348 Practical Modern SCADA Protocols: DNP3, 60870.5 andRelatedSystems Sending a UDP datagram involves very little overhead in that there are no synchronization parameters, no priority options, no sequence numbers, no timers, and no retransmission of packets. The header is small, the protocol is streamlined functionally. The only major drawback is that delivery is not guaranteed. UDP is therefore used for communications that involve broadcasts, for general network announcements, or for real-time data. The UDP header The UDP header is significantly smaller than the TCP header and only contains four fields. Figure 12.20 UDP header format UDP source port (16 bits) is an optional field. When meaningful, it indicates the port of the sending process, and may be assumed to be the port to which a reply should be addressed in the absence of any other information. If not used, a value of zero is inserted. UDP destination port has the same meaning as for the TCP header, and indicates the process on the destination host to which the data is to be sent. UDP message length is the length in bytes of the datagram including the header and the data. The 16-bit UDP checksum is used for validation purposes.
13 Fieldbus andSCADA communications systems Objectives When you have completed study of this chapter you should be able to: 13.1 Introduction • List the main features of Profibus DP, PA and FMS • List the features of Foundations • Give 4 main differences betweeen Foundation Fieldbus and Profibus The two main Fieldbus standards that may impact on SCADA systems are discussed below. These are: 13.2 Profibus • Profibus • Foundation Fieldbus 13.2.1 Introduction to Profibus Profibus is an open standard fieldbus defined by the German DIN 19245 Parts 1 and 2. It is based on a token bus/floating master system. There are three different types of Profibus – FMS, DP and PA. The fieldbus message specification (FMS) is used for general data acquisition systems. DP is used when fast communications is needed. PA is used in areas when intrinsically safe devices and intrinsically safe communications are needed.