License Free Radio Modem Communications With Siemens S7-200 PLC Remotes
The following application note reports on the very first installation worldwide of any license-free radio modems for data communications with the Siemens S7-200 architecture. It appeared as the front page article written by Cascade Control in the January 1996 issue of "InControl," a quarterly newsletter published by Cascade Controls & Electric Supply, the Siemens Distributor based in Seattle, Washington.
TOSCO´s refinery in Ferndale, Washington, was faced with a challenge. The pump and valve system used to direct the transport of various petroleum products throughout their tank farm was deteriorating. This control system, originally installed in the 1950s, consisted of individual push buttons and pilot lights for each of the many pumps and valves. Due to the harsh environment and heavy use over the years, many of the junction boxes and field connections to the pumps and valves had deteriorated to the point of making the system almost inoperable.
Evaluation of the existing system indicated that virtually all field wiring and related components required replacement. It sounds simple, but the fact that these tanks, pumps and valves are spread throughout a tank farm that covers many acres, made the replacement cost very high. in addition, any new control system would be required to interface directly with TOSCO´s sophisticated DCS (distributed control system) This would require all the pumps and valves to be hard wired into expensive DCS inputs and outputs. The estimated labor and material costs quickly exceeded $130,000.
TOSCO´s Project Manager contacted Cascade Controls to determine if any other solutions existed. He had heard about wireless communication to remote I/O from Cascade Controls and wanted more information.
After reviewing the application, Cascade Controls suggested controlling the system with a combination of PLCs and radio modems, thus reducing the highest cost item- field wiring. The main controller would be a Siemens Simatic TI545, utilizing the Field Interface Module, communicating to the tank farm via the Data-Linc SRM frequency hopping spread spectrum 900MHz serial radio modems (no site license required). At each tank cluster, a Remote radio modem was connected to a Simatic S7-200, the new mini-PLC from Siemens, providing direct connection to the pumps and valves in the field. Another cost savings was in the DCS connection to the system via the Simatic TI545 PLC. A Modbus server in the PLC provided the needed interface so the DCS could control and view the status of the remote I/O and thus, the field devices.
The use of Spread Spectrum Radio Technology and the Intelligent I/O capabilities of the Simatic S7-200 and TI454 resulted in a large capital savings on the project. The approximate cost of the PLC system was less than $40,000!
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