1. RS232: The maximum transmission distance is about 15 meters, and the maximum rate is 20kb/s. RS-232 is designed for point-to-point (that is, only one pair of receiving and sending devices) communication, and its driver load is 3 ~ 7kΩ. So RS-232 is suitable for communication between local devices.
2. RS485 / RS422: In order to improve the shortcoming of RS232 communication distance and low rate, RS422 increases the transmission rate to 10Mb/s, and the transmission distance is extended to 4000 feet (when the rate is lower than 100kb/s). The maximum transmission distance is 1219 meters, and the maximum transmission rate is 10Mb/s.
3. RS485: The RS-485 standard is formulated on the basis of RS-422, adding multi-point and two-way communication capabilities.
4. Serial port and COM port refer to the physical interface form (hardware). TTL, RS-232, and RS-485 refer to the level standard (electrical signal) of the serial port.
5. When wiring, generally only connect GND, RX (receive), and TX (transmit). Do not connect power lines such as Vcc to avoid conflicts with the power supply on the target device. (Wiring rules: The TX of the host is connected to the RX of the target device, and the RX of the host is connected to the TX of the target device, but many designers intentionally reverse the markings of RX and TX in order to make the wiring more intuitive, you can try to exchange after connecting a 200-1kΩ resistor. Or No.4 method.)
6. Serial communication generally does not contain a handshake protocol, so you can only connect GND and RX for pure reception, or only GND and TX for pure transmission.
7. The serial port wiring error may burn the chip, for example: 1 pulled high by one TX is poured into 0 pulled low by another TX.(When you are not sure about the definition of the target wiring, you can try to connect the TX and RX of another device with the line determined to be RX to see if data is received.)
8. CH340, PL2303, CP2102, FT232R chips are USB-to-serial (TTL level output) chips, and Windows drivers need to be installed. (Commonly used to add serial ports in notebooks, note that its compatibility is not as good as the onboard serial ports. Preferred choice: FT232R > CP2102 > PL2303 )
9. MAX232 is a bidirectional conversion chip between TTL level and RS232 level. Different pins realize the function of TTL to RS-232 or RS-232 to TTL. (There are many TTL and RS232 conversion chips, and the number of forward and reverse interfaces is different: for example: MAX202, SP213, MAX3232)
10. TTL level standard is low level is 0, high level is 1 (to GND line voltage, standard digital circuit logic). The RS-232 level standard is that the positive level is 0, and the negative level is 1 (not very sensitive to the level of the GND line and the specific value of the voltage).
11. RS-485, RS-422 are similar to RS-232, but use differential signal logic, which is more suitable for anti-interference/high-speed transmission. Skip it here.
12. The D-type 9-pin plug (onboard serial port, male port) behind the desktop computer is RS-232 level. Can be converted to TTL level by MAX232.
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https://www.pcm-cable.com/serial-cable/
