Here is a possible solution.

By means of an asynchronous pulse generator, a counter is incremented by 1 every second until it reaches the value of 1800 (1800s = 30min).
In the arithmetic instruction, the result from the counter x 0.2778 (50.0 ° C / 1800 = 0.2788) is added to the starting value of 200.0 ° C.