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Ethernet vs Modbus

Since its introduction, Ethernet technology continuously improved and it has matured to the point that the cost of deploying an Ethernet network solution is now quite affordable. Today, Ethernet is the communication standard for many systems, so it is no surprise that it’s now recognized as one of the standards for factory networking. 

Conversely, Modbus communication is by far the oldest and most widely used communication protocol in the field of process and factory automation, as well as in SCADA (Supervisory Control and Data Acquisition) systems. However, you may wonder what is the difference between Ethernet and Modbus? Are the two protocols related in any way? How do I know whether to use Ethernet or Modbus? This article will provide you with a detailed comparison between the two communication protocols. 

What is Ethernet? 

Ethernet is primarily defined as a communication protocol commonly used to create Local Area Networks (LANs), Metropolitan Area Networks (MANs), and Wide Area Networks (WANs). It was commercially introduced into the market in 1980 and first standardized as an IEEE 802.3 protocol in 1983. IEEE 802.3 working group refers to a set of standards that define the physical network layer and data link layer’s Media Access Control (MAC) of wired Ethernet. Essentially, the IEEE 802.3 protocol relates to physical media (Ethernet cables, switches, hubs, etc.) over which data is transmitted.

As mentioned, Ethernet networking technology is mainly used to create Local Area Network (LAN), thereby connecting multiple computers or other electronic devices such as scanners, printers, and so on. In a wired Ethernet network, data is transmitted and received through cables; which can be fiber-optic, twisted pair, or coaxial cables. This type of wired connectivity facilitates network communication between two or more different types of networks such as from fiber-optic to copper (coaxial or twisted pair) cables and vice versa.

Note you can create a wired Ethernet network using a variety of network topologies such as star, ring, bus, and Device Level Ring (DLR). In a wireless network, Ethernet communication is achieved through wireless network technology.

Types of Ethernet Communication Protocols 

In an OSI (Open System Interconnection) model, Ethernet is classified as a layer 2 protocol referred to as the data link protocol. The IEEE 802.3 standard defines the data link protocol, dividing it into two sublayers: the Media Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer.

Ethernet specifies how the data will look at the data link layer (Layer 2), including the header and trailer. Devices connected in an Ethernet network are identified by a MAC address, which is normally hard coded to the specific device and rarely changes. As a data link protocol, Ethernet can be used with other protocols for communication purposes. There are two common types of Ethernet communication protocols which include:

A) Ethernet TCP/IP

Ethernet is widely used with the TCP/IP protocol, which resides at Layer 3 and Layer 4. Layer 3 is the network layer whose protocol is the Internet Protocol (IP). The IP layer breaks apart the data in transmission into variable-length packets, referred to as “Ethernet Frames”, these frames can be up to 1,500 bytes long. Each Ethernet frame consists of a header with a source and destination address, and a trailer that contains the error correction data. The Internet Protocol is thus the standard protocol for routing packets across interconnected networks; hence the name Internet.

The most common transport protocol of the Internet is TCP/IP. Ethernet TCP/IP is a set of layered protocols which are used to communicate over Ethernet. They provide a reliable mechanism for data transmission between machines. Ethernet TCP/IP is mainly used in residential and commercial networks (primarily Local Area Networks). The two key protocols (but not only protocols) of Ethernet TCP/IP are:

  • Transmission Control Protocol (TCP): This is a connection-oriented protocol that manages the connection between a server and a client. It runs via Internet Protocol and its responsibility is to make sense of the data being transmitted. For example, through “ports” for different types of data. It also makes sure that the Ethernet packets being transmitted are handled in the right order, and so on.
  • Internet Protocol (IP): Also known as Inter-network, the IP protocol is responsible for moving a data packet from point A to point B. It simply transmits the data packets it is provided with by TCP, User Datagram Protocol (UDP), and other protocols within the TCP/IP suite. It does this by finding the optimum pathway between the source and destination of each data packet being transmitted, thus, its main responsibility is to do the routing. Generally, the IP protocol is used to transmit data between computers using Ethernet links. 

Note, Ethernet transmits data packets(frames) from one network node to the next and it can only guarantee that if a frame is received, then it’s intact. But if a frame goes missing, then it isn’t intact. However, in an Ethernet TCP/IP protocol, the TCP ensures that the entire data set is delivered while intact. 

B) EtherNet/IP

Following a wide and highly successful implementation of Ethernet for commercial applications, Industrial Ethernet has been introduced to support industrial infrastructure. Industrial Ethernet, particularly EtherNet/IP was developed by Rockwell Automation in the late 1990s and released for commercial use in early 2000. Currently, EtherNet/IP is managed by Open DeviceNet Vendors Association (ODVA) and is considered as the best-in-class network for Industrial Internet of Things (IIoT).

Rockwell Automation developed EtherNet/IP as a call to meet the standards required for manufacturing automation. The scope of Internet Protocol (IP) is specifically limited to providing only the functions required to deliver an internet datagram (package of bits) from a source to a destination over a system of interconnected networks. Unlike other host-to-host protocols, the Internet Protocol doesn’t have mechanisms that are useful for augmenting flow control, end-to-end data reliability, data sequencing, and other data services. It mainly capitalizes on the services of its supporting networks to provide specific types and qualities of service.

