Rsko, I would suggest looking at the available Dynamic Simulation Arrays in section 13.4.2 in the Program Application Guide Volume 2 (PAGV2.pdf) for internal arrays/variables that are available to you (also 21.1.2 in the Program Operation Manual (POM.pdf)). Section 24 in the same document will give you an idea of how to model transfer functions in Fortran. The Model Writing section in the POM also has some good information, which I am guessing is what you are looking at now. Finally, most (all?) API routines are available in Fortran. If you take a look at the Application Program Interface document (API.pdf), the Fortran command syntax is listed for each individual API. (i.e. CALL BRANCH_CHNG(IBUS, JBUS, CKT, INTGAR, REALAR, IERR)
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