PLC COMMUNICATIONS HAS EVOLVED as automation has advanced over the years, with hardware requirements and PLC vendors pushing these communication standards to new levels of performance. The present and future is looking bright for PLC communication standards as we look at what the future holds.

One example is EtherNet/IP, as it moves past its 15th birthday, which through the years has expanded into a reliable and popular industrial Ethernet communication network connecting machines, processes and factories. Some keys to its success are the network and transport protocol layers where explicit and implicit messaging is used. EtherNet/IP′s features and terminology will be discussed to help with application of its networks in industrial automation.

Explicit versus Implicit Messaging

PLCs Make the Connection

PLCs must communicate in real-time to a wide variety of devices such as local and remote I/O, motors, drives, servo controllers or other PLCs. An interface to a PC-based HMI or an operator interface terminal (OIT), and possibly the Internet may also be required. There may also be communication to upper level server PCs running various quality, manufacturing and enterprise applications.

All of these communications require two things: a physical connection or layer, and a shared protocol. The physical layer defines the electrical, wiring and connection requirements. The shared protocol is the common language allowing each device to understand what the bits and bytes in the communication messages mean.

Through much of the PLC′s early years, it was common for PLC vendors to use proprietary communications. This provided a robust and well defined communication to the supplier′s family of products, but made connections to other suppliers′ software and hardware difficult.

Proprietary communications

Many of the older, proprietary communication protocols are still in use due to easy connection among a single supplier′s products. And even on newer smaller systems with no plans for expansion or connection to other systems, the proprietary communication option is still a viable approach. These older connections using RS-232C, RS-422 and RS-485 and related protocols are still supported by many suppliers. For example, some PLCs use a serial RS-232C or RS-422 physical layer network such as the DirectNET protocol for communications. This protocol can be used for PLC-to-PLC communication over a point-to-point or multipoint network using standard PLC instructions, or to talk to HMIs. If the control system will include multiple suppliers or communications to other computing systems, standard communication protocols become more important. These standards start at a physical layer.

The physical layer is often confused with the physical medium such as the cable, connectors, network interface cards and wireless transmission hardware. But, the physical layer instead defines the interface requirements necessary for proper connection to the physical medium.

The physical layer defines how to connect the upper data link layer in the Open Systems Interconnection (OSI) communications model within a computer to physical devices. It defines the hardware requirements, schematics and specifications for successful bit-level communication to different devices. The physical layer defines items such as bit rates; transmission electrical, light or radio signals; flow control in asynchronous serial communication; cable types; and the mechanical design of connectors.

With the physical layer mechanical and electrical requirements defined, a variety of different protocols can reside in the physical layer. Ethernet, USB and Bluetooth are the most common physical layer protocols specified in new products and applications, and represent the future direction of PLC communications.

But common physical layer protocols also include:

• Proprietary protocol for a RFID reader using an RS-232 point-to-point connection
• VFD protocol connecting multiple devices on an RS-422/485 multi-drop connection
• Printer protocol over a parallel interface
• Ethernet for connection to a plant or control network
• USB for connection to keyboard
• Bluetooth for connection to a wireless microphone

Protocols travel the physical layer

A protocol defines a set of rules for communication among networked devices on a physical layer. Some common protocols used in the industrial arena include Modbus RTU, EtherNet/IP, Ethernet TCP/IP, Modbus TCP/IP, Profibus DP and Profinet.

One of the more common industrial serial communication protocols is Modbus RTU, developed by Modicon and usually running on an RS-485 network. This is just one of many popular serial protocols, supported by suppliers and used by a wide range of automation professionals. However, performance limitations make serial protocols a poor choice for high-speed and high-performance applications.

Ethernet has become the dominant standard for the physical layer of many industrial protocols such as EtherNet/IP, Ethernet TCP/ IP, Modbus TCP/IP and Profinet due to its performance and other advantages. And unlike serial protocols, multiple Ethernet protocols can run on the same Ethernet physical layer.

