How to connect legacy factory automation systems to Industry 4.0 without disruption
the Siemens SIPLUS unit which features an RS-485 serial interface (Figure 1). In the early days of factory automation, manufacturers linked their PLCs to a central supervisory system using RS-232. This was a wired, serial data link with a maximum throughput of a few hundred kilobits per second (Kbits/s) at best. It used ground voltage to represent digital ‘0’ and ±3 to 15 volts to represent digital ‘1’. Later, RS-422 and RS-485 took wired communications to a more advanced level using differential signaling over a twisted pair cable. The systems allowed one controller to supervise up to 32 PLCs and offered a data rate of up to 10 megabits per second (Mbits/s) over a distance of up to 1,200 meters (m). It is important to note that RS- 232 and RS-485 are standards that specify the physical layer (PHY); they do not specify the communication protocol. In the industrial automation sector, several protocols have been developed to run on the RS-232 or RS-485 PHY. Examples include Modbus Remote Terminal Unit (RTU), Modbus American Standard Code for Information Interchange (ASCII), DF1-Common Industrial Protocol (CIP), DF1-Programmable Controller Communication Commands (PCCC), Point-to-Point Interface protocol (PPI), DirectNET, Coprocessing Communication Module (CCM), and HostLink. The
Figure 3: The 7940124933 Industrial Gateway Communication Device bridges the gap between Industrial Ethernet and up to four RS-232/RS-485 serial networks. The 7940124932 version supports two serial ports. Image source: Weidmüller For more, see ‘Design for Rugged IoT Applications Using Industrial Ethernet-Based Power and Data Networks’. A gateway to Industry 4.0 Updating legacy RS-232 and RS- 485 factory automation systems to Industrial Ethernet is daunting for designers. There could be thousands of PLCs in a large factory and tens of kilometers of wiring. The cost and disruption caused by ripping out old systems for new replacements is not viable for many companies. Yet, without upgrading, a production facility will not be able to take advantage of the productivity gains promised by Industrial Ethernet.
One strategy to limit cost and disruption is to commit to an Industrial Ethernet backbone while retaining legacy serial buses, PLCs, and machines. Then, when machines are replaced or when new machines are added to the factory, they can be specified such that they’re interoperable with the Ethernet backbone. This allows the factory to be gradually updated to the latest communications standards without production interruptions or major cashflow issues. However, such a strategy creates a discontinuity between the RS-232/ RS-485 and Industrial Ethernet networks. This discontinuity can be bridged with an Industrial Gateway Communication Device such as the 7940124932 or the 7940124933 (Figure 3) from Weidmüller. Each gateway is a single solution providing a cost-effective way to move data between PLCs and peripheral devices, using different
Figure 2: Shown is the Industrial Ethernet software stack. Industrial Ethernet protocols such as PROFINET operate in the application layer. Image source: PROFINET
and synchronized production lines. Such an environment requires a deterministic protocol to ensure machine instructions arrive on time, every time, no matter how high the network load. To overcome this challenge, Industrial Ethernet hardware is complemented by customized software. There are several proven Industrial Ethernet protocols available, including Ethernet/ IP, ModbusTCP, and PROFINET. Each is designed to ensure a high level of determinism for industrial automation applications. Standard Ethernet comprises the PHY, data link, network, and transport layers (which use either TCP/IP or UDP/IP as the transport) and can be viewed as a communication mechanism that brings efficiency, speed, and versatility. In contrast, Industrial Ethernet protocols, for example, PROFINET, use the application layer of the Industrial Ethernet stack (Figure 2).
routing and transport, ensuring cloud interoperability, a capability that is well beyond RS-232 and RS- 485 technology. ‘Industrial Ethernet’ describes Ethernet systems adapted for factory use. Such systems are characterized by rugged hardware and industrial standard software. Industrial Ethernet is a proven and mature technology for factory automation that allows a remote supervisor to easily access drives, PLCs, and I/O devices on the manufacturing floor. The infrastructure typically uses line or ring topologies because these help to shorten cable runs (mitigating the impact of electromagnetic interference (EMI)), reduce latency, and build in a degree of redundancy. Standard Ethernet’s communication mechanism is prone to disruption and lost packets, which increase latency and make it unsuitable for the near real-time demands of fast-moving
protocols have been developed and supported by many PLC makers. PLCs proved to be a robust, reliable, and flexible way to bring automation to the shop floor, and RS-485 and its associated industrial protocols offered an inexpensive and simple-to-install network technology. Today, PLCs are typically used to control entire assembly lines, and most industrial automation uses some type of PLC. Many thousands of factory automation installations are based on venerable RS-232 and RS-485 networks. Ethernet enters the factory However, since the turn of the century, Ethernet has provided the most accessible and proven solution for a modern factory network. It is the most widely used wired networking option with extensive vendor support. Ethernet typically uses TCP/IP (part of the Internet Protocol (IP) suite) for
Figure 4: The industrial gateway setup involves connecting the device to an Ethernet switch and a power supply, then connecting a PC to the switch and configuring the gateway via a browser. Image source: Weidmüller
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