Modbus RTU vs TCP: A Comprehensive Comparison of Industrial Protocols
23 May, 2023
Head Engineer monitoring plant data using SCADA, through HMI
Modbus protocols are essential components in the realm of industrial automation and control systems. Designed in 1979 by Modicon (now Schneider Electric), Modbus was developed as a way to enable communication between Programmable Logic Controllers (PLCs) and other devices in industrial networks. Over time, the protocol has evolved, and its versatility has made it a popular choice in various industries, with RTU and TCP protocols being prominent among them.
Introduction to Modbus Protocols
Consider a scenario where you need to monitor or control - multiple freezer temperatures on hospital floors, devices at your home, traffic patterns, or various devices that are spread throughout the industry stations. Thanks to the Modbus protocol that makes this achievable easily, and centrally.
Put simply, the Modbus protocol was developed to easily transmit data between electronic devices. They prove to be beneficial in industrial applications where industrial computers, PLCs, and SCADA systems require analog data from the sensors to make quick decisions. Thankfully, most of these industrial devices support the Modbus protocol.
Being an open-source protocol, it is freely available online and easier to download - hence Modbus communication protocol is the de-facto standard and the most widely used in the control and automation industry.
It is implemented in oil, gas, wind, solar, and various other industries. Because the data is transferred in a single (application) layer, it easily performs activities such as - collecting analog data from various sensors, monitoring it on a single interface, performing communication between industrial and automation devices, etc.
Modbus RTU vs Modbus TCP
There are several types of Modbus protocols used today, the most common being Modbus RTU (Remote Terminal Unit) and Modbus TCP (Transmission Control Protocol). Each of these protocols has distinct characteristics, advantages, and applications that make them suitable for specific environments and use cases. Modbus data is therefore exchanged efficiently through - Modbus serial (using RTU) and through Modbus ethernet layer (using TCP protocol).
Modbus RTU, for instance, utilizes serial communication for data exchange between devices, making it ideal for systems that require simple, robust, and cost-effective communication. Modbus RTU is often used in applications such as building automation, industrial process control, and remote monitoring.
On the other hand, Modbus TCP leverages Ethernet and IP networks for communication, providing increased speed and scalability in more complex and demanding environments. Modbus TCP is commonly found in applications that require high-speed communication such as power generation, oil and gas, and data center management.
The spotlight of this article Modbus RTU vs TCP will be analyzed through various technical and practical nitty-gritty such as - comparing the communication and network frameworks, security concerns, cost considerations, real-world applications, and more. By the end of this discussion, you will have a comprehensive understanding of these two industrial protocols and be better equipped to choose the most suitable option for your specific needs.
Overview of Modbus RTU
Modbus RTU (Remote Terminal Unit) is the most widely used industrial communication protocol that exchanges Modbus data using serial communication for data exchange between devices. Its simplicity, robustness, and cost-effectiveness, make it a popular choice for systems that demand reliable communication without the need for high-speed data transfer. Some common application areas for Modbus RTU include building automation, industrial process control, and remote monitoring.
The Modbus RTU protocol operates over various physical layers such as RS-232, RS-422, and RS-485 (where RS stands for Recommended Standard), allowing it to adapt to different communication requirements. The most common implementation is with RS-485, which supports multi-drop configurations and communication distances up to 1,200 meters. The maximum data rate for Modbus RTU varies depending on the physical layer used, with RS-485 capable of achieving speeds up to 10 Mbps.
Modbus RTU Frame Structure
The Modbus messaging frame structure for RTU is designed to facilitate communication between devices, ensuring that data is organized and transmitted efficiently. Each field within these frames serves a specific purpose, as discussed below.
Address Field: This field occupies 1 byte and contains the address of the device (slave) for whom the message is intended for. Addresses can range from 1 to 247, allowing a maximum of 247 devices on a single network, through repeaters.
Function Code Field: This 1-byte field indicates the type of operation to be performed by the slave device, such as reading or writing data. Function codes can be standard (1-127) or user-defined (128-255).
Data Field: The data field contains the information that needs to be transmitted between devices. Its size varies (up to 252 bytes) depending on the function code and the amount of data involved in the operation.
