how to recover from an intermittent TCP timeout exception #70
Description
What is the proper way to recover from a snap7 exception ISO : An error occurred during recv TCP : Connection timed out?
If I place a db_read() into a try/except clause to keep the application from aborting on TCP timeout, subsequent db_read() become out of sync. It seems somewhere the calls are queued. Probably in the native snap7 thread.
For example: If I have three locations I am reading from a PLC
3,260,4 contains value 100
3,272,4 contains value 200
1,104,4 contains value 300
-the first db_read(3,260,4) call successfully returns a value 100
-second db_read(3,272,4) call times out with TCP timeout exception
-third db_read(1,104,4) call will return value 200 (Not 300 like you would expect)
..
-a fourth call of db_read(3,260,4) will return the value from the third call above (300)
It seems the method calls get spooled into a queue and for every call that times out, the data returned will be delayed that many calls in the future.
Is there a way to throw away the buffer on a tcp timeout? Or is there some other philosophical approach I should be taking? It seems very dangerous to call a function with specific arguments and get the wrong data back. Thanks for any suggestions.