Allen Bradley 1756-Eweb/B 當IO口處于高阻態時，我們也將其稱為浮空輸入狀態，此時其電平是不確定的，既不是高電平也不是低電平。我們可以想象成，單片機在檢測IO口的電平高低時，相當于在CPU里面有一個類似電壓表的東西，并且這個電壓表內阻很大，假設內阻為100MΩ（示意圖如下）。在這里，我們可以把這個電壓表的內阻稱為這個IO口此時的輸入電阻。
現在設想，當我不小心用手碰到了IO端口，而由于人體本身就是阻值很大的導體，周圍有很多電磁波干擾，手上可能存在一些很微弱的電流，這個時候，電壓表的讀數就會發生變化，單片機讀取的電平高低就會變。高阻態表現出來的結果就是外界很小的干擾，都可能導致讀取的電平變化，甚至即使沒有碰這個IO口，它每次讀取的結果也可能不一樣，因為外界的電磁波等可能會干擾到IO口。IO口輸入端不使用時，可通過電阻單獨連接到 VDD或 VSS。
為什么雙向IO口輸入的時候要求是高阻態呢？
我們假設現在有一種輸入設備，等效電路如下圖。內部開關上下切換，它就會輸出高低電平給到單片機的IO口，IO口通過引腳內部電壓表可以檢測出來輸入的是高電平還是低電平。但是這個設備驅動能力很弱，連LED也驅動不了。設備里的100kΩ，可以叫做設備的輸出電阻（同樣也可以近似認為是輸出阻抗）。
Allen Bradley 1756-Eweb/B APPLIED MATERIALS 0010-09306
APPLIED MATERIALS 0010-09316
APPLIED MATERIALS 0010-09319
APPLIED MATERIALS 0010-09335
APPLIED MATERIALS 0010-09340
APPLIED MATERIALS 0010-09340W
APPLIED MATERIALS 0010-09341
APPLIED MATERIALS 0010-09341I
APPLIED MATERIALS 0010-09408
APPLIED MATERIALS 0010-09416
APPLIED MATERIALS 0010-09416
APPLIED MATERIALS 0010-09417
APPLIED MATERIALS 0010-09418
APPLIED MATERIALS 0010-09447
APPLIED MATERIALS 0010-09463
APPLIED MATERIALS 0010-09481
APPLIED MATERIALS 0010-09488
APPLIED MATERIALS 0010-09489
APPLIED MATERIALS 0010-09490
APPLIED MATERIALS 0010-09495
APPLIED MATERIALS 0010-09500
APPLIED MATERIALS 0010-09501
APPLIED MATERIALS 0010-09559
APPLIED MATERIALS 0010-09591
APPLIED MATERIALS 0010-09633
APPLIED MATERIALS 0010-09656
APPLIED MATERIALS 0010-09732
APPLIED MATERIALS 0010-09781
APPLIED MATERIALS 0010-09819
APPLIED MATERIALS 0010-09837
APPLIED MATERIALS 0010-09837
APPLIED MATERIALS 0010-09900
APPLIED MATERIALS 0010-09917
APPLIED MATERIALS 0010-09919
APPLIED MATERIALS 0010-09940
APPLIED MATERIALS 0010-09967
APPLIED MATERIALS 0010-09967
APPLIED MATERIALS 0010-09982
APPLIED MATERIALS 0010-10007
APPLIED MATERIALS 0010-10010
APPLIED MATERIALS 0010-10021
APPLIED MATERIALS 0010-10032
APPLIED MATERIALS 0010-10036
APPLIED MATERIALS 0010-10036
APPLIED MATERIALS 0010-10037
APPLIED MATERIALS 0010-10045
APPLIED MATERIALS 0010-10055
APPLIED MATERIALS 0010-10060
APPLIED MATERIALS 0010-10061
APPLIED MATERIALS 0010-10098W
APPLIED MATERIALS 0010-10106
APPLIED MATERIALS 0010-10121
APPLIED MATERIALS 0010-10128
APPLIED MATERIALS 0010-10141
APPLIED MATERIALS 0010-10141
APPLIED MATERIALS 0010-10157
APPLIED MATERIALS 0010-10177
APPLIED MATERIALS 0010-10214
APPLIED MATERIALS 0010-10257
APPLIED MATERIALS 0010-10259
APPLIED MATERIALS 0010-10275
APPLIED MATERIALS 0010-10277
APPLIED MATERIALS 0010-10293
APPLIED MATERIALS 0010-10312
APPLIED MATERIALS 0010-10331
APPLIED MATERIALS 0010-10371
APPLIED MATERIALS 0010-10419
APPLIED MATERIALS 0010-10436
APPLIED MATERIALS 0010-10449
APPLIED MATERIALS 0010-10451
APPLIED MATERIALS 0010-10455
APPLIED MATERIALS 0010-10474
APPLIED MATERIALS 0010-10517
APPLIED MATERIALS 0010-10521
APPLIED MATERIALS 0010-10579
APPLIED MATERIALS 0010-10620
APPLIED MATERIALS 0010-10761
APPLIED MATERIALS 0010-10892
APPLIED MATERIALS 0010-10957
APPLIED MATERIALS 0010-10957
APPLIED MATERIALS 0010-10973
APPLIED MATERIALS 0010-10973
APPLIED MATERIALS 0010-11000
APPLIED MATERIALS 0010-12282
