WHY ac and dc value in HSTL?

zxswzj · electronics forum beginner Joined: 02 Apr 2009 Posts: 1

Hi, I studied HSTL specification recently days, I am puzzled about why there are dc value and ac value(Vih(dc),Vih(ac),Voh(dc),Voh(ac)), I found a description in a book, "The reason is that many input waveforms will include a certain amount of 'ringing'. The system can be sure that device will switch state a certain amount of time after the input has crossed the ac threshold and not switch back as long as the input stays beyond the dc threshold", can anyone tell me what's the meaning of that?? Thx,

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