I GOT 90% IN MY DIPLOMA..NOW CAN I ATTEND IIST EXAM WITHOUT JEE

Sorry, to say that you cannot join in IIST after completion of Diploma course.To get admission in the IIST, you need to appear for the Joint Entrance Examination (Main) conducted by Central Board of Secondary Education (CBSE), New Delhi and you must qualify it to be eligible to appear for Joint Entrance Examination (Advanced) conducted by IITs and you need to secure the minimum prescribed category-wise marks in JEE (Advanced) as given below:-

(a) General Candidates (GEN) :- At least 20% marks in aggregate and at least 5% marks in each of the three subjects (Physics Chemistry and Mathematics).
(b) Other Backward Community (OBC)  Non Creamy Layer (OBC-NCL) Candidates :- At least 18% marks in aggregate and at least 4.5% marks in each of the three subjects (Physics Chemistry and Mathematics).
(c) Scheduled Caste(SC), Scheduled Tribe(ST) & Persons with Disabilities(PD) Candidates :- At least 10% marks in aggregate and at least 2.5% marks in each of the three subjects (Physics Chemistry and Mathematics).