The encyclopedia · Engineering & Operations · Technical decision · 1985–2007
HART put digital data on 4-20mA wires, so plants upgraded without rewiring
Rosemount's HART superimposed a digital signal on the analog 4-20mA loop, giving old plants diagnostics and configuration without replacing a single cable.
Rosemount (Emerson) · HART Communication Foundation
The solution
For decades process plants carried measurements on 4-20mA current loops, and replacing that wiring for a new digital fieldbus meant an expensive plant-wide project. In the 1980s Rosemount developed HART to add digital communication to the analog loop.
HART superimposes a low-speed digital signal on the 4-20mA current using frequency-shift keying, so the same wires carry the analog process value and digital configuration, diagnostics and extra measurements simultaneously. Revision 5, introduced in 1989, became the huge installed base.
Because existing cables were reused and field crews only needed a handheld master and a little training, adoption was cheap and fast. Rosemount transferred the patents, trademarks and technology to the HART Communication Foundation in 1993, making it an open standard.
Why it worked
- No rewiring: it rode the 4-20mA pairs already in the ground.
- Analog and digital worked at the same time on the same wire.
- Field staff needed minimal training and a handheld tool.
- Open governance (HCF, then IEC 61158) ended fears of vendor lock-in.
What can be applied
The cheapest upgrade path is the one that preserves the installed base: add a new capability to the wire that already exists, and users adopt it without a forklift project.
Aftermath
HART became the de facto standard for smart field devices and part of the IEC 61158 fieldbus standard in 2007; WirelessHART later extended the protocol to wireless without any wiring.
Sources
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