CRC (Cyclic Redundancy Check) Field: The 2 bytes CRC field is used for error detection, ensuring data integrity during transmission. It is calculated based on the content of the entire frame, excluding the CRC field itself.
A typical Modbus frame starts with a silent interval of at least 3.5 times the period of a character, followed by the address field, a function code field, a data field, and finally the CRC field. After the CRC field, another silent interval of a similar period marks the end of the frame.
The data in Modbus (both RTU & TCP) is stored in Modbus registers, which are memory locations for storing different types of data, namely integer, binary, floating-point, custom data types, etc. The register address in Modbus ranges from 0 to 65,535. The most commonly used Modbus registers are Coil, Discrete Inputs, Holding registers, and input registers.
This frame structure equips Modbus RTU with a simple and efficient communication scheme, which contributes to its robustness and reliability in industrial applications.
Modbus RTU Error Detection
Error detection is crucial in any communication protocol to ensure data integrity and reliable operation. Modbus RTU employs a Cyclic Redundancy Check (CRC) mechanism to detect errors that may occur during data transmission. The CRC is a widely used error-detection method known for its simplicity and effectiveness in detecting a wide range of errors, such as single-bit errors, double-bit errors, and burst errors.
In Modbus RTU, the CRC is calculated based on the content of the entire frame, excluding the CRC field itself. The CRC calculation involves treating the frame's content as a binary message and dividing it by a predetermined polynomial, producing a remainder that serves as the CRC value. This value is appended to the end of the frame as a 2-byte CRC field, for transmission.
The receiving device recalculates the CRC based on the received content and compares it with the CRC value included in the frame. If the calculated and received CRC values match, the message is considered error-free, and the device proceeds to process the message. If the CRC values do not match, the receiving device assumes that an error has occurred during transmission and discards the message. The receiving device may also request the sender to retransmit the message, depending on the specific implementation.
However, it is worth noting that, like any error-detection technique, the CRC is not foolproof and may not detect all possible errors. Nevertheless, the CRC's effectiveness in detecting a wide range of errors makes it a suitable choice for the Modbus RTU protocol, contributing to its overall robustness and reliability in industrial applications.
Modbus ASCII
ASCII stands for American Standard Code for Information Exchange. Like RTU, Modbus ASCII is a part of the original Modbus protocol that uses serial communication to transmit data bit by bit over a single communication line.
It uses an ASCII character set as compared to RTU’s binary coding. However, Modbus ASCII is known to utilize almost double the size of the data frame as compared to Modbus RTU, making it slower for transmission compared to RTU. Also, Modbus ASCII uses LRC (Longitudinal Redundancy Check) method for error checking, which makes it less effective as compared to RTU’s CRC method. For this reason, ASCII is used very less and is a depleting type of protocol. Moreover, Modbus ASCII is now less supported by modern devices and interfaces.
Overview of Modbus TCP
Modbus TCP/IP (Transmission Control Protocol) is an adaptation of the Modbus protocol designed for communication over Ethernet and IP networks. As industrial networks have grown more complex and demanding, Modbus TCP has emerged as a popular choice for applications requiring high-speed communication and increased scalability.
Modbus TCP retains the simplicity and ease of use associated with the original Modbus protocol while taking advantage of the benefits offered by Ethernet and IP networks. It offers increased data rates, enhanced network reliability, and the ability to connect a larger number of devices compared to RTU. Additionally, Modbus TCP can leverage existing Ethernet infrastructure, reducing the need for specialized hardware and simplifying network implementation.
Modbus TCP Frame Structure
The Modbus messaging frame structure for TCP is designed to facilitate communication over Ethernet and IP networks. While it shares some similarities with the Modbus RTU frame, Modbus TCP introduces additional fields to accommodate the requirements of Ethernet and IP communication. The Modbus TCP frame consists of the following fields:
MBAP Header: The Modbus Application Protocol (MBAP) header is a 7-byte field unique to Modbus TCP. It contains the information necessary for routing and processing the message within an Ethernet and IP environment. The MBAP header consists of four sub-fields:
Transaction Identifier (2 bytes): This field is used to match a request message with its corresponding response message.
Protocol Identifier (2 bytes): This field is always set to 0 for Modbus communication, indicating that the Modbus protocol is being used.
Length Field (2 bytes): This field specifies the number of bytes remaining in the frame, excluding the MBAP header.
