[SiriusHardware]

To be more specific about pin 21, it isn't totally obvious from the circuit in the immediate vicinity of the PROM what state the _INIT line (pin 21) will be in, but there are clues.

If you follow that _INIT line around the rest of the circuit sheets you'll see that it goes to a lot of chip pins which are active-low clear or reset inputs on the chips in question - (that is, when the _INIT line is in the low (0V) state all those ICs are cleared, or held, or reset). It wouldn't make any sense for all those ICs to be held or reset while the system is running normally, therefore we can deduce that the usual state of the _INIT line is high, or somewhere near +5V. The _INIT line most likely goes low for a short time after power-up or reset, then returns high and stays high.

The way the _INIT line is labelled, with a bar drawn above the letters, means that the line performs its named function when it is taken low, rather than when it is taken high.

You could verify that by just measuring the voltage on pin 21 of that PROM when the system is powered - you should find that it is high / logic 1. If so, your reader also needs to hold that PROM pin high when it is trying to read the PROM.