Hi all,
The datasheet of P89LV51RD2 says that:
"When the Security Bit is activated all parallel programming commands except for Chip-Erase are ignored (thus the device cannot be read). However, ISP reads of the user
Yes, it is the serial number functionality that can disable ISP. When the serial number is programmed you cannot use ISP, except for erase, unless the serial number is entered or the device is erased.