Data from devices connected with modbus rtu does not come from time to time? zero is coming. why?
Try with increasing of Timeout value.
In your code I can see 100 ms which looks small. Try with 1000 or 2000.
Another advise is about task time for application where you execute your COM_MOD_MAST. For such task I always set fixed task time, e.g. T#100ms, and not freewheeling.
Create logic to memorize error status code from COM_MOD_MAST block (done+err for 1 cycle only). You can observe the error code and see the reason for lost in communication.