Today, I would like to share with you some common connection ports and communication protocols of PLC.

1. Difference between RS232 interface and RS485 interface

I. Physical Structure of Interface

1. RS232 interface: one of the computer communication interfaces. Usually, the RS-232 interface appears in the form of 9-pin (DB-9) or 25-pin (DB-25).There are generally two sets of RS-232 ports on personal computers, called COM1 and COM2.

2. RS485 interface: RS485 has no specific physical form, and the interface is adopted according to the actual situation of the project.

II. Electrical characteristics of interface

1. RS232: The signal level value of the transmission level signal interface is high (signal "1" is "-3 V to -15 V",Signal "0" is "3 to 15 V"), which facilitates the chip damage of the interface circuit, due to the incompatibility with TTL levels (0 to "2.0 V").A level shift circuit is required to interface with the TTL circuit. In addition, the anti-interference ability is poor.

2. RS485: transmission differential signal logic "1" is expressed as + (2-6) V by two-line voltage difference;Logic "0" is represented by the voltage difference between the two lines, which is- (2 — 6) V.The interface signal level is lower than RS-232, which is not easy to damage the interface circuit chip, and the level is compatible with TTL level, which is convenient to access the TTL circuit.

III. Length of communication distance

1. RS232: The transmission distance of RS232 is limited. The standard maximum transmission distance is 15 meters, and it is only point-to-point communication.The maximum transfer rate is 20kB/s.

2. RS485: The maximum wireless transmission distance of RS485 is 1200m.The maximum transmission rate is 100Kb/S, and the maximum communication distance can be reached.

Up to 1800 meters using special cables with impedance matching and low attenuation!Over 1200 meters, repeaters (up to 8) can be added, and the transmission distance is close to 10 Km.

4. Can multipoint communication be supported?

RS232: The RS232 interface allows only one transceiver to be connected to the bus and does not support multi-point transceiver capability, so it can only communicate point-to-point.Multipoint communication is not supported.

RS485: The RS485 interface allows up to 128 transceivers to be connected on the bus.That is to say, it has the ability of multi-station communication, so that users can easily establish equipment network with a single RS485 interface.

5. Differences in communication lines

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

RS485: two-core twisted pair, two-core shielded wire, etc. can be used.In low speed, short distance and no interference occasions, ordinary twisted pair can be used. In contrast, in high-speed and long-term transmission,Use a special RS485 cable (STP-120Ω (for RS485 & CAN) pair of 18AWG) and an armored twisted-pair shielded cable (ASTP-120Ω (for RS485 &CAN) pair of 18 AWG) shall be used in harsh environments.

RS232 transmission distance is only 15 meters, what is the use?

In fact, it has a wide range of applications, which can connect various devices, such as monitoring, upgrading or debugging of other devices.You may need to use it. This function is close to USB.As USB interfaces 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 not only realizes higher transmission speed,It also realizes the real plug and play, and the communication distance is within 5 meters.

2. Several common communication protocols

Before we start this question, we should understand a question that I have been confused about before.

RS485 and: RS485 is a physical interface, which is simply hardware.

It is an international standard communication protocol used to exchange data between equipment of different manufacturers (generally used for industrial purposes).The so-called protocol can also be understood as the "language" mentioned above, which is simply software.

In general,The two devices transmit data through a protocol: RS232 is first used as a hardware interface (that is, a serial communication port on an ordinary computer);RS422 is also used and commonly used. RS485, this interface has a long transmission distance and is mostly used in general industrial sites.

The protocol is divided into three modes: RTU, ASCII, and TCP, which was developed later.

The physical hardware interfaces used by the first two (RTU, ASCII) are () communication (RS232, RS422, RS485).TCP is in line with the development trend of today's world), everything can be connected or transmitted through the network.So the TCP mode is produced, and the hardware interface of this mode is the () port.That is, the network port we usually use on our computers.