During Fieldbus System upgrade, Shall we Reuse the 4-20mA Wiring of an Existing Plant

Yes, certainly. Although existing wiring was not designed to carry high-speed digital information, the H1 signal is robust enough to be carried on typical existing wiring.

However, there are two extra considerations:

  • distance is limited to 400m for a twisted pair, 200m for untwisted cable
  • unshielded cable may not be usable if local interference is a problem.
  • The distance limit is a result of the cable’s characteristic impedance being unspecified, so reflections may occur at the ends of the cable, even though terminators are used. (For short cables, any reflection is not much delayed from the original signal, so does not cause serious distortion of the waveform.)