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Last time, we delved into the core elements of the MODBUS protocol. I wonder if you have memorized those key points and successfully established a foothold in the world of MODBUS? If you haven’t fully digested the information yet, make sure to review it soon, as our learning path is interconnected, and we can’t afford to fall behind. Today, we will unveil the mysteries of serial communication and see how it helps the MODBUS protocol facilitate smooth “conversations” between devices. Let’s keep up the pace and explore this fascinating field of knowledge together!
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What is Serial Communication? The “Veteran” of Industrial Communication
Serial communication is not a new concept; it has decades of experience in the field of industrial communication. Imagine the late 1970s when our grandparents might still have been using “brick phones”; serial communication was already out there making its mark. Its original purpose was to enable stable and reliable data transmission between various devices.
In simple terms, serial communication is a method of transmitting data bit by bit, much like how we use courier services, where only one package can be delivered at a time. However, with the right methods, we can ensure that all packages are delivered accurately. Common serial communication interface standards include RS-232, RS-485, and RS-422. RS-232 is typically used for short-distance communication, such as connecting a computer to a printer, while RS-485 and RS-422 are used for multipoint and long-distance communication, making them more popular in industrial automation. However, with the development of Ethernet technology, serial communication has become somewhat “outdated,” yet it remains effective in many small and simple automation systems!
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Exploring the Perfect Collaboration Between Serial Communication and MODBUS
For devices to communicate smoothly using the MODBUS protocol over serial communication, three key elements—physical connection, parameter configuration, and transmission process—are essential. We need to understand all of them to enable devices to “converse” happily.
(1) Physical Connection: The “Handshake Agreement” of Devices
This is like setting the stage for a performance where devices will showcase their “communication ballet.” For RS-232, it is generally a point-to-point connection using three wires (transmit, receive, and ground). For example, a computer directly connected to a printer.For RS-485, a bus topology is used, resembling a long rope where multiple devices connect to this bus. Each device must have two signal lines (A and B) connected to the bus. Additionally, to avoid signal reflection, 120Ω termination resistors must be added at both ends of the bus.
(2) Communication Parameters: The “Secret Code Rules” of Devices
Simply connecting the devices is not enough; they also need a set of “secret code rules,” which are the communication parameters. This is akin to playing a game of Werewolf, where we must establish rules for day and night phases and speaking order for the game to proceed smoothly.
In MODBUS communication over RS-485, the main communication parameters include:
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Baud Rate: This can be understood as the “speaking speed” of the devices, with common rates being 9600bps, 19200bps, etc. If the baud rates are set differently, the master and slave devices will be speaking different languages, leading to communication failure.
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Data Bits: This indicates how many data bits are in each data packet, typically 7 or 8 bits. This is like determining how many characters can be written on a note; if too many or too few are written, the recipient may not understand.
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Stop Bits: The role of stop bits is similar to pausing after completing a sentence, allowing the recipient to know that the sentence has ended. It is usually 1 or 2 bits, providing the receiver with a little time to process the data after transmission.
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Parity Bit: This is the “guardian” of data accuracy, typically offering options for no parity, even parity, and odd parity. For instance, odd parity requires that the total number of 1s in the data bits and parity bit is odd. If not, the receiver knows there was an issue during transmission and requests a resend.
These communication parameters are like the “secret codes” of the devices; only when both the master and slave devices are set with the same parameters can they communicate smoothly, accurately transmitting commands and data to each other.
(3) Data Transmission Process: The “Back-and-Forth” of Devices
Once the physical connection and communication parameters are established, the devices can begin their “dialogue.” This process resembles a carefully choreographed duet, where the master and slave devices ask and answer questions in perfect harmony.
The master device first packages the data to be sent according to the rules of the MODBUS protocol into a complete data frame. The beginning of this data frame is the start bit, akin to knocking on a loud door to signal the slave device, “I am about to speak.” Next is the address field, specifying which slave device is being addressed, similar to calling out someone’s name. Then comes the function code field, informing the slave device, “This is what I want you to do,” such as reading or writing data. Following that is the data field, where the specific content to be transmitted is placed. Finally, there is the parity bit, which serves as the “key” for the slave device to check the integrity of the data.
Upon receiving this data frame, the slave device first unpacks the data according to the communication parameters and checks the parity bit to see if there were any errors. If everything is fine, it begins executing the instructions in the data frame, such as reading data from a specific register or changing the state of a coil. After execution, the slave device will also return the execution result or the read data to the master device following the same rules. The master device, upon receiving the reply, will process it accordingly, such as updating the monitoring interface or adjusting control strategies.
(4) Data Transmission Modes: The “Traffic Rules” Between Devices
In the world of serial communication, data transmission modes are like traffic rules, determining how data flows between devices. These rules are mainly divided into three modes: simplex, half-duplex, and full-duplex.
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Simplex: A single lane allows traffic in only one direction (e.g., television signal broadcasting).
Simplex mode is like a one-way street where data can only be transmitted in one direction. In this mode, data can only flow from the sender to the receiver, and the receiver cannot send data back. It is similar to a radio station broadcasting programs to listeners, where listeners can only receive signals and cannot send signals back to the station.
