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I hope my question is not out of the scope of this site. I have heard about 2-wire signal transmission in control circuits. As I know,2-wire analogue signals will make a 4-20 mA current loop. But what about digital control signals? Do they make a loop as well? What about the reference voltage in digital signals?there must be a third wire?

Here Is the link to a part of weidmuller surge protection catalogue(for Control and instrumentation signals) explaining 2-wire transmission. https://www.dropbox.com/s/c5odx1fm350hnhp/CAT4-4_B_001-172_EN_web-6-7.pdf?dl=0

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  • \$\begingroup\$ What leads you to believe that there should be a third wire? \$\endgroup\$
    – Andy aka
    Jan 28, 2017 at 12:56
  • \$\begingroup\$ I was thinking that there must be a 3rd wire to be connected to the reference potential. @Andyaka \$\endgroup\$ Jan 29, 2017 at 6:21

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The link to the digital signal takes you to some rather cryptic text that suggests, not defines, their 2-wire digital inputs to be differential with a signal ground reference on a third wire, as you deduced.

You can look up differential logic signalling on the Internet. In short, each wire is driven with a voltage with respect to ground. One is driven to a higher voltage while the other is driven to a lower voltage. The actual voltages used depend upon the choice of the designer and compliance with any chosen differential signalling standard, such as RS422 or USB. The signalled logic level depends on which of the two wires is at a higher voltage than the other.

If you get more information on your equipment, please post it and we can look at it further.

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