مهندس : SAM
First : Cyclic (watchdog) interrupts are used for executing blocks at fixed intervals. The
cyclic interrupt OB for the S7-300™ is OB 35.
The default call interval for OB 35 is 100ms. You can change this to a value
within the permitted range of 1ms to 1 minute.
Kindly check PIC
Second : Normally when the CPU is in Run mode, OB1 is executing periodically. If an interrupt happen ( cyclic, time of day, hardware or error interrupt), The processor stop executing OB1 at a break point, and starts to execute the interrupt and after finishing it will execute the OB1 from the break point.
If an interrupt happen and during this interrupt, another interrupt with a higher priority happen, then first, the processor will stop executing OB1 at a break point 1, and execute interrupt, Now another interrupt happen before finishing the first interrupt, the processor will stop executing the interrupt and goes the the second one ( with the higher priority) and after finishing, it will complete the first interrupt and after finishing it will complete executing OB1.