Reset generator
The Mitsumi PST518B is a reset generator, it holds the NeoGeo in reset as long as the power supply is below 4.4V.
It has an open-collector output tied to the RST line (not RESET !), which is shared with the reset button on the home system. The line goes to NEO-B1.
Not to be confused with the watchdog reset.
Internal schematic
Datasheet: File:PST518B.pdf.
Replacement
Not pin-compatible: TL7757CL