Commonly used RS232, RS485 and RS422 agreement, belong to the interface protocols (most) for American association or organization, namely the hardware layer generally recognize an agreement, the agreement of the 0 s and 1 s level, RS232 is - low level 3 v to 15 v, 3 v to 15 v high level; RS485 and RS422 is ~ - 6-2 v v low level, high level 2 v to 6 v, and we are most familiar with TTL level expresses low level 0 v to 0.8 v, 2.4 v to 5 v level high,
We are the most commonly used in serial communication application layer protocol is modbus, many people think that is the modbus RS485, actually otherwise, the hardware level is no necessary link layer protocol and the software protocol, just software layer protocol suitable for that kind of hardware layer (hardware) layer protocol or transmission medium for transmission, modbus can also through the RJ45 transmission, that is to say the modbus transmission can also use the RS232, but one of the main faults can cause the signal distortion, RS232 transmitting, namely 232 protocol is common-mode interference; And not 485, 485 USES the differential transmission, which guarantees the even in the interference signal lines of two core voltage up or down at the same time, and the pressure difference between them will not change, the concrete can go to understand the difference between differential mode signal and the common mode signal, because the 485 hardware layer protocols in line with the modbus reliable transmission concept, so almost most modbus protocol will choose 485 media used in equipment for data transmission,
For KNX agreement, I just learned that its transmission medium has a lot of, can realize signal and power sharing two wires, the implementation way is to use technology to the carrier, simple point is two core to remove power part (not sure how to modulation) voltage is the difference between the signal of the (may be used to implement the constant voltage power supply, part to ensure the power supply voltage distortion),