The difference between RS232 and RS485 interface

Ⅰ. The physical structure of the interface

1. RS232 interface:

RS232 is one of the computer communication interfaces, usually the RS-232 interface appears in the form of 9 pins (DB-9) or 25 pins (DB-25). Generally, there are two sets of RS-232 interfaces on the Industrial computer. They are called COM1 and COM2, respectively.

2. RS485 interface：

RS485 has no specific physical shape, and the interface is adopted according to the actual situation of the project.

Ⅱ. The electronic characteristics of the interface

1. RS232:The signal level value of the transmission level signal interface is relatively high (signal "1" is "-3V to -15V", signal "0" is "3 to 15V"), which is easy to damage the chip of the interface circuit, and because it is not compatible with TTL level (0~“2.0V”), it is necessary to use a level conversion circuit to connect with TTL circuit. In addition, the anti-interference ability is poor.

2. RS485: Transmission differential signal logic "1" is represented by the voltage difference between the two lines as +(2-6)V; logic "0" is represented by the voltage difference between the two lines as -(2-6)V. The interface signal level is lower than that of RS-232, so it is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, which can be easily connected to the TTL circuit.

Ⅲ. The length of communication distance

1. RS232: RS232 has a limited transmission distance, the standard maximum transmission distance is 15 meters, and can only communicate point-to-point, with a maximum transmission rate of 20kb/s.

2. RS485:The maximum wireless transmission distance of RS485 is 1200 meters. The maximum transmission rate is 10Mbps, and the maximum communication distance can be achieved only at the transmission rate of 100kb/s.

Ⅳ．Whether to support multipoint communication

RS232: The RS232 interface only allows one transceiver to be connected to the bus, and cannot support multi-station transceiver capabilities, so it can only communicate point-to-point, and does not support multi-point communication.

RS485: The RS485 interface allows up to 128 transceivers to be connected on the bus. That is, it has the ability of multi-station communication, so that users can use a single RS485 interface to easily establish a device network.

Ⅴ．The difference of communication lines

RS232: Three-core twisted pair, three-core shielded cable, etc. can be used.

RS485: Two-core twisted pair, two-core shielded cable, etc. can be used. In low-speed, short-distance, non-interference occasions, ordinary twisted pair can be used. On the contrary, in the case of high-speed and long-term transmission, a special RS485 cable (STP-120Ω (for RS485 & CAN) with a pair of 18AWG) with impedance matching (generally 120Ω) must be used. In the environment with severe interference, armored twisted pair shielded cable (ASTP-120Ω (for RS485 & CAN) a pair of 18AWG) should be used.

RS232 Features:

RS232 is the serial port, the 9-pin socket at the back of the computer case

Since the RS-232 interface standard appeared earlier, it is inevitable that there are shortcomings, mainly in the following four points:

(1) The signal level of the interface is high, which is easy to damage the chip of the interface circuit, and because it is not compatible with the TTL level, a level conversion circuit is required to connect with the TTL circuit.

(2) The transmission rate is low, in the asynchronous transmission, the baud rate is 20kbps.

(3) The interface uses a signal line and a signal return line to form a common ground transmission form. This common ground transmission is prone to common mode interference, so the anti-noise interference is weak.

(4) The transmission distance is limited, the standard value of the maximum transmission distance is 50 feet, but it can only be used in about 50 meters in practice.

RS-485 Features:

In view of the shortcomings of RS-232, some new interface standards have emerged, and RS-485 is one of them. It has the following characteristics:

1. Electrical characteristics of RS-485: logic "1" is represented by the voltage difference between the two lines as +(2-6) V; logic "0" is represented by the voltage difference between the two lines as -(2-6) V. The interface signal level is lower than that of RS-232-C, so it is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, which can be easily connected with the TTL circuit.

2. The maximum data transmission rate of RS-485 is 10Mbps.

3. The RS-485 interface adopts a combination of a balanced driver and a differential receiver, and the anti-common mode interference ability is enhanced, that is the anti-noise interference is good.

4. The standard value of the maximum transmission distance of the RS-485 interface is 4000 feet, which can actually be up to 3000 meters. In addition, the RS-232-C interface only allows one transceiver to be connected to the bus, that is, single station capability. The RS-485 interface allows up to 128 transceivers to be connected on the bus. That is, it has multi-station capability, so that users can easily establish a device network using a single RS-485 interface. Because the RS-485 interface has good anti-noise interference, long transmission distance and multi-station capability, it becomes the preferred serial interface. Because the half-duplex network composed of the RS485 interface generally only needs two connections, the RS485 interface adopts the shielded twisted pair transmission. The RS485 interface connector adopts DB-9 9-pin socket, and the RS485 interface of the intelligent terminal adopts DB-9 (hole), and the keyboard interface RS485 connected to the keyboard adopts DB-9 (pin).

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