Unit Identifier (1 byte): This field contains the address of the device (slave) that the message is intended for, similar to the address field in Modbus RTU.
Function Code Field: Like Modbus RTU, this 1-byte field indicates the type of operation to be performed by the slave device, such as reading or writing data.
Data Field: The data field contains the information that needs to be transmitted between devices. Its size varies depending on the function code and the amount of data involved in the operation.
In conclusion, a Modbus TCP frame begins with the MBAP header, followed by the function code field and the data field. Are you curious about the CRC field, where did it go? Unlike Modbus RTU, Modbus TCP does not include a CRC field for error detection, as this functionality is provided by the underlying Ethernet and IP layers.
The Modbus TCP frame structure enables efficient communication over Ethernet and IP networks while maintaining the simplicity and ease of use associated with the Modbus protocol. These features make Modbus TCP a powerful and versatile option for industrial communication in complex and demanding environments.
Modbus TCP Error Detection and Correction
As discussed in the previous section, Modbus TCP relies on the underlying Ethernet and IP layers for error detection and correction, instead of using a built-in mechanism like the CRC in Modbus RTU. By utilizing these existing mechanisms, Modbus TCP eliminates the need for additional error detection at the application layer.
The error detection and correction methods employed by Ethernet and IP networks include:
Ethernet Frame Check Sequence (FCS): Ethernet frames include a Frame Check Sequence (FCS) field, which is a 4-byte value calculated using the Cyclic Redundancy Check (CRC-32) algorithm. Similar to the CRC used in Modbus RTU, the FCS is calculated based on the frame's content and serves as a method to detect transmission errors. Ethernet devices discard frames with incorrect FCS values, ensuring that only error-free frames are processed.
IP Header Checksum: The IP header contains a checksum field that provides error detection for the IP header itself. The checksum is calculated by adding the 16-bit words of the IP header, taking the one's complement of the result, and storing the value in the checksum field. Receiving devices verify the IP header checksum and discard packets with incorrect checksum values.
TCP Checksum: In addition to the IP header checksum, TCP also uses a checksum to detect errors in the TCP header and payload. The TCP checksum calculation involves adding the 16-bit words of the TCP header, payload, and a pseudo-header derived from the IP header. The receiving device verifies the TCP checksum and discards segments with incorrect checksum values.
Retransmission: TCP incorporates error recovery mechanisms, such as retransmission of lost or corrupted segments. If a receiving device detects an error or does not receive an expected segment within a specified time, it may request the sender to retransmit the missing data. This mechanism ensures reliable data delivery even in the presence of transmission errors.
By relying on the error detection and correction techniques built into the Ethernet and IP layers, Modbus TCP ensures reliable communication and data integrity in industrial applications. This approach simplifies the Modbus TCP protocol and allows it to take advantage of the robustness and reliability of modern Ethernet and IP networks.
Modbus RTU vs Modbus TCP: Communication Media and Speed
When comparing Modbus RTU and Modbus TCP, it is important to consider the communication media and speed, as they play a significant role in determining the overall performance and suitability of each protocol for different applications.
Modbus RTU Communication Media
Modbus RTU primarily utilizes serial ports such as RS-232 and RS-485. RS-232 is suitable for short-distance communication (up to 50 feet or 15 meters), while RS-485 supports long-distance communication (up to 4000 feet or 1200 meters), with multi-drop configurations, and enables multiple devices to share a single communication line. It transmits data in binary format i.e. 0 & 1.
Serial communication in Modbus RTU is typically slower compared to Ethernet-based communication used by Modbus TCP. RS-232 and RS-485 speeds range from 1.2 kbps to a max of 115 kbps, but the actual communication speed is influenced by factors such as cable length, noise, and the number of devices on the network.
Modbus TCP Communication Media
Modbus TCP utilizes Ethernet-based communication media, which offers several advantages over serial communication, including higher data rates, increased reliability, and widespread adoption across various industries. Ethernet communication supports data rates of 10 Mbps, 100 Mbps, 1 Gbps, and even higher, significantly surpassing the speeds achievable with Modbus RTU. It converts the binary data into a hexadecimal string for easy transmission of data.
