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以太網(wǎng)是一種極其通用的網(wǎng)絡(luò)介質(zhì)，可為自動(dòng)化和過(guò)程控制應(yīng)用提供大量數(shù)據(jù)傳輸。但它本身并不是一個(gè)協(xié)議。為了做任何有用的事情，它需要在數(shù)據(jù)流上留下一個(gè)約定的格式——工業(yè)以太網(wǎng)協(xié)議——它所服務(wù)的系統(tǒng)應(yīng)用層可以理解。
在征服了辦公自動(dòng)化領(lǐng)域之后，以太網(wǎng)和 TCP/IP 現(xiàn)在正在推動(dòng)從集中式控制系統(tǒng)向分布式控制系統(tǒng)的轉(zhuǎn)變，承諾自動(dòng)化和企業(yè)金字塔內(nèi)的無(wú)縫通信。滿(mǎn)足工廠(chǎng)車(chē)間的條件需要適應(yīng)，特別是對(duì)于實(shí)時(shí)行為的要求。HTTP 和 FTP 等非確定性通信協(xié)議提供了一個(gè)有用的開(kāi)端，但事實(shí)證明，創(chuàng)建滿(mǎn)足工業(yè)要求的其他以太網(wǎng)友好應(yīng)用協(xié)議是必要的。
OSI 參考模型
OSI 參考模型示意性地描述和標(biāo)準(zhǔn)化網(wǎng)絡(luò)架構(gòu)中設(shè)備之間的通信。通信所需的功能細(xì)分為七個(gè)功能層。每個(gè)相應(yīng)的較低層通過(guò)定義良好的接口向上面的層提供其服務(wù)。
在用戶(hù)數(shù)據(jù)（應(yīng)用程序）可以通過(guò)以太網(wǎng)介質(zhì)發(fā)送之前，它必須通過(guò)協(xié)議棧從上層到下層向下傳遞，終嵌入到特定下層的幀中（封裝）。數(shù)據(jù)包通過(guò)低（物理介質(zhì)）層發(fā)送后，它包含的數(shù)據(jù)向上通過(guò)接收設(shè)備的所有更高層，直到到達(dá)應(yīng)用層。整個(gè)過(guò)程需要在每一層內(nèi)部進(jìn)行邏輯交互來(lái)完成網(wǎng)絡(luò)連接。
傳輸數(shù)據(jù)的解釋與用戶(hù)相關(guān)，并在應(yīng)用程序協(xié)議中指定（第5-7 層）。在辦公世界中，可以使用被稱(chēng)為 IT 標(biāo)準(zhǔn)的廣泛應(yīng)用協(xié)議，例如 FTP、HTTP 等。彼此不兼容的協(xié)議的工業(yè)通信使用這些層。這些自動(dòng)化協(xié)議可以以不同的方式實(shí)現(xiàn)。自動(dòng)化協(xié)議可以直接安裝在 OSI 參考模型的第 4 層（Modbus/TCP、EtherNet/IP 和 CIPsync、JetSync）上，或者另外，第 3 層和第 4 層將被替換和擴(kuò)展（ETHERNET Powerlink、PROFInet、SERCOS）。
EtherCAT（用于控制自動(dòng)化技術(shù)的以太網(wǎng)）是由德國(guó) Beckhoff 公司開(kāi)發(fā)的基于以太網(wǎng)的自動(dòng)化概念。在這項(xiàng)技術(shù)中，以太網(wǎng)數(shù)據(jù)包不再被接收，然后被解釋并復(fù)制為網(wǎng)絡(luò)內(nèi)每個(gè)模塊的過(guò)程數(shù)據(jù)。相反，以太網(wǎng)幀是動(dòng)態(tài)處理的：對(duì)于網(wǎng)絡(luò)中的每個(gè)模塊，讀取尋址到該模塊的數(shù)據(jù)，同時(shí)將電報(bào)轉(zhuǎn)發(fā)到下一個(gè)模塊。類(lèi)似地，在電報(bào)通過(guò)時(shí)插入輸入數(shù)據(jù)。電報(bào)僅延遲幾納秒，從而提高了系統(tǒng)的實(shí)時(shí)性能。網(wǎng)段中的后一個(gè)模塊將幀發(fā)回以創(chuàng)建邏輯和物理環(huán)結(jié)構(gòu)。所有傳輸?shù)臄?shù)據(jù)都包含完全兼容的以太網(wǎng)幀；
實(shí)時(shí)以太網(wǎng)幀通過(guò)內(nèi)部?jī)?yōu)先系統(tǒng)優(yōu)先于其他數(shù)據(jù)（例如配置或診斷等所需的數(shù)據(jù)）。如果有足夠的時(shí)間或通過(guò)使用特定的服務(wù)通道，配置數(shù)據(jù)將在時(shí)間間隙中傳輸。操作系統(tǒng)的完全維護(hù)的以太網(wǎng)功能實(shí)現(xiàn)了與傳統(tǒng) IP 協(xié)議的兼容性。基于 IEEE 1588 的同步機(jī)制實(shí)現(xiàn)了能夠支持 EtherCAT 下的運(yùn)動(dòng)控制應(yīng)用的性能。
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The interpretation of transferred data is user related and specified in application protocols (Layers 5-7). In the office world a wide range of application protocols, known as IT standards, are available, e.g. FTP, HTTP, etc. Industrial communication with protocols incompatible to each other use these layers. These automation protocols can be realised in different ways. Either the automation protocol is directly mounted on Layer 4 of the OSI reference model (Modbus/TCP, EtherNet/IP and CIPsync, JetSync) or, additionally, Layer 3 and 4 will be replaced and extended (ETHERNET Powerlink, PROFInet, SERCOS).
EtherCAT
EtherCAT (Ethernet for Control Automation Technology) is an Ethernet-based automation concept developed by the German company Beckhoff. In this technology the Ethernet packet is no longer received, then interpreted and copied as process data at each module within the network. Instead, the Ethernet frame is processed on the fly: For each module within the network, data addressed to that module is read, while the telegram is forwarded on to the next module. Similarly, input data is inserted while the telegram passes through. The telegrams are only delayed by a few nanoseconds so increasing performance of the system towards real-time behaviour. The last module within the network segment sends the frame back to create a logical and physical ring structure. All transferred data comprises fully compatible Ethernet frames; each module converts these Ethernet frames to what is effectively an internal fieldbus.
The real-time Ethernet frames have priority over other data (such as those required for configuration or diagnosis, etc) via an internal prioritisation system. Configuration data is transmitted in the time gaps if sufficient time is available or by using a specific service channel. Fully maintained Ethernet functionality of the operating system achieves compatibility with conventional IP protocols. Synchronisation mechanisms based on IEEE 1588 achieve performance capable of supporting motion control applications under EtherCAT.
