Today I will tell you about what are the communication interfaces of PLC? The difference between RS232 interface and RS485 interface.
The difference between RS232 interface and RS485 interface
First, the physical structure of the interface
1. RS232 interface
One of the computer communication interfaces, usually RS-232 interface in the form of 9 pins (DB-9) or 25 pins (DB-25), generally personal computers will have two groups of RS-232 interfaces, called COM1 and COM2.
2.RS485
RS485 does not have a specific physical shape, and the interface is adopted according to the actual situation of the project.
Second, the electronic characteristics of the interface
1.RS232
2.RS485
The 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 RS-232, so it is not easy to damage the chip of the interface circuit, and this level is compatible with the TTL level, which can be easily connected with the TTL circuit.
3. The length of the communication distance
1.RS232
RS232 transmission distance is limited, the maximum transmission distance standard value is 15 meters, and can only communicate point-to-point, the maximum transmission rate is 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 reached at a transmission rate of 100Kb/s.
With impedance matched, low-attenuation special cables can reach 1800 meters! More than 1200 meters, repeaters (up to 8) can be added, so that the transmission distance is close to 10Km.
Fourth, whether it can support multi-point communication
1.RS232
The RS232 interface only allows 1 transceiver to be connected on the bus, and cannot support multi-station transceiver capability, so it can only communicate point-to-point, and does not support multipoint communication.
2.RS485
The RS485 interface on the bus allows up to 128 transceivers to be connected. That is, it has multi-station communication capabilities, so that users can easily establish a device network using a single RS485 interface.
Fifth, the difference in communication lines
1.RS232
Three-core twisted pair, three-core shielded wire, etc. can be used.
2.RS485
Two-core twisted pair, two-core shielded wire, etc. can be used. In low-speed, short-distance, non-interference occasions can use ordinary twisted pair, on the contrary, in high-speed, long-line transmission, impedance matching (generally 120Ω) RS485 special cable (STP-120Ω (for RS485 & CAN) a pair of 18AWG) must be used, and in the environment of bad interference should also use armored twisted pair shielded cable (ASTP-120Ω (for RS485 & CAN) a pair of 18AWG).
Addendum: Since the RS232 transmission distance is only 15 meters short, what is the effect?
In fact, it is very widely used and can be connected to various devices, such as monitoring, other equipment upgrades or debugging, etc. may need it. The function is relatively close to USB, and as USB ports become more common, there will be more conversion devices that convert USB to RS-232 or other interfaces.
More RS-232 devices can be connected through the USB interface, which not only obtains higher transmission speed and realizes true plug and play, but also solves the disadvantage that the USB interface cannot transmit over long distances (USB communication distance is within 5 meters).
Several common communication protocols
Before starting this issue, we should understand an issue that I have been confused about before.
The difference between RS485 and MODBUS
RS485 is a physical interface, simply hardware.
MODBUS is an international standard communication protocol for exchanging data between devices from different manufacturers (generally for industrial purposes); The so-called protocol can also be understood as the "language" that some people say above, simply put, software.
Under normal circumstances, two devices transmit data through the MODBUS protocol: RS232 is first used as a hardware interface, (that is, the serial communication port (serial port) on an ordinary computer); There are also RS422, there are also commonly used RS485, this interface transmission distance is long, in the general industrial site is used more.
THE MODBUS PROTOCOL IS DIVIDED INTO THREE MODES: MODBUS RTU, MODBUS ASCII AND LATER MODBUS TCP.
The physical hardware interfaces used in the first two (MODBUS RTU, MODBUS ASCII) are serial (RS232, RS422, RS485).
MODBUS TCP is to follow the development trend of today's world, everything can be connected to the Ethernet network or the Internet to transmit data. Therefore, the MODBUS TCP mode is produced, and the hardware interface of this mode is the Ethernet port, which is the network port generally used on our computer.