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What is the difference between serial port, COM port, UART port, TTL, RS-232, RS-485

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What is the difference between serial port, COM port, UART port, TTL, RS-232, RS-485

 

 
First of all, serial port, UART port, COM port, USB port refer to the physical interface form


And TTL, RS-232, RS-485 refer to the level standard


Serial port: Serial port is a general term. UART, TTL, RS232, and RS485 all follow similar communication timing protocols, so they are commonly referred to as serial ports.


UART interface: Universal Asynchronous Receiver/Transmitter, UART is a logic circuit for serial transceiver, this part can be independent into a chip, or can be embedded in other chips as a module, there will be a UART module in the microcontroller, SOC, PC .


COM port: It refers to the serial communication port of the D-SUB shape (a connector structure, the connector of the VGA interface is also D-SUB) on the desktop computer or some electronic equipment, and the serial communication sequence and RS232 logic circuit are applied. flat

USB port: Universal serial bus, and serial port are completely two concepts. Although it is also serial communication, because the communication timing and signal level of USB are completely different from those of the serial port.

The same, so it has nothing to do with the serial port. USB is a high-speed communication interface, used for PC to connect various peripherals, U disk, keyboard and mouse, mobile hard disk, and of course "USB to
"Serial port" module. (USB to serial port module is the UART module of the USB interface)

TTL, RS232, RS485 are all representations of logic levels

TTL: TTL refers to bipolar transistor logic circuit. Many "USB to TTL" modules on the market are actually "USB to TTL level serial ports" modules. This signal 0 corresponds to
0V, 1 corresponds to 3.3V or 5V. Compatible with IO level of microcontroller and SOC. However, it is not necessarily a TTL level, because most digital logic is now CMOS
The craftsmanship just follows the TTL argument. When we carry out serial communication, what comes directly from the microcontroller is basically TTL level.

TTL level: full duplex (logic 1: 2.4V--5V logic 0: 0V--0.5V)

1. The hardware block diagram is as follows, TTL is used for communication between two MCUs


2, '0' and '1' indicateRS232: : It is an asynchronous transmission standard interface formulated by the Electronic Industries Association (EIA), and corresponds to the level standard and communication protocol (time
sequence), its level standard: +3V ~ +15V corresponds to 0, -3V ~ -15V corresponds to 1. The logic level of rs232 is not the same as TTL but the protocol is the same.
RS-232 level: full duplex (logic 1: -15V--5V logic 0: +3V--+15V)


1. The hardware block diagram is as follows, TTL is used for communication between MCU and PC

2, '0' and '1' indicate

RS485: RS485 is a serial interface standard. For long-distance transmission, differential transmission is adopted, and differential signals are transmitted, and its anti-interference ability is much stronger than that of RS232.
The voltage difference between the two lines is -(2~6)V, which means 0, and the voltage difference between the two lines is +(2~6)V, which means 1.
RS-485: : Half-duplex, (Logic 1: +2V--+6V Logic 0: -6V---2V) The level here refers to the voltage difference between the AB lines.


1. The hardware block diagram is as follows

2, '0' and '1' indicate

COM port is the serial communication port, referred to as serial port. This is different from the "Universal Serial Bus" of USB and the "SATA" of hard disk.


Generally we see two physical standards. D-type 9-pin plug, and 4-pin DuPont header.

This is a common 4-pin serial port, commonly found on circuit boards, often with DuPont pins on it. Sometimes there is a fifth pin, the 3.3V power terminal.

Since it is reserved on the circuit board, there can be many protocols, depending on the specific equipment.

The serial port mentioned in the embedded generally refers to the UART port, but we often do not know the difference between it and the COM port, as well as the relationship between RS232, TTL, etc. In fact,
UART and COM refer to the physical interface form (hardware), while TTL and RS-232 refer to the level standard (electrical signal).
UART has 4 pins (VCC, GND, RX, TX), with TTL level, low level is 0(0V), high level is 1(3.3V or above).

The following is the D-type 9-pin serial port (in popular terms). It can be seen behind a desktop computer. Remember, there are only two protocols for this interface: RS-232 and RS-485. Won't
are TTL level (unless special application).

USB to TTL serial port PCB board, you can use USB to expand a serial port. The chip is PL2303HX. Various serial ports are often confused on the Internet, but this is indeed possible
In order to download the program to the STC microcontroller.

This is another, CP2102 chip, and also USB to TTL serial port. It is said that it is better than PL2303, but it is not felt in actual use. This small board has more +3.3V power supply
terminals to suit different target circuits

 

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