Siemens SIMATIC ET 200SP Manual page 20

HART function
4.1 How HART works
Structure of the HART protocol
Each HART frame sent from the analog module to the connected field device (request frame)
and each HART frame from the field device (response frame) has the following basic structure.
Table 4-3 Structure of the HART protocol
PREAMBLE STRT
PREAMBLE: Bytes (0xFF) for synchronizing.
STRT: Start character (start delimiter)
ADDR: Addresses of the field device
COM: HART command number
BCNT: Byte count, number of bytes to follow without checksum
STATUS: HART device status. Only present for a response frame.
see following tables "Structure of HART device status"
DATA: Transferred user data / parameters, quantity depending on com‐
mand
CHK: Checksum
With the exception of the preamble bytes, this structure is contained in the communication data
of the HART command interface. See section HART request and response data records
(Page 60).
HART replies always contain data. Together with a HART response, the HART device status
(1st and 2nd status bytes) is sent. Always evaluate the HART device status. If no error is
pending, the HART response is correct.
Structure of HART device status (1st and 2nd status bytes).
Table 4-4
When bit 7 = 1: "Communication error"
Bit 6 = 1
Bit 5 = 1
Bit 4 = 1
Bit 3 = 1
Bit 2 = 0
Bit 1 = 1
Bit 0 = 0
When bit 7 = 0: "Specific in line with response frame"
18
ADDR COM BCNT
1st status byte
Parity error
Overflow
Framing error
Checksum error
Reserved
Overflow in the receive buffer
Reserved
STATUS DATA
Key:
5 to 20 bytes depending on parameter
assignment
1 byte
1 byte; short address or 5 bytes; long ad‐
dress
1 byte
1 byte
1st and 2nd status bytes
0 ... 230 bytes
1 byte
Analog Output Module AQ 4xI HART HF
Manual, 05/2019, A5E35098013-AA
CHK