QUESTION:  
What should you watch out for when using the SFB75 (SALRM) on an ET 200S CPU (IM 151-7)? 

ANSWER:  

1. Version of the ET 200S CPU
The SFB75 "Send alarm to the DP master" is supported by the following versions:

ET 200S CPU	From version	Order number
IM 151-7 CPU	V2.0	6ES7 151-7AA10-0AB0
IM 151-7 F CPU	V2.0	6ES7 151-7FA00-0AB0

Table 1: Minimum version of the ET 200S CPU that supports SFB75 

2. The ET 200S CPU must be incorporated as S7 slave
In order to be able to use the SFB75 (SALRM) for sending an alarm to the master, the ET 200S CPU must be incorporated as S7 slave.

An ET200S CPU is incorporated as an S7 slave by defining it with the master in the same configuration. For this the ET 200S CPU must be incorporated on the master via PROFIBUS-DP > Stations already configured > ET200S > CPU. More information on this is available in section 7.1 of the manual "ET 200S IM 151-7 CPU Interface Module" in Entry ID: 12714722.

Note:
If you incorporate the ET 200S CPU via a GSD file, no alarms can be sent. This means that you cannot use SFB75 on a DP V0 master, like the CP 342-5, for example. In this case, the value W#16#8091 is output in the SFB STATUS.

3. Enabling diagnostics for modules
Detailed diagnostics of a module can only be read out if enabled accordingly.

Fig. 1: Enabling diagnostics taking the example of an analog module

4. Calling the SFB75 affects the SF LED on the slave
If you send a diagnostics alarm via SFB75, the operating system of the ET 200S CPU affects the local module status information (DS1 and SZL ID W#16#xy91) and the LED SF through Bit 0 in Byte 0 of AINFO. However, there is no diagnostics buffer entry in the ET200S CPU and no diagnostics alarm OB is started.

5. SFB75 can send a maximum of 16 bytes of additional alarm information to the master
A maximum value of 16 can be specified for the LEN parameter of SFB75. Thus a maximum of 16 bytes of additional alarm information is possible. But the data record of a module of the ET 200S can be up to 44 bytes long. The information required must therefore be shortened to these 16 bytes. Here the first 4 bytes must not be changed, because OB82 of the master uses them to generate a valid diagnostics buffer entry.

Notes: 
The 16 bytes sent previously are read on the DP master by SFB 54 "RALARM".
Please note that when SFB75 sends, data record 1 for the relevant virtual slot is set to the value set on the LEN parameter. This means that with SFC 59 "RD_REC" on the master not all 44 possible diagnostics bytes can be read.

6. The SFB works asynchronously
The SFB75 "SALRM" works asynchronously, i.e. processing lasts several SFB calls and therefore several OB1 or PROFIBUS cycles. 
With the ET 200S CPU a maximum of 34 SFB calls belonging to different jobs can be processed quasi-simultaneously.

7. The alarm works only on the virtual slots
The DP master does not recognize the setup of the ET200S CPU station. Only virtual slots are configured on the master. An alarm can therefore only be sent via these virtual slots.

8. Effects of the data traffic on the PROFIBUS
Calling the SFB75 in the ET200S CPU generates on the PROFIBUS a high-priority answer message of the I slave (ET200S CPU) to the DP master. The result is that the DP master requests diagnostics from the DP slave. Thereupon the DP slave sends a diagnostics message to the DP master. Attached to this diagnostics message is the data form the AINFO parameter of SFB 75 (call in the I slave). Then the master issues an alarm that the slave acknowledges.