jdm for pic16f84a
My english is bad but y try to explain it.
If you don´t have care, you can only program one time the pic with JDM.
If you define outputs RB6 and RB7, when you try to program again, Ic-prog first give power to Vcc, inmediatly the program in the pic define outputs RB6 and RB7 and when Ic-prog try to program the pic, have a short circuit and you can´t program.
The solucion is take care for define inputs RB6 and RB7.
If the pic is already this programmed, you need a programmer with supply voltage for erase, and later take care with your program.
Regards