Tells about three kinds of RS232 to RS485 circuit design - circuit diagram read every day (254)

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RS232 to RS485 learning electronic professional small partners is certainly no stranger to it, we also know that this circuit has a large search on the Internet. But there is a circuit application that you definitely can't search. This application circuit can quickly realize dual isolation of input and output, and can also realize three isolation applications of input, output and power supply.

Since RS232 to RS485 is a well-known application, let's start with the familiar part. You can see a lot of this RS232 to RS485 conversion module on the Internet, all kinds of manufacturers have. There are common switching modules for learning, as well as industrial grade converters for industrial use.

Figure 1 ordinary conversion module

See if this conversion module has a kind of intimacy? Did you personally disassemble such modules in the past? Most people should be affirmative, so let's take a look at the general schematic inside.

Figure 2 Traditional design approach

The circuit above can only be used in some simple applications, and the RS485 circuit of this circuit is generally affected by the commutation circuit, and the communication baud rate is generally not high, and generally can support up to 9600 bps. At the same time, this type of tampering is not very stable in terms of power supply, and it is prone to communication frame loss.

In order to better solve the communication rate limitation caused by commutation, Guangzhou Zhiyuan Electronics has introduced an RS485 isolated transceiver with automatic commutation function. This transceiver supports up to 500K communication baud rate. The application of this module can solve the communication rate limitation problem well, and for the stability of communication, the circuit needs to be separately powered.

Figure 3 dual isolation design circuit

The scheme in the above figure is an isolation conversion circuit designed by MAX232 chip plus RSM485PHT of Guangzhou Zhiyuan Electronics. The design power supply is not isolated from RS232, and the RS232 and RS485 bus terminals are isolated. This isolation design can effectively suppress RS485. The interference on the bus is conducted to the 232 terminal.

Figure 4 three isolated design circuit

Editor's comment: RS-485 serial bus standard is widely used when the communication distance is required to be tens of meters to kilometer. RS-485 uses balanced transmit and differential receive, so it has the ability to reject common-mode interference. The logic "1" is represented by a voltage difference between the two lines of +(2-6)V; the logic "0" is represented by a voltage difference between the two lines of -(2-6)V. The interface signal level is lower than that of RS-232-C, and it is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, which is convenient for connection with the TTL circuit.
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