APPLIED MATERIALS 0010-13033
APPLIED MATERIALS 0010-13104
APPLIED MATERIALS 0010-13122
APPLIED MATERIALS 0010-13334
APPLIED MATERIALS 0010-13445
APPLIED MATERIALS 0010-13643
APPLIED MATERIALS 0010-13753
APPLIED MATERIALS 0010-13806
APPLIED MATERIALS 0010-13912
APPLIED MATERIALS 0010-13927
APPLIED MATERIALS 0010-13943
APPLIED MATERIALS 0010-13975
APPLIED MATERIALS 0010-14017
APPLIED MATERIALS 0010-15435
APPLIED MATERIALS 0010-15437
APPLIED MATERIALS 0010-16220
APPLIED MATERIALS 0010-17004
APPLIED MATERIALS 0010-17961
APPLIED MATERIALS 0010-18247
APPLIED MATERIALS 0010-20000
APPLIED MATERIALS 0010-20001
APPLIED MATERIALS 0010-20003
APPLIED MATERIALS 0010-20021
APPLIED MATERIALS 0010-20030
APPLIED MATERIALS 0010-20079
APPLIED MATERIALS 0010-20098
APPLIED MATERIALS 0010-20115
APPLIED MATERIALS 0010-20127
APPLIED MATERIALS 0010-20128
APPLIED MATERIALS 0010-20129
APPLIED MATERIALS 0010-20130
APPLIED MATERIALS 0010-20130
APPLIED MATERIALS 0010-20132
APPLIED MATERIALS 0010-20142
APPLIED MATERIALS 0010-20143
APPLIED MATERIALS 0010-20146
APPLIED MATERIALS 0010-20161
APPLIED MATERIALS 0010-20163
APPLIED MATERIALS 0010-20163
APPLIED MATERIALS 0010-20221
APPLIED MATERIALS 0010-20238
APPLIED MATERIALS 0010-20238
APPLIED MATERIALS 0010-20288
APPLIED MATERIALS 0010-20291
APPLIED MATERIALS 0010-20291
APPLIED MATERIALS 0010-20292
APPLIED MATERIALS 0010-20300
APPLIED MATERIALS 0010-20300
APPLIED MATERIALS 0010-20317
APPLIED MATERIALS 0010-20389
APPLIED MATERIALS 0010-20406
APPLIED MATERIALS 0010-20418
APPLIED MATERIALS 0010-20422
APPLIED MATERIALS 0010-20441
APPLIED MATERIALS 0010-20442
APPLIED MATERIALS 0010-20462
APPLIED MATERIALS 0010-20472
APPLIED MATERIALS 0010-20483
APPLIED MATERIALS 0010-20511
APPLIED MATERIALS 0010-20524
APPLIED MATERIALS 0010-20753
APPLIED MATERIALS 0010-20764
APPLIED MATERIALS 0010-20771
APPLIED MATERIALS 0010-20862
APPLIED MATERIALS 0010-20991
APPLIED MATERIALS 0010-21055
APPLIED MATERIALS 0010-21246
APPLIED MATERIALS 0010-21259
APPLIED MATERIALS 0010-21259
APPLIED MATERIALS 0010-21409
APPLIED MATERIALS 0010-21465
APPLIED MATERIALS 0010-21497
APPLIED MATERIALS 0010-21668
APPLIED MATERIALS 0010-21669
APPLIED MATERIALS 0010-21740
APPLIED MATERIALS 0010-21768
APPLIED MATERIALS 0010-21769
APPLIED MATERIALS 0010-21803
APPLIED MATERIALS 0010-22209
APPLIED MATERIALS 0010-25423
APPLIED MATERIALS 0010-25735
APPLIED MATERIALS 0010-28058
APPLIED MATERIALS 0010-30029
APPLIED MATERIALS 0010-30111
APPLIED MATERIALS 0010-30136
APPLIED MATERIALS 0010-30230
APPLIED MATERIALS 0010-30334
APPLIED MATERIALS 0010-30354
APPLIED MATERIALS 0010-30402
APPLIED MATERIALS 0010-30576
APPLIED MATERIALS 0010-30579
APPLIED MATERIALS 0010-30579
APPLIED MATERIALS 0010-30580
APPLIED MATERIALS 0010-30580
APPLIED MATERIALS 0010-30591
APPLIED MATERIALS 0010-30610
APPLIED MATERIALS 0010-30612
APPLIED MATERIALS 0010-30687
APPLIED MATERIALS 0010-35004
APPLIED MATERIALS 0010-35048
APPLIED MATERIALS 0010-35227
APPLIED MATERIALS 0010-35384
APPLIED MATERIALS 0010-35483
APPLIED MATERIALS 0010-35537
在線咨詢 
