Read out a struct with S7+/OPC UA Siemens 1500 CPU

jptdbe · Post by **jptdbe** » Mon Sep 23, 2019 11:24 am

Dear all,

We wanted to communicate with a Step7 Plus or OPC UA driver with an Siemens 1500 CPU.

In the PLC we have defined a struct with 16 bits inside. We want to read out the struct in WinCC OA (like a struct). Which adress config do we need to use?

Thanks a lot!

leoknipp · Post by **leoknipp** » Wed Sep 25, 2019 12:45 pm

This thread seems to be the same like
viewtopic.php?f=16&t=10762

Please have a look at the following thread:
viewtopic.php?f=16&t=10732&p=966029&hil ... ex#p966026

For me it looks like in this thread it is described that the usage of a bit struct is not possible when using the S7+ driver.

Best Regards
Leopold Knipp
Senior Support Specialist

ivan141 · Post by **ivan141** » Fri Oct 11, 2019 2:08 pm

Well, we've extrapolated the repercussions of the inability to subscribe to structs in a real life case where we read about 1500 I/O from the PLC (with S7 classic driver), where the structs in the DB were simply read as integers by WinCCOA (then split to booleans by dp_fct's).

This same project with a OPC-UA client connected to the onboard OPCUA server of an S7-1516 PLC netted us the following results:
-the same project now requires about 4600 I/O, note: we are now only reading booleans we really need (no unused bits subscribed).
-the S7-1500 OPC-UA server cannot cope with the 4600 item load at a 2s internal polling rate (GoodOverload status values all over the place)
The maximum recommended item load for the PLC according to Siemens technical documentation is 2000 subscribed items at a 1s polling rate...

To access any serious amount of data in an S7-1500 you NEED to be able to subscribe to struct nodes somehow (assuming we do not recode everything to use arrays, which would render the code less readable). Subscribing to a struct node should count as 1 item for the OPCUA server if I'm not misinformed, much less than the 16 (and sometimes 32) bits that we need to subscribe now.
Also consider that this is a small project, we can and do fit projects 2 to 3 times bigger than the one mentioned above in S7-1500 plc's.

I'll be trying out the S7plus driver next, I can only hope it performs better so the effort of converting my codebase was not a complete waste of time.

ivan141 · Post by **ivan141** » Mon Oct 14, 2019 11:40 am

The S7plus driver supports up to 8000 subscribed items for a 1516 CPU, better, but still too little for large projects.
Afraid we need a different PLC programming paradigm if we ever want to make the transition from S7 classic to its more modern counterparts.

gschijndel · Post by **gschijndel** » Fri Oct 18, 2019 8:22 am

The (classic) S7 driver could also be used to connect to a S7-1500 in compatibility mode.

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