Communication
74
Default SAP (SAP 0) (Data_Exchange)
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 (see Figure 7 on page 73).
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 will be set to
1 to indicate that a DSAP/SSAP byte follows in the telegram
header.
Outp_Data (Output Data)
DU length: 4 to 28 bytes (depending on the selected PPO message/
Standard telegram type)
Inp_Data (Input Data)
DU length: 4 to 28 bytes (depending on the selected PPO message/
Standard telegram type)
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