On the other hand, EtherNet/IP is a member of a set of network protocols that implement Common Industrial Protocol (CIP). CIP is an open application layer protocol that encompasses an extensive suite of messages and services for a variety of industrial automation applications such as control, synchronization, motion, safety, configuration, and information. It enables users to integrate enterprise-level Ethernet networks with any of the aforementioned manufacturing applications.

Lately, the CIP protocol has been improved to include CIP Security, a new feature that is tasked with providing a secure communication method between devices on a plant floor. Other CIP extensions are CIP Safety, CIP Sync, CIP Motion, and CIP Energy; each of these extensions performs a specific service within the CIP protocol.

Note, EtherNet/IP is an industrial Ethernet which should not be confused with standard Ethernet. Ethernet and Ethernet TCP/IP protocols directly provide efficient real-time communications. Whereas, EtherNet/IP provides such real-time communications in conjunction with a managed Ethernet switch-based infrastructure.

Moreover, while Ethernet works well with LANs and WANs, it’s not very efficient for industrial factories which require more network scope. But EtherNet/IP is able to provide real-time and efficient communications to maintain control of manufacturing automation. It also allows access to real-time data to meet quality and production standards. This is because while standard Ethernet is designed only for base-level use, EtherNet/IP is designed for multiple levels and can be applied to more heavy-duty environments, i.e., the industrial factories.

What is Modbus?

Modbus is an open, message-based communication protocol, designed for transmitting manufacturing data. It was published in 1979 by Modicon as a proprietary way of sending information to and from Modicon’s Programmable Logic Controllers (PLCs) over a serial connection. “Modbus” is a registered trademark of the Modicon brand, which is currently owned by Schneider Electric USA, Inc. But since it’s an open communication protocol, anyone can use Modbus freely or by license.

Simply put, the Modbus protocol can transmit information between electronic devices over serial lines (the original version) or via Ethernet. Modbus serial protocol (the original version) uses a master-slave architecture; where a single master can control the Modbus data transactions, with multiple slaves responding to the requests from the master to read from or write data to the slaves. Modbus communications over the Ethernet use client-server architecture.

Modbus communication protocols are widely used in factory and process automation. For example, for a system that measures humidity and temperature, Modbus provides a common language for devices connected on the same network to communicate the measured data to a PLC or supervisory computer. More specifically, Modbus is conventionally employed as a communication protocol of Supervisory Control and Data Acquisition (SCADA) systems. It is extensively used in SCADA systems for a number of its key benefits, which include:

  • Modbus is an open-source communication protocol. Hence, it can be included in a broader range of device types from any manufacturer or equipment vendor. This provides users with a much wider choice of systems or devices that can be utilized to meet specific applications. 
  • It’s master-slave/client-server messaging structure is very simple, making it less difficult to deploy. For instance, implementing a Modbus network might require just a few days, which can save months of work. Also, no intensive training is required to deploy such a network. 
  • It moves raw bits and words, with very few restrictions on data transmission 
  • It is a polled network protocol, in which a central master requests each device on the network for status information in a sequential loop. 

Types of Modbus Communication Protocols

There are several versions of Modbus communication protocols which exist for either serial connections or Ethernet networks. They include: 

  • Modbus RTU: A serial (RS-232 or RS-485 or RS-422) binary protocol which uses Master-Slave architecture. 
  • Modbus ASCII: A serial ASCII protocol categorized as Master-Slave type. 
  • Modbus TCP/IP: Classic Ethernet TCP/IP communication protocol with a speed rate of either 10 Mbps or 100 Mbps. 
  • JBUS: This protocol consists of a limited set of communication messages of the Modbus RTU protocol. 
  • Modbus PLUS: This is a deterministic token of Local Area Network (LAN), peer-to-peer protocol, with a speed rate of 1 Mbps. 

The two most common Modbus protocol options are: 

A) Modbus ASCII/ RTU 

The original version of Modbus specified two possible data transmission modes: Modbus RTU and Modbus ASCII.  Modbus ASCII modules make use of American Standard Code for Information Interchange (ASCII) characters to begin and end messages; which Modbus messages somewhat more readable. However, the ASCII characters are less efficient and also the LRC (Longitudinal Redundancy Check) error checking used by Modbus ASCII is less effective.  

Conversely, Modbus RTU mode is the most common implementation of the Modbus protocol; often used in automated production facilities and in other industrial applications. This mode uses binary coding and CRC (Cyclic Redundancy Check). Also, Modbus RTU makes use of time gaps (usually 3.5-character times) of silence for data framing.  

Both Modbus RTU and Modbus ASCII are designed to be used with serial devices that support RS-422, RS-485, and RS-232 serial protocols. Note, the two Modbus modes are incompatible so a device configured to use Modbus RTU mode cannot communicate with a device that’s configured for Modbus ASCII mode.  A key distinguishing characteristic of Modbus RTU is its use of binary coding and a very effective CRC error-checking. Modbus ASCII messages on the other hand require twice as many bytes to transmit the same data as a Modbus RTU message. 

