Analog scale conversions 1

[ebwease]

Presently, I input a 4-20 mA signal into my DCS which represents 0 to 700 MW. I need to retransmit that signal over a communication data link as -700 MW to +700 MW. How is this accomplished? Can someone please describe the SAMA logic which would represent this signal conversion?

 

[danw2]

I wasn't awary SAMA did comm protocol standards, but if your signal is 0-700 whatevers then the sign bit is always positive, right? Is the comm protocol more complex than dealing with a sign bit?

Dan

 

[ebwease]

Dan, the SAMA logic does not involve the comm link. I need to make the change to the SAMA logic as a way of informing the DCS vendor the particulars of the logic change. My real confusion is the scaling. If 4 mA is 0 MW and 20 mA is 700 MW then how does one develop a scale factor when the process range becomes -700 to +700?

 

[JLSeagull]

I don't use SAMA diagrams. Consider showing a bias [ + / - ] station. Next to the symbol write "input = 0-700 MW; output = -700 to +700 MW".

 

[IRstuff]

Not sure if your question is making any sense, but all you need to do is:

(current-4ma)*700W/16ma

That should be the conversion factor and you actually would not have any negative values as results, since all your inputs are, by definition, positive.

TTFN

FAQ731-376

 

[ScottyUK]

I think the SAMA block you are looking for is a gain-bias - might be shown as K-Bias or K+B in the bloack diagram - which will have two parameters: parameter 1 is the gain (2 in your case) and parameter 2 is the bias (-700 in your case).


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Sometimes I only open my mouth to swap feet...

 

[GTstartup]

What you are asking is this

You want to make 0% to be 50% and 100% to be 100% so...

Multiply by .5 and add 50%, now your range is -100% to 100% (-700 - +700MW) 4mA = 0MW (50%), 20mA= 700MW (100%)

Is this what you mean, or do you want the actual Sama symbols for adder and multipier too?