Understanding Smart Sensors - Nomads.usp

132 Understanding Smart Sensorscentral processing unit (CPU). 1 Events occurring at a particular device are communicateddirectly between modules. LonWorks control network technologyhas been selected as a standard by Semiconductor Equipment Materials International(SEMI). SEMI standard E-61 specifies LonWorks as a sensor bus forconnecting simple as well as complex sensors and other equipment in semiconductormanufacturing.6.5.3 Other Industrial ProtocolsOther industrial protocols listed in Table 6.2 offer advantages to distributedcontrol systems. A brief summary of the most widely known, frequently mentioned,and better-documented protocols will conclude this section. Theseinclude the highway addressable remote transducer (HART), fieldbus, processfieldbus (Profibus), SERCOS, Topaz, and ARCNet protocols.Rosemount developed the HART protocol in the late 1980s. It has adedicated users group (HART Communication Foundation) with several participatingcompanies. The open multiple master protocol is compatible withthe 4- to 20-mA current loop and also allows digital communication [12].The recent combination of the Interoperable Systems Project Foundation(ISPF) and WorldFIP North America into the Fieldbus Foundation has thepotential to complete a fieldbus specification that has been in development forover 10 years. Fieldbus defines the user layer in the OSI model, making it moreextensive than protocols that address only the physical, data link, and/or applicationlayers (e.g., CAN) [13].Profibus is a German Deutsches Industrial Norm (DIN) standard thatsupports a data rate up to 1.5 Mbps. The deterministic system uses a masterslavehierarchy and has a capacity of 127 nodes. Arbitration is handled by commandresponse. EIA-485 is used as the physical layer [13]. A serial, real-timecommunication system (SERCOS) network typically consists of eight nodesper ring. It operates at a speed of up to 1 Mbps and for a distance of up to 40mat that speed. It is a deterministic multimaster network with commandresponse and broadcast for arbitration. The physical layer is fiber-optic [12].Topaz can handle up to 255 nodes and operate at a speed of 1 MHz. At 1km, the speed decreases to 500 Kbps. The multimaster system is deterministicand uses token passing for arbitration. The physical layer uses EIA-485 interface[12].ARCNet (Attached Resource Computer Network), a trademark ofDatapoint Corp., is a deterministic token-passing protocol [14]. In an1. LonWorks is a trademark of Echelon Corporation.