Communication protocol 121
PROFIBUS SD2 telegram for Default SAP (0) and SAP 58-62
PROFIBUS typically uses SD2 telegrams for DP communication.
The structure of an SD2 telegram is shown below.
Default SAP (SAP 0) (Data_Exch)
This SAP allows the master to send output data to a slave station
and to simultaneously request input data from the same station.
In addition to Default SAP, any master can read the I/O data of any
slave at any time using the “Read_Inputs” (SAP 56) and
“Read_Outputs” (SAP 57) telegrams. These telegrams have
otherwise the same structure as the cyclic Data_Exchange
telegram, but include the DSAP and SSAP bytes. In these
telegrams, the MSB of the DA and SA bytes is set to 1 to indicate
that a DSAP/SSAP byte follows in the telegram header.
Output Data Input Data
Data Outp_Data Inp_Data
DU length 4 to 32 bytes (depending on the selected PPO
message or standard telegram type)
DP header DP trailer
SD LE LEr SD DA SA FC *DSAP *SSAP DU FCS ED
68h x x 68h xx xx x xx xx x… xx 16h
SD = Start delimiter
LE = Length
LEr = Length repeated
DA = Destination address
SA = Source address
FC = Function code
*DSAP = Destination service access point
*SSAP = Source service access point
DU = Data unit for DP services
FCS = Frame checking sequence
ED = End delimiter
*Not present with Default SAP (0)
Data unit
See the descriptions of each SAP on the following pages.
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