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本帖最后由 td456 于 2010-3-25 08:06 编辑 & J; j( J. j" J. B
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Hart protocol is something between analog and digital,it is a digitally enhanced 4-20mA smart instrument communication.
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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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In DCS, HART is still treated as AI(Analog In) signal. D4 K! b: k1 L5 e" C
Anyway it is only a transitional protocol and it is being replaced by Fieldbus or Profibus.) I' d" B D! K4 S5 ?3 x
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Devicenet is only a plant base network(底层网络). It deal with BYTE level information." n( f, p6 ~9 D0 i) a2 k
Devicenet 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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