Harmony 2 problem with SYS_TMR_CallbackPeriodic() - SOLVED
The function SYS_TMR_CallbackPeriodic which appears as part of the Harmony 3 suite when you create a TCP/IP application looks like a useful way for your application to set a Periodic callback.
However, the way it is written, it is clearly intended to be used only once.
It has no checks to warn if a second user tries to use it, and no warnings in the documentation (that I could see).
If - as I did - you try to use it for your application, you overwrite its intended single use - and your TCP stack stops working!
Instead, you must use the underlying SYS_TMR functions such as SYS_TIME_CallbackRegisterMS. These allow multiple timer objects and it all works sweetly.
I am posting this in case it helps another tyro like me waste two days' debugging time.