I go curious and had a look at the datsheet. While Prahlad is absolutely right about the lack of SoftICE details, I found the following:
Following reset, the P89LV51RD2 will either enter the SoftICE mode (if previously enabled via ISP command) or attempt to autobaud to the ISP boot loader. If this autobaud is not successful within about 400 ms, the device will begin execution of the
user code.

This is, in my opinion, the way to enter ISP, no 'funny signals', no 'must be low at reset' etc, just connect the cable, reset, and the ISP takes place.

Erik