B) Modbus/TCP

Also known as Modbus TCP/IP, Modbus/TCP uses a client-server architecture. It was created to allow Modbus RTU/ASCII modes to be transmitted over TCP/IP-based networks. Since its introduction in 1999, Modbus TCP/IP has been gaining ground as a new open specification that is moving Modbus into the 21st century. Modbus TCP/IP messaging is accomplished through the TCP/IP protocol which is acts as the transmission medium. So, Modbus/TCP embeds Modbus messages within TCP/IP frames.

As we already know, TCP/IP frames are used to exchange blocks of data between devices connected over an Ethernet network. The TCP protocol ensures that all the data packets being sent are received. Moreover, the TCP enables multiple requests to be queued or ‘pipelined’ in a buffer while waiting to be serviced. The IP protocol on the other hand ensures that the messages being sent are addressed and routed correctly.

With Modbus/TCP you can set up connections between nodes on a particular network, sending requests through the TCP protocol in a half-duplex fashion. Also, Modbus TCP/IP facilitates a large number of simultaneous connections, in that case, it’s the user’s choice of whether to re-use a lived connection or to reconnect a connection. In addition, Modbus TCP/IP modules have the capability of using identifiers (sequence numbers), though the earlier versions of Modbus/TCP devices did not use them.

Note, Modbus TCP/IP or Modbus TCP is an Ethernet-based protocol. So, if your industrial devices are connected to an Ethernet network, they can communicate with each other using Modbus TCP/IP protocol. Also, if you’re considering using both Modbus ASCII/RTU protocols (via serial connections) and Modbus TCP/IP (via Ethernet networks), then a communication gateway will be a necessity.

For example, the B+B SmartWorx line of Modbus serial servers enables smooth Modbus connectivity via serial and Ethernet networks. They achieve this by translating Modbus TCP/IP to Modbus ASCII/RTU protocols. This allows controllers like PLCs to communicate with input field devices such as meters, sensors, and other instruments.

What Differentiates Ethernet from Modbus?

  • In an Open Systems Interconnect (OSI) model, Ethernet defines two layers (Layer One and Two) of the International Organization for Standardization (ISO). The two Layers are the primary and physical connection layers. Layer Three hosts the Internet Protocol (IP). The IP protocol is used as the Internet addressing scheme through which one device is able to find another device. Above the IP layer is the Transport Layer, which includes two different methods of transmitting data over Ethernet- User Datagram Protocol (UDP) and Transmission Control Protocol (TCP). Modbus protocol resides at the Application Layer, which is the topmost layer. There are other protocols within the Application Layer that contribute significantly to the success of today’s Internet. They include HyperText Transfer Protocol (HTTP) for the transfer of Web pages and File Transfer Protocol (FTP) for transmission of large files. So, while Ethernet Protocol takes up Layer 1 and Layer 2, Modbus is hosted in Layer 5 of an OSI model. 
  • As previously stated, Ethernet is a physical networking protocol that is mainly used for commercial applications. However, even though Ethernet works well with Local Area Networks and Wide Area Networks, it’s not efficient for industrial applications. This is because standard Ethernet is designed for base-level usage only, and cannot be applied to heavy-duty manufacturing environments. However, Modbus is not a type of physical network connection, as with Ethernet. Since Modbus messages are transferred on the top most layer (on top of the physical layers) of an OSI model, it is possible to use Modbus on a variety of different types of networks. That explains why Modbus is often used in systems that collect and analyze data or in applications where multiple control and instrumentation devices transmit signals to a centralized controller or supervisory computer. Also, the non-physical layer property enables Modbus to be employed as an application layer protocol in industrial automation and SCADA (Supervisory Control and Data Acquisition) systems. 
  • Generally, Ethernet is much faster and a lot easier to troubleshoot than Modbus. However, speed is not a complete necessity for most Modbus devices. For example, all the temperature sensors, level sensors, and other transmitters in an industrial setting are not required to report data very fast. The key requirement is that data should be reported after every few seconds. Therefore, faster throughput is hardly an adequate reason to use Ethernet over Modbus RTU. Also, using Ethernet for all of your Modbus communications might be too fast.
  • Ethernet can only transmit information between electronic devices over a network connection. Whereas Modbus messages can be carried on two channels – over a network connection (via the Ethernet), or over serial communication. This flexibility of data transmission is part of the reason why Modbus has continued to have a lasting appeal in the industry. Modbus flexibility is further enhanced by the availability of several types of Modbus serial connections including RS-485, RS-422, and RS-232. Modbus RS-485 is half-duplex, and it indicates values in terms of voltage differences. Modbus RS-232 serial interface allows concurrent, and full-duplex flow of data.

For more information or to discuss which equipment might be best for your application, please visit our website here, or contact us at sales@pdfsupply.com or 1-919-535-3180. 

This entry was posted on January 10th, 2022 and is filed under Education, Technology, Uncategorized. Both comments and pings are currently closed.

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