Multitasking

TonyH110

When using PERS variables shared across say two Tasks I assume they are read/write thread-safe ?

lemster68

IF You mean that the values in all tasks are updated the same?  Then yes.

TonyH110

No meant when two asynchronous Tasks (or threads) simultaneously access a common variable there's usually a system error - have seen it on Staubli robots (a problem I had to fix for a client). This is done by using a mutex or semaphore (ABB has such the WaitTestAndSet instruction). Usually IO is thread-safe, the OS handles collisions - I assume (hope) PERS values are handled like IO

lemster68

In that case, if you do have separate tasks changing values, (which is probably best to try to avoid), the WaitTestAndSet is a good option.  Or, like you mentioned IO, set a bit when one task is modifying a value, and all others must wait for that bit to go low again.