I have several ADuC841 devices that are on a common serial programming bus. This is used for field reprogramming of the device in a much larger system.
A host processor port is asseting the psen line low through a schottky diode and 1K resistor to each ADuC. All other devices are kept idle except the one being programmed. Works well but I wanted to confirm the voltage required on the PSEN pin to get the device to enter program mode. Would like to use a plain vanilla Si signal diode instead of a Schottky but need to determine the voltage margins. The diode serves to isolate the normally low state of psen from disturbing the other devices.