What is the RS-485 interface? Compared with the RS-232-C interface, what are its characteristics?

What is the RS-485 interface? Compared with the RS-232-C interface, what are its characteristics?

A half-duplex network composed of RS485 interfaces generally uses a two-wire system, and mostly uses twisted-pair shielded transmission. This connection mode is a bus-type topology, and up to 32 nodes can be connected on the same bus.

As we all know, at first the data was a simple process quantity of analog signal output, and then the instrument interface was RS232 interface, so that point-to-point communication can be realized, but this way cannot realize the networking function, and then the emergence of RS485 solves this problem. Therefore, this article introduces the RS485 interface in detail.

What is the RS-485 interface? Compared with the RS-232-C interface, what are its characteristics?

Because the RS-232-C interface standard appeared relatively early, it is inevitable that there will be defects, mainly in the following four aspects:

1. Because the interface signal level value is high, and it is easy to damage the interface circuit chip, and because it is incompatible with TTL level, it is necessary to use a level conversion circuit to connect with the TTL circuit.

2. The transmission rate is low. In asynchronous transmission, the baud rate is 20Kbps.

3. The interface adopts a signal line and a signal return line to form a common transmission form. The common transmission is prone to common interference, so the anti-noise interference is weak.

Fourth, the transmission distance is limited, the maximum transmission distance standard value is 50 feet, in fact, only about 50 meters can be used. .

Due to the shortcomings of RS-232-C, some new interface standards continue to appear. 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 wires as +(2-6)V; Logic "0" is represented by the voltage difference between the two wires as -(2-6)V . The interface signal level is lower than RS-232-C, it is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, and it is convenient to connect with the TTL circuit.

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

3. The RS-485 interface is a combination of a balanced driver and a differential receiver, with enhanced anti-common-mode drying capability, that is, good anti-noise interference performance.

4. The standard value of the maximum transmission distance of the RS-485 interface is 4000 feet, which can actually reach 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. In other words, it has a multi-station function, so users can use a single RS-485 interface to easily establish a device network.

5. Because RS-485 interface has good anti-noise interference ability, long transmission distance and multi-station ability make it the first choice for serial interface. Since a half-duplex network composed of RS485 interfaces usually requires only two connection lines, the RS485 interface adopts shielded twisted pair transmission. The RS485 interface connector adopts a 9-pin socket of DB-9, the intelligent terminal RS485 interface adopts DB-9 (hole), and the keyboard interface RS485 connected with the keyboard adopts DB-9 (pin).