you could try using an external display (Apple vision) to see if it will display any information there

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I wouldn't. Even though the Portable uses the same DB-15 video out port as other Macs from that era, the interface is completely different, using a digital signal rather than analog. Unfortunately, as far as I'm aware, no monitors were actually made that support the signal from that port, and connecting any display designed for use on other Macs to that port may cause damage to either the display, the Portable, or both.