Communication Protocols · Free Tool

HART Command List and Lookup

Look up any common HART command number to see what it does, whether it is universal or common practice, and whether it writes to the device.

Universal commandsCommon practiceRead or writeCommand lookup

Command

This command changes the device. On a live loop, put the loop in manual and follow your change procedure first.

Command 35Write primary variable range values

ClassCommon practice

ActionWrite

All listed commands

No	Command	Class	Action
0	Read unique identifier	Universal	Read
1	Read primary variable	Universal	Read
2	Read loop current and percent of range	Universal	Read
3	Read dynamic variables and loop current	Universal	Read
6	Write polling address	Universal	Write
7	Read loop configuration	Universal	Read
8	Read dynamic variable classifications	Universal	Read
9	Read device variables with status	Universal	Read
11	Read unique identifier associated with tag	Universal	Read
12	Read message	Universal	Read
13	Read tag, descriptor, date	Universal	Read
14	Read primary variable transducer information	Universal	Read
15	Read device information (range, damping, transfer function)	Universal	Read
16	Read final assembly number	Universal	Read
17	Write message	Universal	Write
18	Write tag, descriptor, date	Universal	Write
19	Write final assembly number	Universal	Write
20	Read long tag	Universal	Read
21	Read unique identifier associated with long tag	Universal	Read
22	Write long tag	Universal	Write
33	Read device variables	Common practice	Read
34	Write primary variable damping value	Common practice	Write
35	Write primary variable range values	Common practice	Write
36	Set primary variable upper range value	Common practice	Write
37	Set primary variable lower range value	Common practice	Write
38	Reset configuration changed flag	Universal	Write
40	Enter or exit fixed current mode	Common practice	Write
41	Perform self test	Common practice	Command
42	Perform device reset	Common practice	Command
43	Set primary variable zero	Common practice	Write
44	Write primary variable units	Common practice	Write
45	Trim loop current zero	Common practice	Write
46	Trim loop current gain	Common practice	Write
47	Write primary variable transfer function	Common practice	Write
48	Read additional device status	Universal	Read
59	Write number of response preambles	Common practice	Write

HART Command Classes

Universal commands: supported by every HART device
Common practice commands: optional, widely implemented
Device specific commands: defined by each manufacturer (via the DD)

Worked example

Command 35 writes the LRV and URV of the primary variable; command 1 reads the primary variable value and units.

Key insight: a handheld that "sees" a device but cannot change a parameter often lacks the device description (DD) needed for device specific commands, not the universal ones.

Frequently Asked Questions

What is HART command 0?

Read unique identifier: it returns the manufacturer, device type and ID, and is used to find devices on the loop.

Which HART command changes the range?

Command 35 writes the range values; 36 and 37 set URV and LRV from the applied input.

What are device specific commands?

Commands defined by the manufacturer, accessed through the device description file.

Learn more on the blog

HART protocol: how it works

A practical, field focused guide on instrumentationblog.in that explains the theory behind this tool and how engineers apply it on real plants.

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