Ethernet networks can span longer distances using switches and routers, providing greater flexibility in network design and implementation. Additionally, Ethernet communication supports advanced features such as Quality of Service (QoS) and network diagnostics, further enhancing the performance and reliability of Modbus TCP networks.
When comparing Modbus RTU vs Modbus TCP for communication speed, we find Modbus TCP leading ahead, with the leverage of Ethernet TCP/IP which is widely popular in the market. Not only does Modbus TCP has a compact data frame relatively, but its reliance on Ethernet TCP/IP protocol for error correction leaves it the best choice for efficient utilization of available bandwidth. Interestingly, RTU remains the most commonly used choice considering the application requirements in the industry that do not require very high communication speed, while the TCP interface is catching up.
Recommended reading: Industrial Communication Protection for RS-485 and Ethernet
Modbus RTU vs Modbus TCP: Network Topology and Scalability
When evaluating Modbus RTU and Modbus TCP, it's essential to consider the network topology and scalability aspects. Both protocols support different topologies and have varying degrees of scalability, which can influence the choice of protocol based on the specific requirements of a given industrial automation system.
Network Topology and Scalability in Modbus RTU
Modbus RTU networks can be configured in various topologies, such as point-to-point, multi-drop, and multi-point. The most common topology is the multi-drop configuration, where multiple slave devices are connected to a single master device on a single communication line. This can also be called daisy chain topology. This is made possible using RS-485 communication, which supports multi-drop configurations. In this master-slave configuration, up to 32 slave devices (which can be extended to 247 devices) can be connected in a single communication line, generally. However, only one slave can communicate at a time.
Modbus RTU networks also encounter limitations when it comes to network expansion and complexity. For example, as the number of devices increases, the complexity of managing and troubleshooting the network can become cumbersome. Besides the total number of devices, the overall network distance also impacts communication speed and reliability.
Network Topology and Scalability in Modbus TCP
Modbus TCP, being based on Ethernet and IP, supports various network topologies, such as star, tree, ring, and mesh. This allows for greater flexibility in network design, which makes it easier to expand and modify the network as needed. Ethernet switches and routers are of great help in achieving network expansion while spanning long distances.
Also, Modbus TCP can accommodate a higher number of devices (247) on the same line network, with each device having a unique IP address for easy identification and management. The use of IP addressing simplifies device identification and management, making it more straightforward to configure, monitor, and troubleshoot the network. This enhances the scalability and performance of Modbus TCP networks.
Scalability is a critical factor to consider, especially in large industrial automation systems that require the interconnection of numerous devices. Modbus TCP offers superior scalability compared to Modbus RTU due to its reliability on Ethernet which supports various network topologies. If it’s monitoring a few devices, RTU serves this best. However, for a growing business that will involve a myriad of devices requiring advanced monitoring facilities, TCP is the way to go.
Modbus RTU vs Modbus TCP: Network Security
Network security is the most critical factor in industrial automation systems, to protect the integrity and confidentiality of data, as well as to ensure the availability of the system. In this section, we will analyze and compare the network security aspects of Modbus RTU and Modbus TCP, providing insights into their respective strengths and weaknesses.
Modbus RTU Network Security
Modbus RTU relies on serial communication, typically using RS-485 or RS-232 physical layers. Due to its nature, Modbus RTU can be considered more secure than Modbus TCP when it comes to external cyber threats. Serial communication is less prone to remote cyber-attacks, as an attacker would need physical access to the communication line to intercept or manipulate data.
However, Modbus RTU does not inherently include any encryption or authentication mechanisms. This means that if an unauthorized person gains physical access to the communication line, they could potentially eavesdrop on the data being transmitted or even inject malicious messages. To mitigate these risks, additional security measures, such as secure communication channels or physical access control, should be implemented.
Modbus TCP Network Security
Modbus TCP, being based on Ethernet and IP, is more susceptible to cyber threats than Modbus RTU, as it operates on a more widely-used and accessible communication medium. This means that Modbus TCP networks can be targeted remotely by attackers, potentially exposing them to a broader range of threats.
Like Modbus RTU, the Modbus TCP protocol does not natively incorporate any encryption or authentication mechanisms. As a result, the data transmitted over the network is vulnerable to interception, manipulation, or denial of service attacks.
