Here is a link to a schematic on the RS-232 connection:
h**p://chaokhun.kmitl.ac.th/~kswichit/MAX232/MAX232.htm
However, once you have samples from MAXIM-IC (h**p://www.maxim-ic.com/quick_view2.cfm/qv_pk/4078 .. at the bottom you will find SAMPLES CART, and you will need to register, but it is harmless), alow on yor new PCB some room not only for MAX232 but for 74125, which will allow you not only program the DS89C450 but execute program from the same board ..
Download MTK (microcontroller tool kit) from here:
**broken link removed**
..install it and goto HELP where you will find harware connectrions of MAX232, 74125 and microcotroller ..
Any questions ..just ask ..