RS-232, RS-422 and RS-485 are serial data interface standards, which were initially formulated and issued by the Electronic Industry Association (EIA). RS-232 was released in 1962 and named eia-232-e as an industrial standard to ensure the compatibility between products of different manufacturers. The maximum transmission distance is about 15m, the maximum speed is 20KB / s, and RS-232 is designed for point-to-point communication (that is, only one pair of receiving and transmitting equipment). Therefore, RS232 is only suitable for local communication.

RS-422 is developed from RS-232. It is proposed to make up for the deficiency of RS-232. In order to improve the disadvantages of short communication distance and low rate of RS-232, RS-422 defines a balanced communication interface, which increases the transmission rate to 10MB / s, extends the transmission distance to 1200m (when the rate is lower than 100kb / s), and allows up to 10 receivers to be connected on a balanced bus. RS-422 is a unidirectional and balanced transmission specification for single machine transmission and multi machine reception, which is named TIA / eia-422-a standard.

In order to expand the application scope, EIA formulated RS-485 standard on the basis of RS-422 in 1983, which added multi-point and two-way communication capability, that is, it allowed multiple transmitters to connect to the same bus, increased the driving capability and conflict protection characteristics of the transmitter, and expanded the common mode range of the bus, which was later named TIA / eia-485-a standard. Since the proposed standards proposed by EIA are prefixed with RS, it is still customary to prefix the above standards with RS in the field of communication industry. RS-232, RS-422 and RS-485 standards only specify the electrical characteristics of the interface, and do not involve connectors, cables or protocols. On this basis, users can establish their own high-level communication protocol.

Conversion method of RS232 and RS485

There are three ways to use. 1: Simple conversion method. 2: The buffer memory at the receiving end controls the conversion mode. 3: The receiver buffer memory plus RTS signal controls the conversion mode. RS485 control mode of two-wire connection can only adopt (2) or (3) mode. The first method cannot be used. The four wire connection mode can be converted by any of the above three modes.

Figure 2 shows the second conversion mode. Where (a) is the receiving control end of the receiving buffer memory. When PC sends data, (a) the control end can avoid RS485 data conflict. Fig. 3 shows the third conversion mode. (b) in the figure is the RTS control terminal. When the PC receives data, the RTS signal is activated, so the PC sends data is delayed. In short, through the control of signals (a) and (b), the conflict between data transmission and reception can be avoided.

Figure 1: simple conversion mode:

  Conversion method of RS232 and RS485

Figure 2: receiver buffer memory control mode.

  Conversion method of RS232 and RS485

Figure 3: receiver buffer memory plus RTS signal control conversion mode.

  Conversion method of RS232 and RS485

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