mycroes/Sally7

Connection status PLC

Closed this issue · 2 comments

CommissarClippy commented

The PLC.TcpClient.Connected does not return a valid value for whether the PLC is currently connected. The value returns true before data could be actually read. Is there a way to actually check the connection status that the PLC is ready for communication? Currently I am simply repeating the read until I do not receive an exception.

gfoidl commented

From TcpClient.Connected (emphasis by mine):

The Connected property gets the connection state of the Client socket as of the last I/O operation. When it returns false, the Client socket was either never connected, or is no longer connected.

Is there a way to actually check the connection status that the PLC is ready for communication?

This is actually a more general problem of TCP and not specific to Sally7.
Afaik the only way to reliable check if it's ready for communication, is to send a (dummy) request. If it succeeds, then it's ready -- but note that a subsequent request may still fail due to network-errors, etc.

The best is to just try, and in case of failure re-try. Therefore you could use some library like "Polly".

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mycroes commented

In addition to the comment from @gfoidl, if you perform an Open() (and it doesn't throw) and then check the Connected prop it should be true. At that point you actually are connected, I'm not sure if there's any way to check if the PLC is ready for communication.

In our applications we have a check whether exceptions are transient, but once I have successfully read a variable without any exception (retrying on transient exceptions only before that) I store that known-good state along the variable. If a PLC update is performed and there are intermittent address out of range exceptions I just ignore those and assume it'll all be good again at some point.

I'm closing this issue because I think it's all working as expected, but feel free to reopen with some more detail if you need further assistance.