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本帖最后由 td456 于 2010-3-25 08:06 编辑 ) r) I# i) u7 k# I# q5 Q4 S- ?
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1# 移民电工 6 ]! [( k! N1 y6 s
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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; ?0 a4 O4 |) ~In DCS, HART is still treated as AI(Analog In) signal.! n* f) f- v# w1 C* G& t
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.9 a) a( R& Q, ?# d7 V; u! M8 G) c
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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