PLCs (hardware programmable logic controllers) are an industry standard for machine builders. But just like technology, PLCs and machine automation is evolving at a high rate. This has triggered OEMs (Original Equipment Manufacturers) to embrace software-based industrial PLCs or soft PLCs. This is because, soft PLCs offer a wide range of benefits like; precision performance, improved productivity, and the ability to future proof infrastructure using a platform-based approach especially for real-time processes like machine vision and motion control. Though you are obliged to use a vendor’s preferred network protocol when using their hardware PLC, the need to maximize performance still suffices. Meaning that, choosing the right PLC is all about choosing the ideal network protocol that will serve you in the long run as the technological pace and your business demands change. But, since there are different Industrial Ethernet Fieldbus protocol standards in the market, it is hard to know the ideal protocol for your business yet choosing the wrong standard means slower performance and unnecessarily high costs. Here is a definition of five main Ethernet-based protocols to help you determine the efficacy and predict their long term viability.

Key features of five real-time industrial Ethernet-based protocols

EtherCAT

EtherCAT system design is ideal for centralized architectures and simple Fieldbus devices under a master/slave configuration. All devices are networked using a ring formation with a bus master. At every cycle, crucial output data is extracted by Ethernet data packet devices that are sent by a bus master. EtherCAT utilizes the Ethernet telegram structure but utilizes a different operation mode where a single Ethernet telegram runs via all slaves/ stations. The Ethernet telegram data section is divided into general and real-time data sections. Under the real-time section, the process data and headers are defined in sub-telegrams in a bid to boost user data rates in the protocol. Since EtherCAT slaves can only interpret EtherCAT frames, general data is tunneled into EtherCAT frames and guided through slaves. Where the general data is too large, it is distributed across a number of EtherCAT frames. EtherCAT allows the implementation of extremely short cycle durations.

EtherNet/IP

Being a TCP/IP application layer protocol, EtherNet/IP utilizes standard Ethernet data link, transport layers, network, and physical while using CIP (Common Industrial Protocol) over TCP/IP. CIP offers common services and messages set for industrial automation control systems and can be utilized in multiple physical media. Note that, Ethernet/IP is the only real-time method that applies Ethernet standards entirely. Moreover, it is time-based and does not require the control commands to get to field stations within a predetermined time. As such, system performance can be executed independently from the network performance. EtherNet/IP is an application layer protocol that can deliver in real-time yet it features a limited performance range.

Ethernet Powerlink

Ethernet Powerlink is a cyclical protocol that facilitates access to the synchronization of devices and networks. It features a communication cycle divided into the asynchronous phase for ad-hoc data transfer and isochronous phase for time-critical data transfers. As such, it is ideal for both remote and local control designs. Ethernet Powerlink focuses on finding the ideal balance between application area-specific demands and common automation demands. However, it is not broadly adopted by servo drive vendors mainly because servo prices remain high.

Profinet IRT

Profinet IRT utilizes consumer/producer principles and resorts to different services and protocols in order to cover a wide range of performance classes. As such, high priority data is sent through the Ethernet protocol in Ethernet frames and VLAN prioritization while configuration data and diagnostics are sent using UDP/IP. This allows the system to hit cycle times of about 10ms among I/O applications. As an industrial Ethernet, Profinet IRT is widely used by GE and Siemens where it is embedded in their equipment and controllers.

SERCOS III

SERCOS III utilizes Ethernet telegram and Ethernet physics while retaining its SERCOS mechanisms. It is based on a time slot mechanism where bandwidth is reserved for real-time channels (isochronous) and IP channels (asynchronous) data traffic. It operates without switches or hubs. Every station features a special integrated FPGA or ASIC and two communication ports that enable connection via the ring or line topology. Shorter cycle times can be achieved by eliminating the switches but this would affect the network topology flexibility. It further utilizes the summation frame method where the network nodes are deployed in a closed ring or daisy chain.