1.RS-232-C is the American Electronics Industry Association EIA (Electronic Industry Association) to develop a serial physical interface standard. RS is in English, "Recommendation," the abbreviation for the identification number 232, C said that the number changes. RS-232-C bus standard with 25 signal lines including a main channel and a secondary channel, in most cases, the main use of the main channel, duplex communication in general, only a few signal lines can be achieved, such as a Send line, a receiving line and a ground wire. RS-232-C standard specifies the data transfer rate of 50,75 per second, 100,150,300,600,1200,2400,4800,9600,19200 Porter. RS-232-C standard stipulates that drivers allow 2500pF capacitive load capacitance communication distance will be affected by this restriction, for example, using 150pF / m communication cable, the maximum communication distance of 15m; if the cable capacitance per meter decreases the communications distance can be increased. Another reason for the short distance is the RS-232 is a single-ended signal transmission, there is not total to suppress common mode noise and interference problems, it is generally used for communication within 20m.

    2.RS-485 bus, the required distance of a few tens of meters to the 1000 meters, the widely used RS-485 serial bus standards. RS-485 uses differential balanced send and receive, so has the ability to suppress common mode interference. Bus Transceiver with high sensitivity and can detect as low as 200mV of voltage, so signals can be restored in the 1000 meters away. RS-485 half-duplex mode, any time there can be only one point in the sending state, therefore, to send the circuit to be controlled by the enable signal. RS-485 for multi-point interconnection is very convenient for a number of signal lines can be saved.

    3.RS422 bus, RS422 and RS485 is basically the same circuit, is to send and receive differential without digital ground. Rate differential is the same distance away under the conditions of the root causes, this is the fundamental difference between the two with the RS232, because the RS232 is a single-ended input and output, duplex digital ground work O'clock at least Xu Yao Jie Shou Fa Song line and three lines Line (asynchronous transfer), you can also add other control lines to complete synchronization. RS422 full duplex by two pairs of twisted pair to send and receive work independently of each other, but only half-duplex RS485 work, income can not be made simultaneously, but it only needs a pair of twisted pair. RS422 and RS485 can be transmitted in the 1,200 meters under 19kpbs.