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Old January 22nd, 2005, 07:06 PM   #14
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Bitmore is offline
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Join Date: Dec 2003
Location: Automation Alley, Oakland County, Michigan
Posts: 477
Yes, in a way that you are thinking, you must have been taught electronics! A sinking device in the field is like an open collector output device so it needs a collector load and +Vcc hence 3 wire sinking positive reference. (Sinking devices can also be open emmiter, 3 wire negative reference)
A sourcing device is an H biased transister circuit that has it's own internal load and power supply (Vcc/Vee ie. 4 wire) and the signal is "a produced" output whether you connect a load (I/O card) or not!
Now back to the positive or negative references...
Nothing more than open collector for positive reference and open emmiter for negative reference. The device still needs one of the reference voltages (+ or-)to complete the field devices' internal circuitry.
The analog current input cares nothing about the field devices current signal except that it must flow through the terminals of the input card.
Now on to single ended or differential inputs...
Are you ready yaba?
"A good scientist is a person with original ideas. A good engineer is a person who makes a design that works with as few original ideas as possible. There are no prima donnas in engineering." Freeman Dyson

Last edited by Bitmore; January 22nd, 2005 at 07:29 PM.
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