Web Servers for Allen-Bradley PLCs – Part 1

If you’re like the typical factory floor engineer, you’re probably not very familiar with web server technology though you use it every day. It’s hard to imagine a modern website or software application that wouldn’t use it.

Web Server Technology and The Internet

Web server technology in the IT world is the glue that holds much of the internet together. Simplicity is the key to it. All web servers, the websites you use every day, use a very simple, two command protocol[1] called HyperText Transfer Protocol (HTTP). It’s used whenever you type a URL into a browser or click on a link.

The idea behind HTTP is that a client device forms a request for some data and sends that request to a server (a web server) using the IP Address or the name of the website – www.rtautomation.com for example. On the internet, your browser is a web client forming requests that the computer hosting the website answers. Every time you click a link, another file is transferred from the webserver to your browser. In internet communications, that file is in HyperText Markup Language (HTML). HTML contains both the source content and the formatting commands that say where each part of the web page should be placed on the screen and how it should be formatted.

HTTP powers this. There are two primary commands that HTTP uses to move this content: the GET and the PUT command. The GET command moves data from the webserver to your browser web client. A command to get more information on a particular ginzo knife might look like “GET www.ginzo.com/knife1209” The PUT command moves data from your browser client to a web server. If you are using a paint coverage calculator, there might be a PUT command like “PUT www.paintcalc.com/?amt=7000/?months=36.” The PUT command sends that data to the webserver where it can make a calculation. The web server might send more HTML content back to be displayed as a result of that input.

HTTP is simply the process of moving these commands back and forth between your browser web client and the webserver on the computer hosting the website you are on.

Web Server Technology and Programmable Controllers

It is a whole different ballgame when you want to use web server technology for industrial devices, and especially PLCs. These devices are not off-the-shelf computers hosting websites. They primarily communicate using Modbus TCP, EtherNet/IP, and PROFINET IO. How does this technology apply to them?

First, why would we want to use web servers with industrial devices? The answer to that is, once we get out of the control system, the rest of the world is using web servers. There are many tools, programming languages and applications that have the ability to use web servers. Having an industrial device that supports web server technology makes its data easily available to lots of different devices.

How many devices want data in an A-B PLC? A massive number – and with web server technology, that is possible. That’s what I will talk about in part two of this article.

PS – If you want to see a device that can turn industrial PLCs (A-B PLCs) into web servers, take a look at the Real Time Automation gateway 460ETCWI.

[1] There are more than two commands, but GET and PUT comprise the majority of operations. The rest are not pertinent to this overview discussion.