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S7NetPlus / s7netplus

S7.NET+ -- A .NET library to connect to Siemens Step7 devices
MIT License
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Snap7 server only supports reading 20 dataItems in a single request #157

Open mycroes opened 6 years ago

mycroes commented 6 years ago

Snap7 server only supports read of up to 20 dataItems in a single request. I'm not sure why they chose this, but I did email the author of Snap7 to see if I can get an explanation.

Although at first this doesn't really seem to matter much, it doesn't allow us to create unit tests for more than 20 DataItems in a single request. One way to fix this might be to add server support to S7NetPlus or Sally7, but honestly I think Sally7 is the easier candidate here since all the structs are already mapped.

rapha-dev commented 6 years ago

So Snap7 limits this in server support? Then we should add server support or, as you proposed, Sally7 (server) as we can get more than 20 DataItems out of PLCs (except 240b PDUs).

killnine commented 6 years ago

I think @mycroes is saying it'd be easier to add server support to Sally7 right now than to S7N but it doesn't exist yet, correct?

What's the use case for this? Is it causing testing challenges?

rapha-dev commented 6 years ago

@killnine It will affect UnitTests. Currently max. 20 DataItems can be passed to Snap7 server, S7 protocol (and S7-400, 1200 and 1500) can handle more. S7netplus should allow maximum possible DataItems and so a UnitTest should proove that (PDU constraints).

killnine commented 6 years ago

I'm just throwing it out there that it'd be nice to have no external dependencies.... 😄

But maybe that's something for a future release.

thoj commented 6 years ago

This is the same issue I encountered. Tests with real cpus with more than 20 data items work fine if the CPU negotiates a higher pdu size. I suggest that we just use the negotiated pdu size and only enforce the 20 dataitem limit on snap7 tests.