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本帖最后由 td456 于 2010-3-25 08:06 编辑
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* {3 \, A0 x/ @/ w( K 1# 移民电工 3 L$ R2 f: e# k$ V% t' B
Hart protocol is something between analog and digital,it is a digitally enhanced 4-20mA smart instrument communication.
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: o* @- D5 s' m, m1 M"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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1 `+ F: M s {, {9 n' s: iIn DCS, HART is still treated as AI(Analog In) signal./ t7 M- b# I4 X- ^9 v
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.$ l. M d! v2 X0 y! {8 f1 W
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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