What is the Difference Between RS232 and RS485?

RS232 and RS485 are common weak current interfaces, to understand what is the difference between RS232 and RS485 today.

The physical structure of the RS232 interface and RS485 interface

1.  what is the RS232 interface

One of the computer communication interfaces, the RS-232 interface usually comes in the form of 9 pins (DB-9) or 25 pins (DB-25). There are two groups of RS-232 interfaces on personal computers, which are called COM1 and COM2 respectively.

Rs232 interface is a standard for serial communication developed in 1970 by the U.S. Electronics Industry Association (EIA) in conjunction with Bell systems, modem manufacturers, and computer terminal manufacturers. Its full name is the Technical Standard for Serial Binary Data Interchange Interface between Data Terminal Devices (DTE) and Data Communications Devices (DCE). The standard specifies a 25-pin DB25 connector.

The signal content of each pin of the connector is specified, as are the levels of various signals. Generally, only 2 (RDX), 3 (TXD), and 7 (GND) pins are used in the serial port of DB25. With the continuous improvement of the equipment, db25 pins are rarely seen now. Instead of db9, the pins used in DB9 are different from db25, namely 2 (RDX), 3 (TXD), and 5 (GND). So the RS232 interface is now called DB9.

The connector shall be a 25-core plug holder model DB-25. Some devices connect to the RS-232 interface of the PC because they do not use the transmission control signal of the other side, they only need three interface lines, namely “send data”, “receive data” and “signal ground”.Therefore, the 9-core plug seat of DB-9 is adopted, and the transmission line is a shielded twisted pair.


2.  What is the RS485 interface

RS485 without specific physical shape, according to the actual situation of the project and the use of the interface.

RS485 interface composed of the half-duplex network is generally a two-wire system (there was a four-wire connection method, can only achieve point-to-point communication mode, now rarely used), the use of shielded twisted pair transmission. The connection mode is bus-type topology, and 32 nodes can be connected on the same bus at most. The RS485 communication network is generally adopted in the master-slave communication mode, that is, a host with multiple slaves.

In many cases, connecting the RS-485 communication link is simply A pair of twisted pairs connecting the “A” and “B” terminals of each interface. The RS485 interface connector USES DB-9’s 9-core socket, the RS485 interface with the intelligent terminal USES DB-9 (hole), and the keyboard interface RS485 USES DB-9 (pin).

What is the difference between RS232 and RS485 communication interface?

1. Electronic properties of interfaces

A. RS232 interface: the signal level of the transmission level signal interface is relatively high (signal “1” is “-3V to -15V”, signal “0” is “3-15V”). It is easy to damage the chip of the interface circuit because it is related to TTL level (0~ “& LT;0.8 v, 1 ~ “& gt;2.0V “) is not compatible, so level conversion circuit is needed to connect with TTL circuit. In addition, the anti-interference ability is poor.

B.  RS485 interface: Transmission difference signal logic “1” is expressed as + (2-6) V between two lines; 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 rS-232, it is not easy to damage the chip of the interface circuit, and the level is compatible with TTL level, so it is convenient to connect with TTL circuit.


2. Communication distance

A.  RS232 interface: RS232 has limited transmission distance, the maximum transmission distance standard value is 15 meters, and can only communicate point-to-point, and the maximum transmission rate is 20kB/s.

B. RS485 interface: The maximum wireless transmission distance of RS485 is 1200 meters. The maximum transmission rate is 10Mbps, and the maximum communication distance can only be reached at the transmission rate of 100Kb/S.

With the use of impedance matching, the low attenuation of the special cable can reach 1800 meters! Over 1200 meters, Repeaters (up to 8) can be added, so that the transmission distance is close to 10Km.

3. Can support multi-point communication

RS232: The RS232 interface is only allowed to connect 1 transceiver on the bus and cannot support multi-station transceiver capability, so it can only point-to-point communication, does not support multi-point communication.

RS485: The RS485 interface is allowed to connect up to 128 transceivers on the bus. Namely has the multi-station communication ability, so the user can use the single RS485 interface to conveniently set up the equipment network.


4. Differences in communication lines

RS232: Three-core twisted pair, three-core shielded wire, etc.

RS485: Can use two – core twisted pairs, two-core shielding wire, etc. In the situation of low speed, short, no interference CAN use ordinary twisted pair, on the other hand, at high speed, long transmission, it must adopt impedance matching (usually 120 Ω) of RS485 special cable (STP – 120 Ω (for RS485 & CAN) a pair of 18 AWG), and interference in the bad environment also should use type sheathed twisted-pair shielded cable (ASTP – 120 Ω (for RS485 & CAN) a pair of 18 AWG).

Supplement: namely however RS232 transmission distance is only 15 meters so short, so what effect does it have?

In fact, it is widely used, can be connected to a variety of devices, such as monitoring, other equipment upgrades or debugging may need to use it. As USB ports become more common, there will be more devices that convert USB to RS-232 or other interfaces.

More RS-232 devices can be connected through the USB interface, which can not only obtain higher transmission speed and realize true plug and play, but also solve the disadvantage that the USB interface cannot transmit remotely (USB communication distance is within 5 meters).

We know that there is a lot of what is difference between RS232 and RS485. They are used in different applications, and the actual application needs to decide which interface to use according to their own needs.

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