1746-P3（A-B）
1746-P3 接受USB數據信號時也必須利用1個差模輸入接受器。當2個差模數據輸入以地電位作為參考，并且處于0．8-2．5V這樣的范圍之間時，接受器具有的靈敏度至少200mV，這稱為共模輸入電壓范圍。當差模信號線不在共模范圍之內時，也要求能進行正確的數據接受。如果在沒有損壞并以本地地電位作電位參考的條件下，接收器所能接收的穩態電平輸入電壓應該位于一o．5-3．8V之間。另外對于不同的接收器而言，每一條信號線都必須有1個單端接收器，這樣接收器必須具有1個位于0。8～2．0V之間（TTL輸入）這樣的開關閥值電壓。
3 USB接口工作電路主要參數
3．1 數據編碼／解碼
在傳輸信號時，USB應用了NRZI編碼方式。在NRZI編碼中，“1”由不出現電平變化來表示，而“0”由電平發生變化來表示。圖4給出一個數據流和等同的NRZI碼流，其中高電平代表數據線上的J狀態，而圖示代表了NRZI編碼過程。一串“0”會使得NRZI數據每比特周期都會出現跳變，而一串連“1”則使得數據中長時間不會出現變化。
3.2 比特填充
為了保證信號有足夠的變化，在USB上發送一個分組數據時，傳輸設備要進行比特填充。如圖5所示，對于USB上進行NRZI編碼之前，在數據流內每6個連“1”之后都應該插入1個“0”，從而在NRZI數據流中強制加入1個變化。這樣在邏輯上至少每7個比特周期，接收器就會接收到1個數據變化，以保證數據和時鐘相互鎖定。接收器必須對NRZI數據進行解碼，識別填充比特并丟失他們。比特填充sync（同步）模式開始進行，并貫穿于整個傳輸過程中。用于終止該sync方式的數據“1”，將作為一個序列中的第1個數據而加以計數。比特填充總是被強制執行，不會有什么例外。如果比特填充原則需要的話，即使該比特是分組結束（EOP）信號之前的最后一個比特，也會在最后面插入-個“0”比特。
3．3 數據信號速率
全速數據速率的標稱值是12Mb／s。對于全功能設備的數據速率容差為±0．25％。為了符合幀時間間隔精度的需要，主控制器的精度必須保證其優于±0．05％。對于低速率名譽上為1．5Mb／s，所允許的頻率容差為±1．5％。這一誤差包括下列原因所產生的不準確性：最初的頻率的正確度、晶體容性負載、提高給振蕩器的電壓、溫度和老化。低速率的的抖動應該小于10ns，這一容差允許在低速率設備中實用價格較低振蕩器。
1746-P3
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PRISTINE!!! Allen Bradley 1785-LT/1785LT/ B
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