i am driving 741 with a zenor and preset circuit Iam getting 5v at output of 741 opamp ..... instead of taking 9v from external source, i wish to take from MAX232 , is it ok

1) a zener is NOT good enough for this purpose, you need a device specifically designed for reference use, (check Maxim for "voltage references")
2) Still using the 741? that is not the most steady OP-amp today - as a matter of fact it is an order of magnitude worse than what should be used for precision circuits.
3) If you are driving the voltage reference with a mA or less, you may get by with using the UART 232 for 9V, otherwise use a separate 232. DO NOT drive the voltage reference with less than optimum current to "save a 232" that will defeat the purpose.

If you are only interested in 6-8 bits of precision, ignore the above.

Erik