To enhance the security of Modbus TCP networks, various techniques can be employed. One approach is to use Virtual Private Networks (VPNs) to create secure communication tunnels between devices, encrypting the data transmitted over the network. Another option is to implement network segmentation, isolating critical devices or systems from the rest of the network to limit potential attack surfaces.
Firewalls and intrusion detection systems can also be used to monitor and filter network traffic, detecting and blocking potential threats. Additionally, proper access control, secure password management, and regular security updates for network devices and software should be established as part of a comprehensive security strategy.
Comparing the network security aspects of Modbus RTU and Modbus TCP, it becomes apparent that each protocol has its own set of challenges and advantages. Modbus RTU benefits from the inherent security provided by serial communication, making it less prone to remote cyber-attacks. However, it lacks native encryption and authentication mechanisms, which can expose the network to physical security breaches.
On the other hand, Modbus TCP networks are more susceptible to remote cyber threats due to their reliance on Ethernet and IP. While the protocol itself does not include built-in encryption or authentication features, a variety of security techniques can be employed to enhance the overall security of the network.
Ultimately, the choice between Modbus RTU and Modbus TCP will depend on the specific security requirements of the system, as well as the ability to implement additional security measures to address the inherent vulnerabilities of each protocol.
Modbus Security
In August 2018, Modbus Security was published, considering the lack of proper security implementation that leaves the messages vulnerable to hackers. It is also called Modbus TCP TLS (Transport Layer Security) as it’s used over the existing TCP/IP layer, to add security over the ongoing communications. Using the cryptographic code keys, Modbus TLS verifies the message between the client and server. Besides, the devices on the server also exchange the codes and keys for further verification.
This prevents the hackers from sending false messages from clients or devices or overloading the system with messages to hack it.
Though Modbus TLS adds strong security to the existing scenario, it nonetheless isn’t considered a perfect system.
For this reason, companies using Modbus are always recommended to implement the best network security practices to ensure safety from security hacks.
Modbus RTU vs Modbus TCP: Ease of Integration
The ease of integration of Modbus RTU and Modbus TCP protocols into existing systems and infrastructure plays a crucial role in the selection process. This section will delve into the factors that influence the integration process and provide a comparison of both protocols in terms of their ease of integration.
Modbus RTU Integration
Modbus RTU, as a serial communication protocol, can be easily integrated into systems that already use RS-485 or RS-232 interfaces. Devices with serial communication capabilities (like Modbus RTU devices) are often found in legacy systems, making Modbus RTU an attractive choice for retrofitting or upgrading existing installations.
One advantage of Modbus RTU is its simplicity, which results in lower implementation costs and reduced complexity. It requires fewer resources and less processing power than Modbus TCP, making it suitable for devices with limited computational capabilities.
However, there are some challenges when integrating Modbus RTU into larger systems. The protocol requires a dedicated communication channel for each connection, which can limit the scalability of the network. Additionally, Modbus RTU networks may suffer from signal degradation and noise issues when transmitting data over long distances or in electrically noisy environments.
Modbus TCP Integration
Modbus TCP is designed for Ethernet-based communication, which is the prevalent communication medium in modern industrial automation systems. Due to the widespread availability of Ethernet-compatible devices and infrastructure, the integration of Modbus TCP into new installations becomes more straightforward.
Another advantage of Modbus TCP is its compatibility with the standard TCP/IP stack, which allows for seamless integration with existing IP networks. This compatibility enables Modbus TCP devices to communicate over local networks, as well as remotely over the Internet, providing greater flexibility in network architecture.
However, the integration of Modbus TCP into systems with limited resources or legacy devices can be more challenging. Modbus TCP requires more processing power and memory than Modbus RTU, which may necessitate the use of more powerful or specialized hardware.
Comparing the ease of integration of Modbus RTU and Modbus TCP reveals that RTU is a good choice where implementation is simple, and cost is a concern. Besides, many industries with legacy systems will find RTU as the best retrofit. On the other hand, TCP could be the standard Modbus choice considering the widespread usage of modern Ethernet-based systems. However, its higher resource requirements may not be integrable with various limited devices.
Modbus Gateways
Just like language a translator helps people speaking different languages understand each other. Similarly, Modbus gateways help in exchanging data between different devices that use different protocols for communication.
