First, The Physical Structure Of The Interface
1. RS232 interface: one of the computer communication interface, usually RS-232 interface in the form of 9-pin (DB-9) or 25-pin (DB-25)), there are generally two groups of personal computers on the RS-232 port, called COM1 and COM2.
2. RS485 interface: RS485 does not have a specific physical form, according to the actual situation of the project using the interface.
Second, The Electrical Characteristics Of The Interface
1. RS232: Transmission level signal interface signal level value is high (signal “1” for the “-3V to -15V”, signal “0” for the “3 to 15V”, “0” for the “3 to 15V”, “0” for the “3 to 15V”, “0” for the “3 to 15V”, “0” for the “3 to 15V”. “3 to 15V”), to facilitate the interface circuit chip damage, due to incompatibility with the TTL level (0 ~ “2.0V”), the need to use the level conversion circuit to connect with the TTL circuit. In addition, poor anti-interference capability.
2. RS485: Transmission of differential signals logic “1” with two lines of voltage difference expressed as + (2-6) V; logic “0” with two lines of voltage difference between the two lines for – (2-6) V said. ) V said. Interface signal level is lower than RS-232, not easy to damage the interface circuit chip, level compatible with TTL level, can be easily accessed TTL circuit.
Third, The Length Of The Communication Distance
1. RS232: RS232 transmission distance is limited, the standard maximum transmission distance of 15 metres, and only point-to-point communication, the maximum transmission rate of up to 20kB/s.
2. RS485: The maximum wireless transmission distance of RS485 is 1200 metres. Maximum transmission rate of 100Kb / S transmission rate, the maximum communication distance achievable.
Use impedance matching low attenuation special cable, up to 1800 metres! Above 1200 metres, repeaters (up to 8) can be added, and the transmission distance is close to 10Km.
Fourth, Can It Support Multi-point Communication?
RS232: RS232 interface allows only one transceiver to be connected to the bus, does not support the ability to send and receive multiple points, so it can only point-to-point communication, does not support multi-point communication.
RS485: RS485 interface allows up to 128 transceivers to be connected to 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 network of devices.
Fifth, The Difference Between The Communication Lines
RS232: You can use three-core twisted-pair cable, three-core shielded wire, etc..
RS485: Two-core twisted-pair cable, two-core shielded wire, etc.. In low-speed, short-distance, non-interference occasions, you can use ordinary twisted-pair cable. On the contrary, in high-speed and long-term transmission, the use of special RS485 cable (STP-120Ω (for RS485 & CAN) a pair of 18AWG) and armoured twisted pair shielded cable (ASTP-120Ω (for RS485 & CAN) a pair of 18AWG) should be used in harsh environments.
Supplementary: RS232 transmission distance is only 15 metres, what’s the use?
In fact, it has a wide range of applications, can be connected to a variety of devices, such as monitoring, other equipment upgrades or debugging, etc., may need to use it. The function is close to USB. as the USB interface is more and more common, the USB to RS-232 or other interfaces will be more and more conversion devices.
Through the USB interface can be connected to more RS-232 devices, not only to achieve higher transmission speeds, but also to achieve a true plug and play, the communication distance within 5 metres.
Several Commonly Used Communication Protocols
Before starting this issue, we should understand a problem that I have been confused before.
RS485 and RS485 is the physical interface, simply put, the hardware.
Is an international standard communication protocol for the exchange of data between devices of different manufacturers (generally used for industrial purposes); the so-called protocol can also be understood as someone said above “language”, simply put, is the software.
In general, two devices through a protocol to transmit data: RS232 first as a hardware interface (i.e., ordinary computers on the serial communications port (port)); RS422 is also used, commonly used, but also. RS485, this interface transmission distance, mostly used in general industrial site.
The protocol is divided into three modes: RTU, ASCII and later developed TCP.