Siemens S7-300 Installation And Operating Manual page 290

Analog modules
9.3 SM 336; F-AI 6 x 0/4 ... 20 mA HART
Structure of the command data record
The figure below shows the command data record with which you can write a command to a
client's transfer area. The HART analog module sends the command to the connected HART
field device.
Figure 9-48
Notes on the command
The same client cannot send another command before reading the reply to the previous
command.
Notes on the reply
When the reply data record is read, you must make sure that a current reply data record has
arrived.
If the processing status is "successful" or "faulty", the data record contains current reply data
or error displays.
290
Command data record of the HART analog module
Installation and Operating Manual, 01/2010, A5E00085586-10
Fail-safe signal modules