maziar navahan wrote:
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other hand :
4-20ma loop only use in analog communication its old!

Yes, Maziar and so am I but both 4-20 and I are still kicking.

Many industrial devices are still only available with 4-20 interfaces. Also, I do not know how often I have heard in the control industry: "this newfangled digital thang is something that we do not understand and thus will not use". Since the demand for rs-485 devices is much less for the above reason you will often find that a rs-485 device (such as a tachometer) will cost you ten times what a similar 4-20 will cost you. The curse of the 4-20 is here to stay.

Erik