The difference between Modbus RTU and TCP is easy to understand at a glance!

The Modbus protocol is like the "standard language" between industrial devices, allowing instruments and meters produced by different manufacturers to communicate with each other. Modbus RTU and Modbus TCP are two different accents of this Mandarin language, each with its own applicable scenarios.

Basic Understanding: Two brothers'“Birth background”

Imagine, Modbus RTUandModbus TCPThey are two brothers from the same family:

• Modbus RTU​ It's from home“Big Brother”, mature and steady. It is mainly inSerial line (For example, commonRS-485bus) Go to work, The way devices are connected is like an old-fashioned telephone line, String them together one by one .
• Modbus TCP​ then it is“young and promising”My younger brother, Flexible and efficient. It runs onEthernetUp,All devices are connected to the switch, Like the current local area network, More modern management .

Core difference: Detailed comparison of the five dimensions

1. Transmission method and hardware interface: Let's go“road”different

• RTULet's go“country road” (Serial bus) : It depends onRS-485/RS-232This type of serial port, Use a dedicated serial port cable (Usually two core shielded twisted pair cable) connected device. The network structure is“Hand in Hand”ofBus Type, A primary device (Master) Carrying multiple slave devices (Slave) . Just like broadcasting, Host shouting, Designated slave response.
• TCPLet's go“expressway” (Ethernet) : It uses standard Ethernet cables (RJ45interface) And the switch, All devices are equipped withstar-shapedHow to connect to the network center. The communication parties areclient (Client) And the server (Server) ​ of, It's more like a one-on-one phone conversation .

2. Data packaging method: “package”The packaging is different

The core that both need to convey“Content” (That is to say, function codes and data) It's actually the same, But the packaging methods vary greatly.

• RTUpackage (data frame) : Compact and small
  • slave address: 1byte, Indicate which device the data packet is sent to.
  • function code: 1byte, Tell the other person what to do (For example, reading data or writing data) .
  • data domain: Indefinite length, It is a specific instruction or data content.
  • CRCVerification code: 2byte, Used to check if there are any errors during data transmission . RTUSending direct binary data .
• TCPpackage (data frame) : Added new packaging
  • MBAPMessage header: 7byte, Equivalent to a new express envelope, It contains transaction identifiers (Used for matching requests and responses) , Protocol identification, Information such as length and unit identifier .
  • Function code+data field: This part andRTUThe core content of the agreement is exactly the same, Installed on itMBAPIn this new envelope.
  • canceledCRCVerification: BecauseTCPThe protocol itself is a reliable connection, Built in checksum retransmission mechanism, So there is no need for additionalCRCVerified it .

Speed and distance: Who is“Super Boy”, Who can“journey”

The table below visually displays the differences in performance and connectivity between them:

Reliability and real-time performance: Who is better“stable”, Who is better“timely”

• reliability:
  • RTU​ dependenceCRCVerificationTo check for errors. But under the bus topology, A node or line failure may affect the entire network .
  • TCP​ rely onTCPConfirmation and retransmission mechanism of the protocol itselfTo ensure reliable delivery of data. Under star topology, A single point of failure usually does not affect the entire network .
• real-time:
  • RTU​ ofLow protocol overhead, Low and stable latency, The response is very timely, Suitable for demanding real-time control .
  • TCP​ Due tohandshake process, Confirmation mechanism and greater protocol overhead, The delay is relatively high and may fluctuate, But fast enough for most monitoring scenarios .

Cost and Application Scenarios: How to choose the most“cost-effective”

• cost:
  • RTU: related equipment (If equipped with a serial portPLC, sensor) The cost is usually low, The wiring is relatively simple and economical .
  • TCP: needEthernet infrastructure (switch, Devices with Ethernet ports) , Initial investment may be higher .
• Typical application scenarios:
  • ChoiceRTUplay the leading role: There are not many devices available, Relatively concentrated distribution, Not far away. (For example, in a workshop or a control cabinet) , And forHigh real-time requirements, limited budgetthe scene. For example, smallPLCsystem, Sensor clusters around CNC machine tools .
  • ChoiceTCPplay the leading role: Large number of devices, widely distributed (Even across regions) , needremote monitoring, Related to the upper level information system(such asMES) IntegrationFuture oriented projects. For example, Whole plant data collection for smart factories, Monitoring of cross regional pipeline networks .

IIIPractical selection: Which one should I use?

Simply put, You can follow this principle:

• Pursuing cost savings, stable, real-time control, The devices are all close together? → Modbus RTU​ It's your good choice.
• Pursuing fast speed, Easy to expand, remote management, Devices are scattered and even require internet access? → Modbus TCP​ More advantageous.