1. The serial port and COM port refer to the physical interface form (hardware), while TTL, RS-232, and RS-485 refer to the level standards (electrical signals).
2. When connecting devices, generally only GND, RX, and TX are connected. The Vcc or +3.3V power line is 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 that use USB to extend the serial port (TTL level).
4. The MAX232 chip is a specialized bidirectional conversion chip between TTL levels and RS-232 levels, 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 be skipped for now.
Serial Port, COM Port:
The COM port is a 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 physical standards: the D-type 9-pin connector and the 4-pin DuPont connector.
This is a common 4-pin serial port that is often seen on circuit boards, often 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, there can be many protocols, depending on the specific device.
The following is the D-type 9-pin serial port (commonly referred to). It can be seen at the back of desktop computers.
Remember, the protocols for this type of interface are only two: RS-232 and RS-485. It will not be TTL level (unless for special applications).
We generally only connect the RXD and TXD pins, plus GND.
The picture below is a small board for USB to TTL serial, which can extend a serial port using USB. The chip is PL2303HX.
Various serial ports are often confused online, but this one can indeed be used to download programs to the STC microcontroller.
This is another one, with a CP2102 chip, also a USB to TTL serial. It is said to be better than PL2303, but I haven’t felt the difference in actual use. This small board has an additional +3.3V power pin to suit 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 connector)?
Just connect a MAX232 to convert it.
You can also build a simple comparator circuit to achieve the TTL to RS-232 function, as shown in the figure below.
How to convert RS-232 to TTL? You need to think a bit. Of course, someone has already thought of making a finished product. Take a close look at the picture below, USB is converted to TTL serial through PL2303, and the four holes in the middle can be extended, then converted to RS-232 level through MAX232, leading to the 9-pin serial port.
The following is another model: the level conversion still uses MAX232.
You might buy something like this: it looks like it only has one chip inside.
But remember one thing, as long as it is a D-type 9-pin serial port, it will not be TTL level; unless specified, it is assumed to be RS-232.
Therefore, this wire, regardless of its internal structure, is a USB to RS-232 serial port wire.
It is important to emphasize that a device’s serial port can be judged as TTL or RS-232 based on the extended serial port wire, thus determining the connection method and whether a conversion circuit is needed.
