RS232 is one of the most widely used serial interfaces for communication with PCs. It is defined as a single-ended standard that increases communication distance in low-speed serial communication . RS232 is designed for point-to-point (i.e., using only one pair of transmitting and receiving devices) communication, and its driver load is 3 to 7 kΩ.
RS232 is a single-ended signal transmission, which has problems such as ground noise and inability to suppress common-mode interference. Therefore, it is generally used for communication within 20m.
A complete RS232 interface has 22 wires and uses a standard 25-pin connector. RS232 uses logic: logic "1": -5V to -15V, logic "0": +5V to +15V.
When communication distances range from tens of meters to thousands of meters, the RS485 serial bus standard is widely used. RS485 employs balanced transmission and differential reception, thus possessing the ability to suppress common-mode interference . Furthermore, the bus transceiver has high sensitivity, capable of detecting voltages as low as 200mV, allowing the transmitted signal to be recovered even at distances exceeding one kilometer.
RS485 operates in half-duplex mode, meaning only one point can be transmitting at any given time. Therefore, the transmitting circuit must be controlled by an enable signal. RS485 is very convenient for multi-point interconnection, saving many signal lines. RS485 can be used to network and form distributed systems, allowing up to 32 drivers and 32 receivers to be connected in parallel .
RS422 is a two-way line signal transmission. Through the transmission line driver, logic levels are converted into potential differences to complete the information transmission at the origin; through the transmission line receiver, the potential differences are converted into logic levels to realize the information reception at the terminal.
RS485 and RS422 circuits are fundamentally similar in principle, both using differential transmission and reception, and do not require a digital ground . Differential operation is the fundamental reason for their greater transmission distance at the same data rate, which is the key difference between them and RS232. RS232 is single-ended input/output, requiring at least three lines (digital ground, transmit, and receive) for full-duplex operation (asynchronous transmission), and can also add other control lines for synchronization. RS422 can operate in full-duplex mode with two twisted pairs of wires, allowing transmission and reception to proceed independently, while RS485 can only operate in half-duplex mode, meaning transmission and reception cannot occur simultaneously, but it only requires one twisted pair of wires.
RS422 has a longer transmission distance and faster speed than RS232, with a maximum transmission rate of 10 Mbit/s. At this rate, the allowable cable length is 120 meters. If a lower transmission rate is used, such as 9000 baud, the maximum distance can reach 1200 meters.
Each RS422 channel requires two signal lines; one is a logic "1", and the other is a logic "0". The circuit consists of a transmitter, a balanced connection cable, a cable termination load, and a receiver. Only one transmitter is allowed in the circuit, but multiple receivers are permitted; therefore, point-to-point communication is typically used.
RJ45 interfaces are typically used for data transmission, with the most common application being network interface cards (NICs).
RJ45 is one type of connector among various types (for example, RJ11 is also a type of connector, but it's used for telephones). There are two different ways to arrange the RJ45 connector pattern: one is orange-white, orange, green-white, blue, blue-white, green, brown-white, brown; the other is green-white, green, orange-white, blue, blue-white, orange, brown-white, brown. Therefore, there are two types of cables using RJ45 connectors: straight-through cables and crossover cables. The RJ45 network cable plug, also known as a crystal head, has eight pins and is widely used for connecting network cables (called Category 5 cables or twisted-pair cables) between network devices in local area networks (LANs) and ADSL broadband internet access users.
In practical applications, there are two connection methods (wire sequence) for RJ45 plugs and network cables. The method for identifying the pin numbers of an RJ45 network cable plug is as follows: Hold the plug with the end with 8 small gold-plated contacts facing upwards and the end with the large rectangular opening into which the network cable is inserted facing downwards. At the same time, point the side without the thin plastic locking pin towards your eyes. Starting from the first small gold-plated contact on the left, the pins are numbered sequentially as pin 1, pin 2, ..., pin 8.
In the past, communication between PCs and smart devices relied mainly on RS232, RS485, and Ethernet, depending on the device's interface specifications. However, RS232 and RS485 only represent the physical media layer and link layer of communication. To achieve bidirectional data access, a custom communication application had to be written. But most of these programs do not conform to ISO/OSI standards, can only perform relatively simple functions, are suitable for a single device type, and lack versatility .
In a network of devices connected via RS232 or RS485, if the number of devices exceeds two, RS485 must be used as the communication medium. Communication between devices in an RS485 network is only possible through a "master" device, typically a PC. Only one master device is allowed in such a network, with the rest being slave devices. Fieldbus technology, based on the ISO/OSI model, has a complete software support system and can solve problems related to bus control, collision detection, and link maintenance. Fieldbus devices automatically form a network, without a master/slave distinction or allowing multiple masters. Products from different manufacturers are interchangeable at the same level, and devices are interoperable .
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