be80be:
I no how it works Have you ever used one I worked where this was used to control mixing valves read fluid temperature sensorsAnalog current loops are used where a device must be monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time.
Given its analog nature, current loops are easier to understand and debug than more complicated digital fieldbuses, requiring only a handheld digital multimeter in most situations. Using fieldbuses and solving related problems usually requires much more education and understanding than required by simple current loop systems.
Additional digital communication to the device can be added to current loop using HART Protocol. Digital process buses such as FOUNDATION Fieldbus and Profibus may replace analog current loops.
Yes I worked in a oil refinery for 29 years as an instrumentation analyst. We had thousands and thousands of current loops. Most used the true analog 4.-20ma current loops. Some use the Honeywell 'smart meters' in DE mode where they did drive the current loop in true digital serial data mode rather then analog mode. And then there were the Rosemont transmitters (that developed the original HART digital protocol) that could use the current loop as analog but at the same time superimpose a tiny FSK ac signal onto the loop so that digital communications could be had between the remote field transmitting device and the control house controller device while still sending the analog measurement value on the same 4-20ma current loop.
So what was your point about an old IBM PC that could send serial data with 4-20ma current loops. Did you really mean that is the same method this thread has been dealing with?
Lefty