Siemens SIRIUS M200D Manual page 165

The figure below illustrates how a data set is read using the example of data set DS 200,
"Diagnostics":
Figure A-2
①
The CPU sends the CTT2 data request (using command 44: CTT2 request_write) from the send buffer to the AS-i
master via the command interface.
The header contains the slave address and the number of string bytes.
②
The AS-i master accepts the request and forwards it to the addressed slave (code 18: acyclic vendor specific read
service request).
M200D AS-Interface Standard
Manual, 08/2014, A5E02548958A/RS-AA/004
Reading CTT2 data sets
A.1 Acyclic data transfer via CTT2
Appendix
165