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| Wysłany: Pią 18:13, 26 Maj 2006 Temat postu: Standardy SCSI |
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| Standardy SCSI
Zaawansowany technologicznie typ połączenia komputera z urządzeniami zewnętrznymi takimi jak: streamery, CD–ROM-y, dyski twarde, skanery itp. Standard SCSI umożliwia połączenie w łańcuch do jednego kontrolera 7, a w przypadku wersji rozszerzonej WIDE SCSI nawet do 16 urządzeń (łącznie z kontrolerem). Dla porównania standard EIDE obsługuje tylko 4 urządzenia dzięki czemu SCSI jest szczególnie przydatny w przypadku dysków twardych, gdyż umożliwia jednoczesne połączenie więcej niż 4 takich urządzeń a każdy z nich może mieć pojemność nawet do 50 GB. SCSI oferuje również szybszy transfer danych między urządzeniami, dochodzący do 80 MB/s (EIDE - 66 MB/s). Standard SCSI jak i wykorzystujące go urządzenia używane są głównie w komputerach Macintosch oraz szybkich serwerach sieciowych i urządzeniach archiwizujących. Rzadziej w domowych pecetach gdyż urządzenia komunikujące się za pomocą tego standardu są zwykle dwukrotnie droższe od takich samych ale wykorzystujących inne standardy jak EIDE czy USB. Ogólnie SCSI składa się z 4 części: kontrolera SCSI montowanego zazwyczaj jako karta rozszerzająca (ISA lub PCI), lub gotowego elementu wbudowanego bezpośrednio do płyty głównej, kabla połączeniowego SCSI, samego urządzenia (np. dysku twardego) i tzw. terminatorów w postaci zworek lub dodatkowych złącz, które umieszczane są na dwóch końcach łańcucha połączonych urządzeń SCSI. Dzięki nim kontroler otrzymuje informację gdzie są ostatnie urządzenia łańcucha przez co może sprawnie obsługiwać przepływ danych między poszczególnymi urządzeniami a komputerem. Standard SCSI od czasu swego powstania uległ kilkakrotnie modyfikacjom przez co możemy mieć doczynienia z kilkoma różnymi wersjami tego samego standardu. Jest to szczególnie kłopotliwe w przypadku dobierania i łączenia ze sobą urządzeń obsługujących różne wersje SCSI. Oto zestawienie najważniejszych standardów SCSI: SCSI-I – Leciwy standard wraz z ujednoliconym nieco później Common Command Set (CIS) pracuje na bazie ośmiobitowej magistrali danych i oferuje maksymalną prędkość przesyłania danych około 3 MB/s.. SCSI-1 był pierwszym standardem SCSI stworzonym w 1986 r. Oferował on wsparcie dla asynchronicznych i synchronicznych urządzeń SCSI. W asynchronicznym transferze danych dane są przesyłane w nieregularnych odstępach. Sygnał start i stop określa początek i koniec bajta. W synchronicznym transferze danych, dane są synchronizowane impulsami z komputera. Ten rodzaj transferu jest szybszy. Jak już wspomniano we wprowadzeniu SCSI-1 obsługuje do siedmiu urządzeń. Maksymalny transfer danych to 5 MB/s. SCSI-II – SCSI-2 został znacząco poprawiony. Do SCSI-1dodano następujące narzędzia: - Ulepszone złącze (mniejsze) - Fast SCSI-2 : zwiększenie taktowania zegara szyny SCSI ze zwiększeniem transferu danych do 10MB/sec. - Wide SCSI : podwojenie szerokości szyny (szerokości pasma) z 8 do16 bitów, co umożliwiło podwojenie transferu danych (20MB/sec). - Sprawdzanie parzystości szyny SCSI (SCSI bus Parity Checking ) Kombinacja SCSI i Wide SCSI (Fast Wide SCSI) umożliwia transfer z maksymalną prędkością 20 MB/sec. Oferuje jasno zdefiniowany zestaw poleceń i listę parametrów. Dzięki liście urządzeń uniknięto wiele problemów z napędami CD, MO, wymiennymi dyskami, skanerami itp. Magistrala SCSI-II używa 50 żyłowego kabla SCSI-A SCSI-3 Standard SCSI-3 nie jest jeszcze sfinalizowany. Jednakże wiadomo już, że dodane zostaną następujące narzędzia. Transfer danych 40 MB/sec., wstecznie kompatybilny z SCSI-2 i SCSI-1. - Wsparcie dla Fiber Channel - Wsparcie IEEE P1394 (FireWire) - Serial Storage Architecture (SSA) : szybsze transfery, więcej urządzeń, dłuższy kabel, ale brak kompatybilności wstecz ze SCSI-2 lub SCSI-1. Fast-SCSI – Jest przyszłością standardu. Pozwala na transfer danych do 10 MB/s, co osiągnięto m.in. podnosząc częstotliwość taktowania magistrali. Wide-SCSI – To 16-bitowa wersja Fast SCSI, Maksymalnie 20 MB /s może być przesyłane synchronicznie za pośrednictwem 68-żyłowego kabla SCSI-B. Dzięki temu, że do adaptera Wide-SCSI na ogół można podłączyć zarówno SCSI-A, jak i SCSI-B, kontroler ten może jednocześnie obsługiwać urządzenia SCSI-II, Fast SCSI oraz Wide - SCSI. Ultra-SCSI – nazywany również Fast-20-SCSI, stosuje większą prędkość taktowania sygnału przesyłanego zwykłym kablem 50-żyłowym i osiąga wydajność 20 MB/s. Dla urządzeń zewnętrznych wymagany jest specjalny kabel połączeniowy. 6-bitowy wariant Ultra-SCSI umożliwia transfer 40 MB/s poprzez kabel SCSI-B nazywany jest Ultra-Wide-SCSI lub Fast-40-SCSI; transfer danych 40 MB/sec., wstecznie kompatybilny z SCSI-2 i SCSI-1. - Wsparcie dla Fiber Channel - Wsparcie IEEE P1394 (FireWire) - Serial Storage Architecture (SSA) : szybsze transfery, więcej urządzeń, dłuższy kabel, ale brak kompatybilności wstecz ze SCSI-2 lub SCSI-1. Ultra2 SCSI: wprowadzono technologię Low Voltage Differencial, pozwalającą na zwiększenia maksymalnej odległości do ~12 m. Prędkość transferu 40-80 MB/s, Ultra3 SCSI (Ultra160 SCSI): maksymalny transfer 160 MB/s, dodano funkcje wspomagające wykrywanie i usuwanie przekłamań. Ultra4 SCSI (Ultra320 SCSI): maksymalny transfer 320 MB/s. Każde z urządzeń SCSI musi mieć przyporządkowany własny adres tzw. ID (od 1 do 15). Dokonuje się tego przełączając odpowiednią zworkę lub pozycję przełącznika (DIP) w urządzeniu SCSI. |
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