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According to Emerson,<ref>Emerson ''https://web.archive.org/web/20181107104043/https://www.automation.com/automation-news/industry/emerson-proves-advancements-in-eddl-electronic-device-description-language-technology'', rev. 2005-04-14</ref> due to the huge installation base of 4–20 mA systems throughout the world, the HART Protocol is one of the most popular industrial protocols today. HART protocol has made a good transition protocol for users who wished to use the legacy 4–20 mA signals, but wanted to implement a "smart" protocol.
==OSI layer==
{| class="wikitable" style="margin: 1em auto 1em auto;"
|+ OSI model
|-
! colspan="4" |
! Wired FSK/PSK & RS485
! Wireless 2.4GHz
|-
! colspan="3" | Layer
! [[Protocol data unit]] (PDU)
! colspan="2" | HART
|-
! rowspan="4" | Host<br />layers
| style="background:#d8ec9b;" | 7
| style="background:#d8ec9b;" | [[Application layer|Application]]
| style="background:#d8ec9c;" rowspan="3" | [[Data (computing)|Data]]
| style="background:#d8ec9c;" colspan="2" | Command-oriented. Data types and application procedures.
|-
| style="background:#d8ec9b;" | 6
| style="background:#d8ec9b;" | [[Presentation layer|Presentation]]
| style="background:#d8ec9b;" colspan="2"|
|-
| style="background:#d8ec9b;" | 5
| style="background:#d8ec9b;" | [[Session layer|Session]]
| style="background:#d8ec9b;" colspan="2"|
|-
| style="background:#e7ed9c;" | 4
| style="background:#e7ed9c;" | [[Transport layer|Transport]]
| style="background:#e7ed9c;" | [[Packet segmentation|Segment]], [[datagram]]
| style="background:#e7ed9c;" colspan="2"| Auto-segmented transfer of large data sets, negotiated segment sizes
|-
! rowspan="3" | Media<br />layers
| style="background:#eddc9c;" | 3
| style="background:#eddc9c;" | [[Network layer|Network]]
| style="background:#eddc9c;" | [[Network packet|Packet]]
| style="background:#eddc9c;" |
| style="background:#eddc9c;" | Wireless [[Mesh_networking|mesh network]], power-optimized
|-
| style="background:#e9c189;" | 2
| style="background:#e9c189;" | [[Data link layer|Data link]]
| style="background:#e9c189;" | [[Frame (networking)|Frame]]
| style="background:#e9c189;" | [[Binary operation|Binary]], [[byte]]-oriented, token passing, master/slave
| style="background:#e9c189;" | Wireless, time synced, [[Time-division_multiple_access|TDMA]], [[Automatic repeat request|ARQ]]
|-
| style="background:#e9988a;" | 1
| style="background:#e9988a;" | [[Physical layer|Physical]]
| style="background:#e9988a;" | [[Bit]], [[Symbol rate#Symbols|symbol]]
| style="background:#e9988a;" | Analog and digital signaling, normally 4–20mA or RS485 copper wiring.
| style="background:#e9988a;" | 2.4GHz wireless, 802.15.4-based radios, 10[[dBm]] Tx power
|}
==History==
| 