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我們新研制出一種用于紙漿白度測定的智能白度儀，該儀表采用西門子公司生產的S7-224 CPU結合軟件功能進行數據處理，具有較好的可靠性、重復性、精確度和穩(wěn)定性。為了使其能與其它智能設備之間實現通信，我們將通過現場總線技術把它連接到網絡中，使它成為造紙廠現場總線控制系統(tǒng)的一部分。

2.現場總線的選擇

  2.1現場總線技術的選擇

  現場總線集控制技術、計算機技術、通信技術于一體，是當今自動化領域技術發(fā)展的熱點之一。這項技術可將多個智能控制儀表連接成網絡系統(tǒng)，實現分布式計算機控制，提高檢測和控制精度，改善系統(tǒng)的動態(tài)響應速度，提高系統(tǒng)安全性，從而得到了廣泛的應用。

  近年來，幾種有影響的現場總線技術已在一些特定的應用領域里顯示出各自的優(yōu)勢。在此，我們選定了現為德國和歐洲標準的PROFIBUS（Process Fieldbus）現場總線。一方面由于它是目前數據傳輸率快的一種現場總線，傳輸率高可達12Mbps，另一方面它能夠很好的與西門子的傳動設備相配合，并與西門子PLC（Programmable Logic Controller）結合緊密且組態(tài)簡便。

  2.2 PROFIBUS現場總線簡介

  參照ISO/OSI參考模型,PROFIBUS現場總線只包含物理層、數據鏈路層和應用層。由PROFIBUS-DP，PROFIBUS-FMS， PROFIBUS-PA組成PROFIBUS系列。

  PROFIBUS-DP：經過優(yōu)化的高速、廉價的通信連接，專為自動控制系統(tǒng)和設備級分散I/O之間通信設計，使用PROFIBUS-DP模塊可取代價格昂貴的24V或0～20mA并行信號線。用于分布式控制系統(tǒng)的高速數據傳輸。

  PROFIBUS-FMS ：解決車間級通用性通信任務，提供大量的通信服務，完成中等傳輸速度的循環(huán)和非循環(huán)通信任務，用于紡織工業(yè)、樓宇自動化、電氣傳動、傳感器和執(zhí)行器、可編程序控制器、低壓開關設備等一般自動化控制。

  PROFIBUS-PA專為過程自動化設計，標準的本質安全的傳輸技術，實現了IEC1158-2中規(guī)定的通信規(guī)程，用于對安全性要求高的場合及由總線供電的站點。

  本設計中采用的是PROFIBUS-DP標準，PROFIBUS-DP（或DP標準）是由歐洲標準EN50170定義的一種遠程I/O通信協(xié)議。DP表示分布式外圍設備，亦即遠程I/O。PROFIBUS-DP還用到了應用層接口（ALI），ALI將過程對象轉換為通信對象。

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