Three web server architectures aid plant-floor, enterprise integration
Driven by manufacturing's desire to connect the factory floor with the rest of the enterprise, web-enabled PLCs and Ethernet are giving users new ways to view and control factory data using embedded web servers. When selecting a web-enabled automation device, users can choose from three distinct system architectures.
Driven by manufacturing’s desire to connect the factory floor with the rest of the enterprise, web-enabled PLCs and Ethernet are giving users new ways to view and control factory data using embedded web servers. When selecting a web-enabled automation device, users can choose from three distinct system architectures.
Each architecture’s performance capabilities depend on the location of the web server within the system hardware. One architecture places a web server in the CPU of the PLC; a second installs a server on a separate board in the PLC backplane; and a third places it on a computer outside the PLC.
External web server
When installed in a separate computer, a web server must rely on a data collection process, which typically uses a network, such as Profibus or DeviceNet, to communicate with factory automation devices. This architecture usually employs an OPC/DDE server as the PLC interface. One advantage of this method is that users only need to buy several software components, but they can store many web pages on the PC’s disk drives. Disadvantages include higher system costs and lower potential reliability due to additional components. Processing cycle time may be slower because two separate networks are being used.
A web server on a PLC’s backplane is typically placed on an Ethernet board, executing on the same CPU with the TCP/IP stacks. Several processors can perform this task. Some of the best choices range from a lower-cost 80186 to Motorola’s 860T for higher performance implementations. The Ethernet board requires at least 1 MB of flash RAM to store a few simple web pages, and many tens of megabytes for applications containing complex pages.
For example, Schneider Electric Automation’s (North Andover, Mass.) first Web-enabled PLC uses an Ethernet board that provides high performance and allows users to load complicated web pages. Its CPU is a 40-MHz 860T Ethernet CPU with 16 MB of bursting SDRAM and 8 MB of flash ROM. Memory access is optimized to limit wait states and memory access delays. It also incorporates 100-Mbps Ethernet, both twisted pair and fiber, into the module.
Schneider’s Web-enabled PLC also features an ultra-high-performance TCP/IP stack, which was developed to work in parallel with the standard stack for high-speed cyclic I/O scanning. To handle access to PLC data, the Web-enabled PLC includes a set of Java beans to collect data from the PLC and help create displays that improve flexibility and access. It allows users to create almost any HMI with a web browser, eliminating the need for expensive software packages.
Another advantage of backplane-based web server architecture is that its high-speed access to the PLC across the backplane doesn’t impact PLC performance. The separate board provides space on board memory supporting several complex web pages.
Server on CPU
The third architecture puts a web server directly on the PLC’s CPU. This solution is less costly because it eliminates the need for a separate board and CPU. However, it requires significant innovation to maintain determinism and proper PLC performance, while also managing TCP/IP stacks and the web server.
Schneider uses this approach in a second Web-enabled PLC with a board designed for fast memory timing, so the CPU isn’t delayed by waiting for memory access. This PLC places an Ethernet stack and HTTP server in a PLC without impacting determinism of the logic solving function. Memory is limited to 512 K SRAM and 256 K flash RAM due to the chip’s addressing limitations. The board’s size is limited by the PLC’s form factor, which requires the board to fit in a 4.75 x 2-in. space.
Today, users are implementing applications with varying size, complexity, and performance requirements, as well as related web pages. Fortunately, these three web server architectures can help implement a solution that best meets their needs.
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Richard Baker led the Schneider Electric Automation (North Andover, Mass.) design engineering team that developed its Web-enabled PLC.
Web server architectures
PC with web browser
Web server on personal computer
Data collection task (OPC/DDE)
PC with web browser
Ethernet board with web server
PC with web browser
PLC with Ethernet and web server running on PLC CPU
Source: Control Engineering with data from Schneider Electric Automation