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本帖最后由 td456 于 2010-3-25 08:06 编辑 ! E0 | i( i2 ]4 G7 K% j
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2 |) A# E% n9 KHart protocol is something between analog and digital,it is a digitally enhanced 4-20mA smart instrument communication./ b3 d# e( e8 d0 Q
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"The HART protocol makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20 mA. Since the digital FSK signal is phase continuous, it does not interfere with the 4-20 mA signal. A logical “1” is represented by a frequency of 1200 Hz and a logical “0” is represented by a frequency of 2200 Hz"
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. l; f( r9 J" }- q. }( FIn DCS, HART is still treated as AI(Analog In) signal.1 }4 k2 b, U' E- A0 @! n9 R: l* Y
Anyway it is only a transitional protocol and it is being replaced by Fieldbus or Profibus.
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Devicenet is only a plant base network(底层网络). It deal with BYTE level information.
?- j4 S) V( U& D. z& e: `3 ~- HDevicenet can not be a backbone of a plant network but Fieldbus can. There is difference between Devicenet and Fieldbus. They deal with difference level of communication. |
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