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Half-Duplex: A two-way lane, but only one direction can be used at a time (e.g., walkie-talkies).
Half-duplex mode is like a two-way street where there is a rule that only one direction can be used at a time. This means that devices can send and receive data, but not simultaneously. It is like a walkie-talkie, where pressing the talk button allows you to speak, and releasing it allows you to listen, but you cannot talk and listen at the same time.
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Full-Duplex: A two-way lane where both directions can operate simultaneously (e.g., data exchange in automated production lines).
Full-duplex mode is like a two-lane road where data can be transmitted in both directions simultaneously. Devices can send and receive data at the same time, just like a phone call where both parties can speak and hear each other simultaneously.
This data transmission process is quite fascinating, isn’t it? Through serial communication, the master and slave devices can achieve efficient “dialogue” under the guidance of the MODBUS protocol, allowing the entire industrial system to operate in an orderly manner.
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Transmission Methods Based on Serial Communication
In MODBUS protocol’s serial communication transmission, there are mainly two methods: MODBUS RTU and MODBUS ASCII. They are like twin brothers, looking similar but having different characteristics and applicable scenarios in practice, and we need to distinguish them well.
(1) MODBUS RTU
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Characteristics: It is like the “efficiency expert,” with compact data transmission and high efficiency. During transmission, data is encoded in binary, with each byte containing actual data information without unnecessary “packaging.”
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Applicable Scenarios: When communication between devices requires speed, and the communication distance is relatively short with a large amount of data to be transmitted, MODBUS RTU is the preferred choice. For example, in a small factory, if you want to monitor the operating status of inverters in real-time and make quick adjustments, MODBUS RTU is the way to go.
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Disadvantages: However, it has high requirements for the quality of communication lines. If there is interference on the line, it is like encountering obstacles on a fast highway, and data can easily become erroneous.
(2) MODBUS ASCII
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Characteristics: In contrast, MODBUS ASCII is like the “steady type,” converting data into ASCII characters before transmission. Even if the communication line is not very stable, it can ensure data integrity. Each byte is represented by two ASCII characters, effectively adding a layer of “protection” to the data.
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Applicable Scenarios: It is suitable for scenarios where the quality of communication lines is poor, or where communication between devices does not require high speed but demands high reliability of data. For example, in some old factories where communication lines are aging and prone to interference, using MODBUS ASCII can prevent data loss or errors.
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Disadvantages: The transmission efficiency is lower because each byte must be represented by two characters, which increases the data volume and naturally slows down the transmission speed.
In practical applications, if the communication distance is short, the environment is less prone to interference, and speed is required, then choose MODBUS RTU; if the communication environment is complex, with more interference, and speed is not a priority but data accuracy is essential, then use MODBUS ASCII.
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Practical Application Case
We need to look at practical cases to clarify these concepts. Let’s consider an automated irrigation system.
In a vast farmland, there is a control center (master device), which acts as the “command center” of the entire irrigation system. In the fields, there are many soil moisture sensors (slave devices) and irrigation valves (slave devices). These sensors and valves are connected to the control center via RS-485 serial communication.
The soil moisture sensors store the detected soil moisture data in their registers. If the control center wants to know the soil moisture status, it sends a command to read the register of the corresponding sensor according to the MODBUS protocol. After receiving the command, the sensor returns its moisture data to the control center via the RS-485 bus.
Once the control center receives the moisture data, it compares it with the set moisture range. If it finds that the soil moisture in a certain area is too low, it sends a command to the corresponding irrigation valve to open, watering the farmland. If the moisture is adequate or too high, it sends a command to close the irrigation valve, stopping the watering.
In this case, RS-485 serial communication acts like the “nervous system” of the farmland, tightly connecting the control center and various devices. Under the guidance of the MODBUS protocol, the devices automatically perform irrigation operations based on the actual soil moisture conditions, conserving water resources while ensuring that crops thrive.
The perfect combination of RS-485 serial communication and the MODBUS protocol enables such an intelligent automated irrigation system, making farming high-tech!
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Summary and Practice
Alright, that’s all for today regarding the mysteries of serial communication! Let’s recap: serial communication is the “veteran” in the field of industrial communication. Through simple physical connections and reasonable communication parameter settings, combined with the MODBUS protocol, it enables smooth communication between devices. It has advantages such as strong anti-interference capability, long transmission distance, and multipoint communication ability, but also limitations like relatively slow transmission speed and restricted network topology. However, in suitable scenarios, serial communication can still play a significant role in helping us achieve intelligent and efficient automation control.
If you can thoroughly understand the knowledge points discussed today, you will have made significant progress in the MODBUS protocol and industrial communication field! However, just understanding the theory is not enough; you need to practice hands-on. Look for small devices that support the MODBUS protocol and communication around you, set up a simple communication network, and use upper computer software or programming tools to try to achieve communication between the master and slave devices based on today’s knowledge. Trust me, practice makes perfect; as long as you are willing to get hands-on, you will soon master serial communication and gain valuable experience for handling complex industrial communication projects in the future.
If you encounter any issues during practice or have any questions about today’s content, don’t hesitate to leave a comment, and let’s brainstorm together to solve the problems! See you next time, and remember to ponder the mysteries of serial communication!