electrician or instrument?

移民电工

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  注意，自动化控制不等于PLC。电动气动表单个回路控制也是自动化控制。PLC只是一种设备，就象DCS一样。控制（从某一方面讲）分两大类：顺序控制和连续控制。PLC多用于顺序控制，象汽车装配，包装企业，连锁紧急停车等等。 而石油化工的四大物理量的控制大多是用DCS
! ]2 u  J  ~! [$ X9 w, g    NorthernWolfy 发表于 2010-3-24 09:59
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you are right, right now in Oil sand most place we use Filedbus and some place you can find out HART, DeviceNet and so on. They are all Digital but You still can find out update old system they use Analogy ,
depend on company and cost.

犀利哥哥

correct!

td456

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0 J- j+ e# H5 b, k) z( W# y' fHart 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"

( v) _) j6 s. C+ x  U+ F7 Z8 \1 B  GIn DCS, HART is still treated as AI(Analog In) signal.
Anyway it is only a transitional protocol and it is being replaced by Fieldbus or Profibus.

Devicenet is only  a plant base network(底层网络). It deal with BYTE level information.
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.

NorthernWolfy

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http://www.edmontonchina.cn/viewthread.php?tid=272089&extra=page%3D1&page=2
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  l, _6 E7 d2 o' B( R
- L0 m5 ?: j4 \3 l  U8 i2 U8 M9 ?  注意，自动化控制不等于PLC。电动气动表单个回路控制也是自动化控制。PLC只是一种设备，就象DCS一样。控制（从某一方面讲） ...
移民电工 发表于 2010-3-24 23:43

$ A/ N8 I4 n7 q# t( d/ U* E"Filedbus","HART", "DeviceNet" 只是不同的物理层连接和通讯协议。他们跟数字信号没有关系. 模拟量和数字量的相互转换是在PLC或DCS中（输入输出卡，A/D和D/A）处理的. HART协议时将4-20mA信号加载到一个高频信号上传输的，原来的仪表有四根线，信号线和电源线，HART只有两根线。

td456

4# NorthernWolfy
' B0 B: x5 ^2 q4 zYou are correct!
) ]6 j8 k: N1 [' T' h5 x* u When we talk about Digital or Analog, we are talking about signal   processing .

otttc

dingding

lele2002

4# NorthernWolfy

lele2002

4# NorthernWolfy
lele2002 发表于 2010-3-28 22:07

HART协议时将4-20mA信号加载到一个高频信号上传输的，原来的仪表有四根线，信号线和电源线，HART只有两根线。

lele2002

This call " loop powered"