Consider a device that uses serial communication using Modbus RTU/ASCII. However, you would like to integrate this into a network that uses Modbus TCP/IP over ethernet. Modbus Gateways makes this possible.
Not just for Modbus devices, but it can also be used to connect non-Modbus devices to the Modbus network, or vice versa. Besides protocol conversion, Modbus Gateways also perform data mapping, interface conversion, filtering, etc.
Modbus RTU vs Modbus TCP: Cost Considerations
When choosing between Modbus RTU and Modbus TCP, cost considerations play a significant role in the decision-making process. In this section, we will examine the various factors that contribute to the overall costs of implementing and maintaining both protocols.
Modbus RTU Cost Factors
Modbus RTU, as a serial communication protocol, is generally considered to be more cost-effective than Modbus TCP in certain scenarios. The hardware required for Modbus RTU communication, such as RS-485 or RS-232 transceivers, is typically less expensive than the hardware necessary for Ethernet-based communication.
Additionally, Modbus RTU's simplicity and lower resource requirements can lead to savings in terms of computational resources and memory. This means that less expensive and less powerful devices can be used in a Modbus RTU network, reducing overall hardware costs.
However, there are some cost-related drawbacks to Modbus RTU. The need for dedicated communication channels for each connection can increase wiring and installation costs in larger systems. Moreover, the potential for signal degradation and noise interference in long-distance or electrically noisy environments may necessitate the use of repeaters or additional shielding, further increasing the overall cost.
Modbus TCP Cost Factors
Modbus TCP, being an Ethernet-based protocol, generally requires more expensive hardware than Modbus RTU, such as Ethernet switches, routers, and network interface cards. Furthermore, the higher processing power and memory requirements of Modbus TCP may necessitate the use of more powerful devices, which can increase hardware costs.
However, Modbus TCP benefits from the ubiquity of Ethernet-based infrastructure, which can result in lower installation costs due to the widespread availability of Ethernet cables and devices. The use of standard TCP/IP communication also allows for the sharing of communication channels, reducing the need for dedicated wiring and lowering installation costs.
In addition, Modbus TCP's compatibility with existing IP networks enables remote access and monitoring, potentially reducing maintenance and troubleshooting costs by allowing remote diagnostics and updates.
When comparing the cost considerations of Modbus RTU and Modbus TCP, it is essential to take into account factors such as hardware, installation, and maintenance expenses. Evaluating based on these factors with the help of system designers, the best choice can be estimated between the two.
Modbus RTU vs Modbus TCP: Real-World Applications
When it comes to real-world applications, the choice between Modbus RTU and Modbus TCP often depends on the specific requirements of the system, the environment, and other factors. In this section, we will explore typical use cases for both Modbus RTU and Modbus TCP, highlighting their strengths and weaknesses in different scenarios.
Modbus RTU in Industrial Automation
Modbus RTU is widely used in industrial automation applications due to its simplicity, reliability, and cost-effectiveness. It is particularly well-suited for small-scale systems or for connecting devices within a localized area, such as a factory floor or a single building.
For instance, Modbus RTU can be employed to connect Programmable Logic Controllers (PLCs) to various sensors, actuators, and other devices in a production line. This allows for the monitoring and control of processes such as temperature regulation, motor speed control, or fluid level monitoring. Its deterministic nature, which ensures a fixed response time, is essential in time-sensitive applications where quick decision-making and control are crucial.
Modbus TCP in Building Automation and Smart Grids
Modbus TCP is an ideal choice for applications that require extensive networking, remote access, or integration with other IP-based systems. In building automation, for example, Modbus TCP can facilitate communication between various building systems, such as heating, ventilation, air conditioning (HVAC), lighting, and security systems. This allows for centralized monitoring and control, improving energy efficiency and optimizing building operations.
Similarly, in smart grid applications, Modbus TCP can enable communication between different components of the electrical grid, including power generation, transmission, and distribution systems. This facilitates real-time monitoring, control, and data analysis, helping to optimize grid performance, enhance reliability, and improve energy efficiency.
Hybrid Applications
In some cases, a hybrid approach that combines both Modbus RTU and Modbus TCP may be the most suitable solution. For example, in a large industrial facility, Modbus RTU can be used for local communication between devices within individual production lines or sections, while Modbus TCP can be utilized for higher-level communication between different sections or for remote access and monitoring.
