Btw, have you checked your DS232 if it is really driven by the serial port, you can connect one into DTR I think and check it when you connect via hyperterminal if the pin state changes state on the TTL side.
If you use MTK and 11,0952MHz quarz the proper communication between PC and DS89C450/4.. can be established at 14,400bps.. so configure your serial port for that speed.
Before you try to establish communication DO NOT FORGET aboyt setting TARGET CONFIGURATION Flash Security to Level 1 (if you leave Level4 you will lock the security bits).
With all this done you can press CTR+L = connect to loader (or TARGET ...) and you will see the following:
DS89C450 LOADER VERSION 2.1 COPYRIGHT (C) 2002 DALLAS SEMICONDUCTOR<0D><0A>
> 89C450 LOADER VERSION 2.1 COPYRIGHT (C) 2002 DALLAS SEMICONDUCTOR
> R<0D><0A>
R<0D><0A>
LB:00 OCR:FF ACON:1F CKCON:01 P0:FF P1:FF P2:FF P3:FF FCNTL:B1<0D><0A>
>W CKCON 0<0D><0A>
W CKCON 0<0D><0A>
<0D><0A>
> W ACON E0<0D><0A>
W ACON E0<0D><0A>
<0D><0A>
> W OCR 0<0D><0A>
W OCR 0<0D><0A>
<0D><0A>
> W LB 7<0D><0A>
W LB 7<0D><0A>
<0D><0A>
>