Looking inside a 1970s PROM chip that stores data in microscopic fuses (2019)
5 hours ago
- The MMI 5300 is a 1024-bit PROM from the early 1970s, programmed once by blowing fuses.
- Data is stored in a 33×33 array of diodes and fuses; intact fuses represent 1, blown fuses represent 0.
- Address decoding splits into column selection (A3-A7) and output selection (A0-A2) to access 256 4-bit words.
- Programming requires high-voltage pulses to melt Nichrome fuses with precise timing to avoid damage.
- Testing was difficult due to irreversibility; an extra test row and column were included for verification.
- Unused transistors on the die allowed the same layout to be used for different variants (e.g., open-collector vs. tri-state).
- PROMs were replaced by EPROMs and later EEPROMs; modern flash offers billions more storage at lower cost.