Hello, I want to use user-defined model with fortran

I use PSS/E V34.2.0

and, I want to make user-defined model

So, I will learn about fortran.

but I don't know what fortran version is used in PSS/E after version 33.0

fortran77? fortran90? fortran95?

here is an example fortran source code

.flx file

  INTEGER   IARG,   ISLOT
  INTRINSIC MAX
  EXTERNAL  BADMID
  INTEGER   I,      J,      K,      JJ,     KK,
 *          IB,     IBUS,   IBPTR
  REAL      VERROR, VR,     DEL2,   FDBK,   SS,    R_TF_KF
  LOGICAL   HIGH,   LOW,    NEW
  CHARACTER CB1*6,  CB2*6
  CHARACTER IM*2,   VLTI*4
  IF (MODE.EQ.8) GO TO 1500                   ! GET DATA DESCRIPTIONS
  I=IARG                                      ! MACHINE ARRAY INDEX
  J=STRTIN(1,ISLOT)                           ! STARTING 'CON'
  K=STRTIN(2,ISLOT)                           ! STARenter code hereTING 'STATE'
  IF (MODE.GT.4) GO TO 1000
  IB=NUMTRM(I)                               ! RETURN IF OFF-LINE
  IF (IB<=0) RETURN                        ! OR SVS OR INDUCTION MACHINE

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I think this example is made by fortran77

so, I have to learn fortran77?

please tell me the answer TT