Some reasons why your code isn't working are;
1) Immediate constants in Assembler must be preceeded with a (numeric) 0 when the constant starts with a letter. #ECh must be coded #0ECh.
2) Your START: routine flows right into your ISR and causes a RETI to execute without an interrupt.
3) Additional routines UP: and CONV: end in RETI, must use RET.
4) You have no MAIN routine to interrupt from. Reduce the size of your ISR (interrupt service routine) to keep it as small as possible and place most of the code in a MAIN routine.

Hal
5) Placing the Stack Pointer at 60h leaves little room for your stack. Unless you really, really need it that high, I would leave it where it defaults to (07h) until you are more comfortable with Assembler.