Figure 1: Equipment connects to the Internet through a local interface unit.
The server
The Web server/data service device—a “thin server”—provides the connection between the machine or process and the Internet. A connection to an existing PLC or proprietary controller is required to extract data or enable control over the equipment. Most PLCs, including products from AB, Schneider/Modicon and Siemens, support at least a serial connection using communication protocols available from the PLC vendor or third-party suppliers. In many cases, the communication driver is available from the thin server vendor as part of the embedded software application.
Equipment that uses a standard PLC protocol or supports another open standard protocol, such as Modbus RTU or TCP, greatly simplifies the job of connecting the thin server. However, this is not always the case.
If the equipment uses a non-standard PLC or a proprietary controller, a connection to the thin server device is still possible, as long as the controller supports some kind of serial protocol and the vendor can provide protocol documentation. Then, either the thin server vendor or a third-party developer will be involved for driver implementation.
In the extreme case, the monitored equipment may have a controller with no external communication port, or the protocol may be unavailable for some reason. If so, it may be necessary to instrument the equipment or process externally using additional sensors and appropriate I/O to monitor strategic data or equipment control points. Choose those that support a standard communication protocol (e.g. Modbus or an open ASCII protocol) and can be connected to the thin server. Alternatively, an I/O device may be selected that includes an integral thin server function. If the I/O device includes an integrated server, the separate thin server may not be required.
Linking
After installing a thin server and proper driver in the equipment enclosure, the next step requires configuring the thin server to link the equipment communication data and control (inputs and outputs) to the network. Details depend on the features and configuration software provided with the thin server. For example, a gateway server may do nothing more than map PLC registers to network variables or a remote PC connection. Sophisticated thin servers may allow configuration of a Web page to view equipment data, enable alarm monitoring and data trending, add standard connectivity interfaces, such as OPC, and may even support paging a response team when programmable equipment fault conditions arise. Make sure that the selected thin server allows for future expansion.
The Internet or Intranet (LAN) connection to the thin server may include a standard hardwired Ethernet line, a modem/phone line (dialup) or a wireless connection (802.11b Ethernet, for example). The type of thin server and existing plant network infrastructure—or lack of network infrastructure—usually determines the connection.
If a hardwired LAN already exists near the equipment, a network connection is relatively simple with the addition of a cable drop. If complex cable runs or long distances make a hardwired connection difficult, a wireless connection may be the best alternative.
However, when selecting wireless connections, be sure to evaluate the proposed installation site carefully and consider factors, such as range and signal integrity. Look for a wireless thin server that supports a wireless LAN interface to simplify installation.