It has become common to use Ethernet to interconnect several devices such as PLCs, HMIs, field I/O and valve banks. Plus, the communication remains fast while talking to several dissimilar devices on the same cable, due to the very high speed of Ethernet as compared to older serial networks.

Today′s physical, wired layer is moving to Ethernet for most control system communications with EtherNet/IP becoming a very popular industrial protocol. But with this protocol, explicit and implicit messaging should be understood to optimize network operation.

EtherNet/IP messaging modes

To help optimize a real-time EtherNet/IP network, it′s important to select a controller supporting explicit messaging as a client or server, and implicit (real I/O) messaging as a scanner or adapter. With explicit messaging, the controller is called the client and the field devices are called servers. With implicit messaging, the controller is called the I/O Scanner and the field devices are called I/O Adapters.

While a controller generally supports both explicit and implicit modes as a client, server, scanner or adapter, the choice often depends on the field device itself. Many times, field devices only support one messaging mode.

Explicit messaging is often the better choice if the application requires large amounts of data. In this case, explicit messaging can save bandwidth as data is only transmitted when needed or requested.

Implicit messing is typically the best choice for real-time, high speed applications. Think of the “i” in implicit as the “I” in I/O messaging, which of course requires high speed as I/O is generally used for real-time control.

Explicit messaging requires programming in the controller for setup as the data must be explicitly requested. Programming is required to request the data, provide handshaking, acknowledge the data, and move the data where it′s needed in the controller.

Real-time implicit messaging, by comparison, is quickly configured with little or no programming. The controller is configured as a scanner to send and receive data, and to connect to a remote EtherNet/IP device, all by filling in the blanks in the controller programming software. The configuration defines what the data is and where it will be in the controller data table. Once configured and with the controller up and running, the data appears in controller memory without handshaking or data handling.

Controller outputs are sent to field devices, local bits are set, and integers and floating point words are written as the data is automatically sent to the device based on program scan. Data is transferred at the specified rate, typically in the 5 to 20ms range.

This diagram highlights some of EtherNet/IP′s explicit and implicit message modes, terminology and usage. Explicit messaging handles requests and definition of information, while implicit messaging is data only, and no protocol information is included (also known as I/O Messaging).

Explicit messaging applications

With the EtherNet/IP protocol, the explicit message connection is a client/server relationship. A client such as a PLC requests data from a server and the VFD, for example, sends the information back to the controller.

Since the request for data issued by the client uses TCP/IP services, the server has the information necessary to explicitly respond to the message. The client/controller requests the data and specifies how it is formatted, and Explicit versus Implicit Messaging the server/VFD provides the data, formatted as specified.

Typically, explicit messaging is used in applications that are not time critical. The monitoring and configuring ability common to explicit messaging works well in applications where the client can send data requests or configuration parameters anytime, and the server can respond when available.

Implicit messaging applications

With the EtherNet/IP protocol, implicit messaging is commonly used for time-critical control applications. Implicit messaging is sometimes called I/O messaging since a common application is communication between a controller and remote I/O. Because the I/O scanner and I/O adapter are pre-configured to implicitly know the data format and communication requirements, implicit messaging is significantly more efficient when compared to explicit messaging.

Since the data has been pre-defined, real-time implicit messaging basically just copies the data with little overhead information required in the message. Neither end of the message communication link needs to be told what the data is as the meaning of the data is implicit or implied. Both the controller and field device know what each bit and byte mean.

Developed to assist creation of detailed interface standards, the Open Systems Interconnection (OSI) standard is a common reference model showing how applications can communicate over a network.

Connected messaging

Other terms often used with respect to Ethernet/IP messaging are unconnected and connected. Explicit messages usually are unconnected messages. Implicit messages are typically connected messages configured in advance for real-time I/O messaging. These connected messages also use features built into each device to enable this high speed connection.

With the popularity of Ethernet communication in industrial applications, continued improvements and additional standardization should be expected in the future. For today, in simple applications, a proprietary protocol may be the best solution. However, when the application requires high speed, extensive connectivity and scalability, an Ethernet protocol is likely the best solution.

Rick Folea is Senior Technical Marketing Specialist at Automation Direct.