1. The serial port and COM port refer to the physical interface form (hardware). TTL, RS-232, and RS-485 refer to the level standards (electrical signals).
2. When connecting devices, usually only GND, RX, and TX are connected. Vcc or +3.3V power lines are generally not connected to avoid conflicts with the power supply of the target device.
3. The PL2303 and CP2102 chips are USB to TTL serial chips, used to expand the serial port (TTL level) via USB.
4. The MAX232 chip is a dedicated bidirectional conversion chip between TTL level and RS-232 level, capable of converting TTL to RS-232 and vice versa.
5. The TTL standard has a low level of 0 and a high level of 1 (+5V level). The RS-232 standard has a positive level of 0 and a negative level of 1 (±15V level).
6. RS-485 is similar to RS-232 but uses differential signaling with negative logic. This will not be discussed here.
Serial Port, COM Port:
The COM port is the serial communication port, abbreviated as serial port. This is different from USB’s “Universal Serial Bus” and the hard disk’s “SATA”.
Generally, we see two types of physical standards: D-type 9-pin connectors and 4-pin Dupont connectors.
This is a common 4-pin serial port, often found on circuit boards, usually with Dupont pins on top. Sometimes there is a fifth pin for the 3.3V power supply.
Since this is reserved on the circuit board, the protocols can vary widely depending on the specific device.
The following is a D-type 9-pin serial port (commonly referred to). It can be seen at the back of desktop computers.
Remember, this type of interface only has two protocols: RS-232 and RS-485. It will not be TTL level (unless for special applications).
The definition of the 9-pin serial port can be referenced here: http://wenku.baidu.com/view/5c170c6925c52cc58bd6be6e.html
Generally, we only connect the RXD and TXD pins, plus GND.
The image below shows a USB to TTL serial board, which can expand a serial port via USB. The chip is PL2303HX.
There is often confusion about various serial ports online, but this can indeed be used to download programs to the STC microcontroller.
This is another type, using the CP2102 chip, also a USB to TTL serial port. It is said to be better than the PL2303, but I haven’t noticed any difference in actual use. This board includes a +3.3V power terminal to accommodate different target circuits.
All the above mentioned are USB to TTL serial ports. What if the target device has an RS-232 serial port (D-type 9-pin interface)?
Just connect a MAX232 chip to convert it.
You can also build a simple comparator circuit to achieve the function of TTL to RS-232 as shown in the figure below.
How to convert RS-232 to TTL? You will need to think a bit. Of course, someone has already thought of making a product. Looking closely at the image, USB is converted to TTL serial via PL2303, and the four holes in the middle can be connected out, then converted to RS-232 level via MAX232, leading to the 9-pin serial port.
Below is another model: the level conversion still uses MAX232.
You might buy something that looks like it only has a simple chip inside.
But remember, as long as it is a D-type 9-pin serial port, it will not be TTL level. Unless specifically stated, it is assumed to be RS-232.
So this cable, regardless of its internal structure, is a USB to RS-232 serial cable.
It is important to emphasize that the serial port of a device can be determined by the lead-out serial cable to identify whether it is TTL or RS-232, thus deciding the connection method and whether a conversion circuit is needed.
