Wireless Systems Design

WLAN Follows Ethernet Into Industrial Control

To simplify life, we prefer to think of wireless and wired-Ethernet local-area networks as being two versions of the same thing. But these networks are inherently different. For instance, the configuration of wireless is totally unlike that of Ethernet. When bridging existing industrial-control equipment to a LAN, there are significant differences in the installation and deployment of wireless-local-area-network (WLAN) bridges. There's no "plug" on wireless devices, so other means of location, identification, and configuration must be designed in. Here, standard Internet web interfaces allow convenient configuration and installation interfaces. They also can augment or replace custom man-machine-interface (MMI) programs.

Unlike Ethernet, which uses a hub/switch architecture, WLANs use access points (APs). Each AP has a Basic Service Set Identifier (BSSID) that allows identification to an individual AP. For 802.11b, up to 14 channels exist. A collection of APs, which are all tied to the same wired-network segment, share a Service Set Identifier (SSID). That SSID can serve as a common-readable description of the network access.

Say Bob's company has named its SSID "BOB." Each AP connected to Bob's network would have the "BOB" SSID. Each client device intended to connect to Bob's network also would be configured to "BOB." As a result, the client device would connect to the AP on Bob's network regardless of the AP channel. It could roam from AP to AP on Bob's network.

Some companies are beginning to provide WLAN-node modules that are complete wireless clients. An integration-ready WLAN node provides an industry-standard UART serial interface that's easily adaptable to RS-232, RS-422, or RS-485. Using this communication interface, companies can tie directly to an unused UART in their industrial equipment. To augment integration, a WLAN node can provide simple digital input/output capabilities. This combination enables the quick integration of wireless capabilities into existing equipment without extensive programming or redesign.

Without a port to attach to, however, it may be difficult to locate a wireless client in order to configure it. Ultimately, each client may have a fixed IP address. But these addresses are system dependent and may be unknown until installation time. During installation, using dynamic IP configuration tools like DHCP enables much quicker configuration. It also makes it much harder to locate the client. If special features are built into both the client and corresponding software on desktop computers, these features can enable the quick location and configuration of wireless client devices.

All of the necessary WLAN-node client-device configuration parameters are available through a standard web-browser interface. This aspect provides for a fairly sophisticated graphical user interface without additional programs on the desktop. WLAN-node client devices have additional capabilities that can help an engineer develop complex, dynamic web pages. These pages, in turn, can function as a simple MMI. Using the power of web-page graphic capabilities along with OEM-specific JavaScript or Java applets, a web page can interact with an industrial-control device. This device will be able to issue commands over the serial port and receive responses. It also will be capable of interpreting, formatting, and presenting the responses via a browser. This method provides a portable, yet sophisticated MMI that is independent of the desktop-computer operating system.

For a more sophisticated MMI, applications written on a desktop computer can directly query the WLAN-node client device using Telnet. It also can query the industrial-control device over the serial interface. This feature allows sophisticated interaction between the application and industrial-control device for complex interactions, logging, or time-based statistics. In addition, a Windows DLL helps application developers simplify the locating and configuring of the WLAN-node client. It also enables communication with the industrial-control device. This aspect frees up Visual C/C++, C#, and BASIC application developers. They can then concentrate on developing the application and not the WLAN node.

Clearly, WLANs can embed the remote control and monitoring of industrial equipment. Of course, Wi-Fi bridge devices are more complicated than Ethernet devices to deploy and operate. But intelligent and highly integrated wireless-LAN-node modules can simplify that deployment. WLANs also are easier to install and set up.

By simplifying the design process, a WLAN module helps engineers realize proof of concept without expensive prototyping. They also can use industry-standard hardware, software, and communication-protocol platforms. This modular approach speeds development. It also accelerates product introduction by using tested/validated off-the-shelf products.

By providing nonvolatile configurations, much of the complexity of WLAN clients can be managed when the system is integrated or deployed. In addition, automated discovery applications allow the quick identification of individual client devices. Web-based interfaces permit configuration and operation by a number of operating-system platforms. Customized applications can even be developed using a custom DLL. WLAN-node modules truly have arrived for industrial control. Now, industries can develop programs for the operation and statistics of industrial-control systems.

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