This hybrid approach allows organizations to leverage the strengths of both protocols: the simplicity, cost-effectiveness, and deterministic nature of Modbus RTU for local device communication, and the networking capabilities, remote access, and integration potential of Modbus TCP for more extensive communication requirements.
Conclusion
In conclusion, Modbus RTU and Modbus TCP are both widely used communication protocols in industrial automation and control systems. While RTU is the most popular so far, the recognized benefits of TCP are narrowing this gap. Ultimately, the choice between these two protocols depends on various factors which we discussed above and has been summarized for you in the table below. By thoroughly evaluating these factors, system designers can determine the best protocol for their specific application and optimize system performance.
Factors | Modbus RTU | Modbus Ethernet TCP/IP |
Cable Type | RS 232, RS-422, RS-485; Designed for Serial Communication | Ethernet TCP/IP; Designed for communication over ethernet with TCP/IP protocols |
Max Speed | 115 Kbps; Often sufficient for small-scale industry usages where master-slave devices are nearby. | 100 Mbps; Suitable for applications requiring large data handling, faster data transmission, and real-time performance. |
Length | Upto 1200 meters | Upto 100 Meters |
Topology | Daisy Chain (supports upto 32 slaves without repeaters and 247 with repeaters) | Star, Tree, Daisy Chain, etc. (up to 247 servers) |
Master-Slave Communication | Single Master | Multiple Clients |
Security | Difficult as compared to TCP. Can be hacked after gaining physical access. | More vulnerable as compared to RTU. Can be hacked remotely by hackers. |
Integration | Integrable with existing RTU systems and legal systems. Suitable with low-level hardware requirements application, also with master-slave having close proximity between them. | Widely integrable due to the widespread Ethernet technology. Ideal for devices spread at far distances and at multiple locations, requiring remote monitoring and control. |
Cost | Low hardware and infrastructure costs. However, wiring cost increases with the number of devices and maintenance factors. | Cheaper cabling costs as easier to install, and with existing ethernet infrastructure setup. However, requires advanced equipment and troubleshooting, which are high on cost |
Application | Devices in close proximity, legacy systems, a wide range of field devices, oil and gas industries, etc. | Distributed systems at multiple locations, modern industrial automation requiring ethernet communication, SCADA systems for real-time monitoring, etc. |
Table 1. Comparison of Modbus RTU vs TCP
Frequently Asked Questions (FAQs)
Can Modbus RTU and Modbus TCP coexist in the same system?
Yes, Modbus RTU and Modbus TCP can coexist in the same system using gateways or protocol converters. These devices facilitate communication between devices using different protocols, enabling seamless integration of Modbus RTU and Modbus TCP within the same network.
Is it possible to convert an existing Modbus RTU system to Modbus TCP?
Converting an existing Modbus RTU system to Modbus TCP is possible, but it requires significant changes to the communication infrastructure and the devices used in the system. Ethernet switches and routers need to replace RS-485 transceivers, and devices may require firmware updates or replacement to support Modbus TCP.
How many devices can be connected to a Modbus RTU network?
A Modbus RTU network can support up to 32 devices without using repeaters, and 247 devices with repeaters, including one master device and up to 246 slave devices. However, practical limitations, such as cable length, signal degradation, and response time may limit the number of devices in a real-world implementation.
What is a wireless Modbus?
Wireless Modbus TCP is a great alternative for industries where the master and slave devices are situated at far distances. This saves the cable cost (especially with soaring metal costs), labor charges, maintenance costs, etc. However, the transmitters and receivers required for communication do not come for free. Based on the application, the wireless frequency and module are determined, and hence the cost.
Is Fieldbus the same as Modbus?
While both are used in industrial automation, they differ in architecture, features, and protocol specification. Fieldbus is more implemented in industrial control systems and employs a group of protocols (such as FOUNDATION Fieldbus, Profibus-PA, and DeviceNet), each for a specific purpose. They provide advanced features and support decentralized control, allowing multiple devices to communicate over a single bus. Modbus in comparison is known for its versatility, robustness, and simplicity that follows a Master-slave architecture (where a master initiates the communication).