What is the HART Protocol?
The HART protocol superimposes a small digital signal on the 4 to 20 mA loop, allowing configuration and diagnostics without disturbing the analog value.
HART Protocol Explained
HART uses frequency shift keying: 1200 Hz for a 1 and 2200 Hz for a 0, at 1200 bit/s and about ± 0.5 mA. The signal averages to zero, so the analog reading is unaffected.
Handheld communicators or asset management software read tags, ranges, diagnostics and extra variables.
HART Protocol: Typical Values
| Signal | FSK 1200 / 2200 Hz, 1200 bit/s |
| Loop resistance | at least 250 Ω (typically 230 to 1100 Ω) |
| Multidrop | up to 15 devices at fixed 4 mA (older revisions) |
Calculators for HART Protocol
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Frequently Asked Questions
What is the HART Protocol?
The HART protocol superimposes a small digital signal on the 4 to 20 mA loop, allowing configuration and diagnostics without disturbing the analog value.
Why can my HART communicator not talk to the transmitter?
Often too little loop resistance, or the communicator is connected across the power supply instead of across the resistor or transmitter.
What is HART 7?
A revision adding features such as WirelessHART and longer tags.
Learn more on the blog
HART protocol: how it works and how to use a HART communicator
A practical guide on instrumentationblog.in with more detail on HART protocol and how it is applied on real plants.
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