Hi,

I got a source file of an self written FB for a device (compact slave). In this FB the data from the device is read/written with SFC14/15. I wonder now why SFC14/15 is used. I mean, why the data is not taken from the process image. I read the help of those SFCs and also the chapter "data consistency", but I don't understand why the data could be inconsistent when reading from the process image.

Btw...the block is for an S7-400 CPU. It's called in an cyclic interrupt (OB35 = 100ms). The device's I/O data is updated in a PIP which is assigned to OB35. The device's data is 20 bytes IN and OUT (only one submodul in HW-config). Is it really useful to use SFC14/15 in that case?

If course, I would have to split the data, in e.g. in the symbol table, as I can read/write max. only a DWORD from/to the process image. But this doesn't matter in my opinion as the data from to the device is organized in WORDs.

Another question is: If SFC14/15 is used, does it mean the data is read/written from the device in the moment of calling SFC14/15? Or is it also taken from the process image (but at once)?

Would be nice if anyone could share his knowledge.

Hi,

He uses the SFCs on the OB35 in order not to call always the PUT/GET instructions.  He also joint the data in DWORDS in order to transfer completely blocks.

Hi,

thanks for your reply, but honestly said, it's not really giving answers to my questions...

PUT/GET is not an option as the device can't act as a S7-data server (it's just a frequency converter).

Actually in HW-config the device has only one slot configured where all bytes (20 bytes for PPO type 6) are included. As with SFC14/15 always the entire slot is read/written, all 20 bytes are read/written at once. It would also be possible to configure the same PPO type with 5 slots containing each a DWORD. In that case I could read the data via the process image...It isn't necessary to read/write this 20 bytes consistent. It's enough to read a WORD (or a DWORD) consistent.

Still don't know what the advantage of using SFC14/15 is, in compare to reading the data from the process image... Also still would like to know if SFC14/15 is interrupting the code execution to read the data immediately...

Hello,

I recommend everytime simple Step7 Professional TIA documentation (also V14SP1 f.ex.), in this case doesn't matter if you don't have this version, it's only perfect documentation about some required infos. If you search for process image or SFC 14, you can find answers to your questions, also you'll improve your knowledge about problematic. Try that...

 Hi,

mopS04

hi,

using the DPRD_DAT and DPWR_DAT does not mean the the process-image is not consistent. The process-image is always consistent. But when you use SFC14/15 you have an actual access to this adresses. The process-image of inputs is reading before the OB1 cycle and the process- image of outouts is writing after OB1 cycle. So when you have a long cycle-time, the process-image-inputs are not actual and might be as old as the cycle-time is. Even the outputs of the process-image would be just written after OB1-cycle is over.

hope this helps,

regards, 

Your contribution is confirming what I expected in regard to "when the data is updated".  Also I found this FAQ now, saying it's more performant (...transfer commands provides a particularly convenient and high-performance method of access (low runtime load)) to read from the process image. My conclusion is now: If it sufficient in regard to the update time, better read/write from/to the process image instead of using SFC14/15. Btw, we are working with PIPs. The block will be put in OB35 and the PIP is assigned to OB35, too. So the inputs are read before processing OB35 and the outputs are written after processing OB35. We will have a lot of instances and I fear wasting to much processing time, when always reading/writing with SFC14/15.

Thanks guys