[TRNSYS-users] How to compose a new Type component for iterative calculation?

[Yun]

Dear TRNSYS user

I want to compose a new component deals with the re-cyclic data 
flow.  It means that the component should produce the optimized 
output for met the pre-defined range.  For an example, the 
operating rate(Actual value) of an exhaust fan in Type157(COMIS)
 should have to be controlled according to not only the indoor 
pollutant concentration but also the infiltration airflow rate 
for optimizing the operating condition of fan in terms of energy
 saving.  In this case, the pollutant concentration from Type157
 should be linked to the inputs of the component that pursues 
the optimized condition of fan for met the standard of pollutant
 concentration.  Of course, the iterative calculation for 
checking the changed pollutant concentration and infiltration 
airflow rate under the time varying operating condition of fan.
The question is, first of all, the iterative calculation can be 
occurred at each time step?
And do I have to use DO LOOP or some special statements(for 
example, needs to use a subroutine or some variables) when I 
compose FORTRAN file?
(Indeed, I am a beginner of making a new component.  Easy reply 
may be delightful..)
Any advice, information will be helpful to me.
Thank you.

Many regards