Basic Hart Message Format is indicated as below
Starting Characters
| Message Type | Short Frame | Long Frame |
| Master to slave | 02 | 82 |
| Slave to master | 06 | 86 |
| Burst message from slave | 01 | 81 |
Address
The address field contains both the master (host) and slave (field device) addresses for the message. These are contained in a single byte in the short frame format, or in five bytes in the long frame format.
In both formats, the most-significant bit is usually the single-bit address of the master device taking part in the transaction. Only two masters are allowed – for example a control system and a hand-held communicator. The most-significant bit of the address field distinguishes between these: primary masters (control systems or other permanently-connected hosts) use address 1, secondary masters use address 0. Burst messages are an exception – in these, the most-significant bit is set alternately to 0 and 1; this gives each master, in turn, an opportunity to interrupt the burst mode operation.
Also in both formats, the next-most-significant bit is set to 1 to indicate that this message comes from a field device in burst mode (which does not necessarily mean that this is itself a burst message).
In the short frame format, slave devices have polling addresses in the range 0 to 15. This number is included in binary form as the least-significant half of the single address byte. In the long frame format, the polling address is not used; instead, the remaining 38 bits of the five-byte address field hold the slave’s “unique identifier” as an address
Summary
HART is a master-slave protocol, with up to two active masters (a control system and a hand-held communicator, for example). Up to 15 slave devices can be connected to a single multidrop pair of wires.
Each message includes the addresses of its source and its destination. Two forms of slave addressing are used: a short form for older devices (and for initial device identification), and a long form, based on the unique identifier, for newer (HART Revision 5) devices.
The field device’s status is included in every response message, indicating its continued good health. Parity checking and the checksum allow corruption of the message itself to be detected.
For typical commands, two transactions can be made each second; in burst mode, three messages can be sent each second.
