)V Express 0, The pressure difference between the two lines is +(2~6)V Express 1.RS-485 The transmission distance is tens of meters to thousands of meters .RS-485 The interface on the bus is allowed to connect up to 128 A transceiver .

RS-485： Half duplex 、（ Logic 1：+2V ~ +6V, Logic 0：-6V ~ -2V） The level here refers to AB The voltage difference between the two wires .

Connection diagram

Level indication

5 Conversion chip

5.1 USB turn TTL

PL2303、CP2102 The chip is USB turn TTL Serial port chip , use USB To expand the serial port （TTL level ）.USB turn TTL The serial port is shown in the figure below ：

5.2  TTL And RS-232 Interturn

MAX232 The chip is TTL Level vs RS232 Special bidirectional conversion chip for level , Sure TTL turn RS-232, It's fine too RS-232 turn TTL. Here's the picture ：

Reference resources

A serial port 、TTL、232、485 difference _IOT-SYM The blog of -CSDN Blog _232 485 difference

A serial port 、COM mouth 、UART mouth , TTL、RS-232、RS-485 Detailed explanation _ level