Looking inside a 1970s PROM chip that stores data in microscopic fuses (2019)

A detailed teardown of the MMI 5300, a 1970s bipolar PROM chip that stored 1024 bits using microscopic Nichrome fuses. The chip was programmed by blowing individual fuses with high-voltage pulses, with intact fuses representing 1s and blown fuses representing 0s. The die photo reveals a 33×33 fuse-diode array, address decode logic using multiple-emitter transistors, DTL multiplexers, and an extra row/column for factory testing. Blown fuses leave a crack roughly 700nm wide — barely visible under a microscope. The same silicon die was used for both open-collector and tri-state output variants by changing only the metal layer wiring. At $70 for 128 bytes in 1971, the chip illustrates how dramatically storage density and cost have improved over five decades.

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