The difference between RS232, RS485 and USB serial interfaces

Difference between RS232, RS485 and USB

RS232 and RS485 have always been the common interfaces of weak current. The two serial ports have a lot of basic knowledge, including electrical characteristics, mechanical characteristics, link layer, transmission control and other aspects, this article will compare two common interfaces from a simpler perspective and from the differences between the two sides. And from the current practical point of view, such as RS232 and RS485 conversion, and USB conversion, etc.

1. Differences between RS232 and RS485

(1) The physical structure of the interface

RS232 interfaces appear in the form of 9 pins (DB9) or 25 pins (DB25). Generally, there are two sets of RS232 interfaces on personal computers, which are called COM1 and COM2 respectively.

The connector uses a 25-core socket of db25. The RS232 interface connected by some devices and PCs only needs three interface lines, namely “sending data”, “receiving data” and “signal ground”, because they do not use the transmission control signal of the other party “. Therefore, the 9-core socket of DB9 is adopted, and the transmission line adopts shielded twisted pair.

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

(2) The transmission distance of the interface

The maximum standard value of RS232 transmission distance is 15 meters, and only point-to-point communication is allowed, and the maximum transmission rate is 20kb/s.

The maximum transmission distance of RS485 is 1200 meters and the maximum transmission rate is 10Mbps. Only at the transmission rate of 100kB/s can the maximum communication distance be reached. If special cables with impedance matching and low attenuation are adopted, they can reach 1800 meters and exceed 1200 meters, repeaters (up to 8) can be added, thus the transmission distance is close to 10Km.

(3) The number of communication interfaces

As mentioned above, the RS232 interface can only connect one transceiver on the bus and does not support multi-station sending and receiving, so it can only communicate point-to-point.

RS485 interface allows up to 128 transceiver connections on the bus. That is, it has multi-station communication capability, so that users can conveniently establish equipment network by using a single RS485 interface.

The difference between RS232, RS485 and USB serial interfaces

(4) The electronic characteristics of the interface

RS232 transmission level signal: the signal level value of the interface is relatively high, which is easy to damage the chip of the interface circuit, and because it is not compatible with TTL level, it is necessary to use level conversion circuit to connect with TTL circuit. Other anti-interference ability is poor.

RS485 transmits differential signals: logic “1” is expressed as +(2-6) V of voltage difference between two lines; the logic “0” is represented by the voltage difference between the two lines as-(2-6) V. If the level of the interface signal is lower than that of the RS-232, it is not easy to damage the chip of the interface circuit. Moreover, this level is compatible with TTL level and can be easily connected with TTL circuit.

(5) Communication line differences of interfaces

RS232 generally adopts three-core twisted pair, three-core shielded wire, etc.

RS485 adopts two-core twisted pair, two-core shielded wire, etc. Ordinary twisted pairs can be used in low-speed, short-distance and non-interference occasions. On the contrary, RS485 special cables with impedance matching (generally 120Ω) must be used in high-speed and long-line transmission, however, armored twisted pair shielded cables should also be used in harsh interference environment.

In general, RS232 has not many advantages, and its transmission distance is only 15 meters, but it has a great effect. For example, it may be used in monitoring, upgrading and debugging of other equipment, the function is relatively connected with USB, so now there is a USB conversion port, which not only obtains higher transmission speed, but also solves the defect that USB interface cannot transmit remotely.

2. Conversion between RS232 and USB

The difference between RS232, RS485 and USB serial interfaces

USB to RS232 adapter

This can connect the USB port on PC or notebook computer to old RS232 equipment, such as PDA, GPS system, etc. These USB-to-serial converters have DB9 male connectors for RS232 connection and USB A male connectors for PC or laptop connection.

The difference between RS232, RS485 and USB serial interfaces

USB to RS232 adapter, 4 RS232 DB9 male

If one interface is not enough, four can be used together to facilitate data transmission.

3. Conversion between RS232 and RS485

The difference between RS232, RS485 and USB serial interfaces

RS232 to RS485 interface converter, DB9 male/DB9 female, Port Power supply

Ports are used for power supply, which is characterized by automatic transmission data control (SDC). When data exists on one side of the RS-232, SDC automatically enables the RS-485 driver. Additional features include point-to-point dual-line half-duplex support and point-to-multipoint dual-line half-duplex operations.

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