{"id":1559,"date":"2009-12-12T02:37:16","date_gmt":"2009-12-12T02:37:16","guid":{"rendered":"http:\/\/instrumentacionycontrol.net\/?p=1559"},"modified":"2020-04-23T12:33:34","modified_gmt":"2020-04-23T17:33:34","slug":"determining-the-scan-interval-in-scada-systems","status":"publish","type":"post","link":"https:\/\/instrumentacionycontrol.net\/en\/determinando-el-intervalo-del-scan-en-sistemas-scada\/","title":{"rendered":"Determining the Scan Interval in SCADA Systems"},"content":{"rendered":"

One of the factors that determines the scan interval is the number of RTUs to be scanned. An estimate can easily be made during the SCADA system design stage.<\/p>\n\n\n\n

A second factor to consider is the amount of data to be sent in each conversation. Depending on the controls implemented in each RTU, the number of bits to be sent can be determined.<\/p>\n\n\n\n

For ON\/OFF signals used in solenoid valves, motors, pushbuttons, lamps, etc., one bit per device will be required. For pressure indicators, flow meters, modulating valve opening percentages, etc., a 16-bit word will be required. In some cases, two 16-bit words will be required when the resolution of the field equipment demands it.<\/p>\n\n\n\n

To simplify and obtain a safe factor, you can select the RTU with the highest number of control points and multiply by the number of RTUs to be installed in the system.<\/p>\n\n\n\n

Remember that a conversation is usually a two-way data transfer. It's important to include the time taken by the master station to communicate with each RTU. This can include both the time required for the master station to query the RTU for data and the time it takes to assign new commands to the RTU. At this point, it's also advisable to analyze which RTU has the highest number of conversation transactions and multiply that number by the total number of RTUs to obtain a reliable estimate of the number of transactions.<\/p>\n\n\n\n

The third factor is communication speed. This is the number of bits per second that can be transmitted over a communication medium. Depending on the communication protocol and the physical medium, the system's communication speed can vary. Using a telephone line, we can work in the range of 300 to 2,400 bps. With an industrial protocol, we could reach 19,200 bps. However, if we consider using a protocol like TCP\/IP, speeds of up to 10 Mbps can be achieved. This latter speed can be useful for systems where the economically feasible time is on the order of seconds.<\/p>\n\n\n\n

A fourth criterion is communication efficiency, which should be considered as the ratio of time spent transmitting the data of interest to the total time spent communicating. For example, part of the message might include the RTU address, which is not relevant data. Similarly, there are errors in the verification process and the algorithms used for correction. Sometimes, transmitting irrelevant data takes more time than transmitting the relevant data.<\/p>\n\n\n\n

Some data has been calculated for specific means of communication, for example, the efficiency for a dedicated telephone line is 70%, for a radio modem it is 40%, and for a dial-up telephone line it is less than 1%.<\/p>\n\n\n\n

Example:<\/strong><\/b><\/p>\n\n\n\n

Calculate the scan interval for the following SCADA system:<\/p>\n\n\n\n