TTL level: The standard protocol is 5V is 1 and 0V is 0. Early microcontrollers are mainly powered by 5v. In order to reduce power consumption, 2/3 of 5v, that is, 3.3v, is selected as the power supply for the microcontroller. For example, STM32 basically They are all powered by 3.3V and compatible with 5V TTL level.
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RS232 : The interface used by the RS232 standard is a 9-pin or 25-pin D-type plug. The commonly used one is a 9-pin plug, which uses full-duplex communication
logic. The level of "1" is -5V~-15V, and the level of logic "0" is +5V~+15V. The purpose of selecting this electrical standard is to improve the anti-interference ability and increase the communication distance. RS-232 has a noise margin of 2V, and the receiver will be able to recognize signals as high as +3V as a logic "0" and signals as low as -3V as a logic "1".
Use max232 chip to convert TTL level into RS232 level
RS485 : In order to expand the 232 communication distance and communication speed, the differential transmission RS485 was born, so it is half-duplex communication. +2V~+6V is logic 1,
-6V~-2V is logic 0.
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