Serial port-rs232-rs485-ttl

A serial port 、RS-232

  Engineer explains serial port ：RS-232、RS-422、RS-485 The difference between

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

First , A serial port 、UART mouth 、COM mouth 、USB Port refers to the form of physical interface ( Hardware ). and TTL、RS-232、RS-485 It refers to the level standard of ( Electrical signals ).

A serial port ： Serial port is a general term ,UART、TTL、RS232、RS485 They all follow similar communication timing protocols , So they are called serial ports .

UART Interface ： Universal asynchronous transceiver （Universal Asynchronous 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 .

COM mouth ： Especially on a desktop computer or some electronic device D-SUB shape ( A connector structure ,VGA The connector of the interface is also D-SUB) The serial communication port of , The serial communication timing and RS232 The logic level of .

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 ）

TTL： When we communicate through serial port , Basically, those directly from the single chip microcomputer are TTL level .
TTL level ： full duplex （ Logic 1:2.4V-5V; Logic 0:0V-5V）
The hardware block diagram is as follows ,TTL For two MCU Inter communication .

 

RS-232： full duplex （ Logic 1:-15V-5V; Logic 0:3V-15V）
The hardware block diagram is as follows ,RS-232 be used for MCU and PC Inter machine communication .

 

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

A serial port

Generally, there are two standards ：D type 9 Pin plug 、4 Needle Du Bangtou .

1. common 4 Pin serial port ：

It's common on circuit boards , Often with DuPont pins . And sometimes there's a fifth needle ,3.3V The power side .

Embedded serial port , Generally refer to UART mouth .UART Yes 4 individual pin（VCC、GND、RX、TX）, With TTL level , The low level is 0(0V), The high level is 1(3.3V Or more ).

2.D type 9 Pin serial port （DB9）：

There are only two protocols for this interface ：RS-232 and RS-485, Will not be TTL. Generally, only RXD、TXD Two stitches , Plus GND.

USB turn TTL：

USB turn RS-232：

The basic principle of serial communication

Reference resources 【STM32】 The basic principle of serial communication
The communication mode between devices is generally ： Parallel communication 、 There are two kinds of serial communication .
Serial communication classification ：
1. It is divided into ：
· Simplex ： Only data transmission in one direction is supported ;
· Half duplex ： Allow data to be transmitted in both directions , But only one direction is allowed at a certain time , It's actually a kind of direction switching simplex communication ;
· full duplex ： Allows data to be transmitted in both directions at the same time .

2. It can be divided into ：
· Synchronous communication ： With clock synchronous signal transmission , Such as SPI、IIC communication interface .
· asynchronous communication ： No clock synchronization , Such as UART（ Universal asynchronous transceiver ）、 Single bus .

In synchronous communication , A signal line will be used above the transceiver to transmit signals , Driven by the clock signal, the two sides coordinate , Synchronous data . for example , In communication, both parties usually uniformly stipulate to sample the data line on the rising or falling edge of the clock signal .

In asynchronous communication, clock signal is not used for data synchronization , They directly insert some signal bits for synchronization into the data signal , Or package the subject data , Transmit data in the format of data frames . In communication, both parties need to regulate the data transmission rate （ That's baud rate ） etc. , To better synchronize . Common baud rates are 4800bps、9600bps、115200bps etc. .

Connection between two chips ：TTL Level communication .
Chip and PC machine （ Or host computer ） Connected to a ： Usually PC machine （ Or host computer ） It uses RS232 Interface , So to make the chip and PC The machine RS232 Interface direct communication , It needs to be realized through level conversion circuit first TTL Level vs RS232 Conversion between levels .