Siemens Simatic S7-300 CPU 31xC Series Installation And Operating Instructions Manual page 202

Debugging functions, diagnostics and troubleshooting
10.7 Diagnostics of DP CPUs
Interrupts with another DP master
When CPU 31x-2 operates with another DP master, an image of these interrupts is created
in its device-specific diagnostic data. You have to post-process the relevant diagnostic
events in the DP master's user program.
Note
In order to allow the evaluation of diagnostics and process interrupts by means of device-
specific diagnostics using a different DP master, please note that:
The DP master should be able to save the diagnostics messages to its ring buffer. For
example, if the DP master can not save the diagnostic messages, only the last incoming
diagnostic message would be saved.
In your user program, you have to poll the relevant bits in the device-specific diagnostic data
in cyclic intervals. Make allowances for the PROFIBUS DP bus cycle time, for example, to
be able to poll these bits at least once and in synchronism to the bus cycle time.
With an IM 308-C operating in DP master mode, you cannot utilize process interrupts in
device-specific diagnostics, because only incoming events are reported, rather than outgoing
events.
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S7-300, CPU 31xC and CPU 31x: Installation
Operating Instructions, Edition 08/2004, A5E00105492-05