Description of a RS232/RS485/RS422 interface converter

Description of a RS232/RS485/RS422 interface converter

DM485 series converters are standard RS232/RS485/RS422 converters that complete the electrical conversion between serial port RS232 and RS485/RS422, extending the maximum communication distance to 1200 meters (4000 feet).

DM485 adopts the basic three-wire system: (TXD, RXD, GND). It does not require any software driver when used. The product adopts industrial-grade product design (applicable temperature range -40℃～+85℃), contains electrostatic discharge protection, and has anti-lightning function. It is suitable for various industrial and civil occasions.

1. Principle Description

The RS232 interface is a communication protocol used for short-distance (maximum 30-60 meters), slow-speed, point-to-point communication. In RS232, one signal only uses one signal line and adopts a ground voltage reference method. Therefore, after long-distance transmission, there is a difference in the ground voltage between the sending and receiving ends, which can easily cause communication errors or speed reduction.

The RS485/422 interface uses a different method: each signal is transmitted using a twisted pair (two signal wires), and the voltage difference between the two wires is used to represent the digital signal. For example, one of the twisted pairs is marked as A (positive) and the other is marked as B (negative). When A is a positive voltage (usually +5V) and B is a negative voltage (usually 0), it represents a signal "1"; conversely, when A is a negative voltage and B is a positive voltage, it represents a signal "0". RS485/422 allows a communication distance of up to 1200 meters, and a transmission rate of 2.5MB/s can be achieved with a suitable circuit.

RS422 and RS485 use the same communication protocol, but they are different: RS422 is usually used as an extension of RS232 communication. It uses two twisted pairs, and data can be transmitted in both directions at the same time (full-duplex). RS485 uses one twisted pair, and input and output signals cannot be carried out at the same time (half-duplex). RS485 can be used for multi-point communication, and multiple devices can be connected on one signal line. It usually adopts a master/slave structure.

In the DM series converters, only three wires of the COM port are used: TXD, RXD, and GND (they are pins 3, 2, and 5 of the D-type interface, respectively). TXD is the signal input line, RXD is the signal output line, and GND is the ground.

In RS485 (half-duplex) communication, when sending a signal, the RS232 signal output by TXD is output by the transmitter (A and B) and converted into an RS485 signal; when receiving a signal, the RS485 signal is received by the receiver (A and B) and converted into an RS232 signal and transmitted to RXD.

In RS422 (full-duplex) communication, because the input and output of the signal are separated, two twisted pairs are required to transmit the input and output signals, marked as A, B, Y, Z (A is the positive pole of the input signal, B is the negative pole of the input signal, Y is the positive pole of the output signal, and Z is the negative pole of the output signal). When sending a signal, the RS232 signal output by TXD is output by the transmitter (A and B) and converted into an RS485/RS422 signal; when receiving a signal, the RS485/RS422 signal is received by the receiver (Y and Z) and transmitted to the RXD of the serial port.

Figure 1: Schematic diagram of DM series RS232/RS485/RS422 interface converter (half-duplex)

Figure 2: Schematic diagram of DM series RS232/RS485/RS422 interface converter (full duplex)

2. Typical connection method

The output end of DM485 can connect up to 256 RS485 interfaces, and the typical connection method is shown in Figure 3 (half-duplex) and Figure 4 (full-duplex). On the two farthest terminals, a 120-ohm resistor is required between the input and output ends. When connecting the RS485/RS422 interface, the output end of interface A is connected to the input end of interface B, and the positive pole of the twisted pair is connected to the positive pole and the negative pole is connected to the negative pole; the input end of interface A is connected to the output end of interface B, and the positive pole of the twisted pair is connected to the positive pole and the negative pole is connected to the negative pole.

Figure 2: Typical connection method of DM485H (half-duplex)

Figure 4: Typical connection method of DM485F (full-duplex)

3. Specific Application

One end of the DM485 series converter is a 9-pin D-type interface, which can be directly connected to the RS232 standard interface; the other end is a terminal block, that is, the RS485/RS422 interface. In the RS485 interface (half-duplex), two groups of pins are used on the terminal block: two groups of inputs (B, A, two groups are short-circuited internally, for system expansion). The converter is connected to the RS232 standard serial port through the D-type interface and to the RS485 interface through the terminal block. In the RS422 interface (full-duplex), three groups of pins are used on the terminal block: one group of outputs (Y, Z), two groups of inputs (B, A, two groups are short-circuited internally, for expansion). The converter is connected to the RS232 standard serial port through the D-type interface and to the RS485/RS422 interface through the terminal block.

Figure 5: Specific application wiring diagram of DM485H

Figure 6: DM485F specific application wiring diagram

IV. Precautions

1. In long-distance transmission, be sure to use twisted pair cables

2. When transmitting across multiple terminals, a 120 ohm resistor must be connected between the input and output terminals of the two terminals farthest away.