Practical TCP/IP and Ethernet Networking- P52: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter.: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more. | Process automation 237 architectures for Internet and ATM and is easy to troubleshoot with tools such as TELNET. Considering all the advantages of TCP IP the overhead of 40 bytes per packet is a small price to pay. Application layer data structures An ideal solution for the implementation of the application layer is the MODBUS a vendor neutral data representation protocol. MODBUS is referred to as every vendor s second choice but every integrator s first choice . Reasons for this include the fact that the specification is open and published and that the minimal implementation involves only two messages. It is easy to adapt existing serial interface software for MODBUS and also very easy to perform automatic protocol translation. Although most implementations of the MODBUS protocol are used on low-speed point-to-point serial links or twisted pair multidrop networks it has been adapted successfully for radio microwave public switch telephone infrared and almost any other communication mechanism conceivable. Despite the simplicity of MODBUS it performs its intended function extremely well. Embedded web servers Once all computers and control devices are connected via a seamless Internet-compatible network it becomes possible to use web servers to make plant information available to operators. This can be done in two different ways. Firstly a control device can incorporate its own local web server. This means that information accessible only to that device can be reported in a legible form as a set of web pages and therefore displayed on any computer on the intranet extranet or Internet by means of a web browser. Alternatively a general-purpose computer can act as a web server and gather data for individual web page requests by generating the native MODBUS requests used by the control devices. These requests can then be sent out over the MODBUS TCP network and will interrogate either the control devices directly if they have TCP IP interfaces or via simple
