programmable read-only memory
A type of read-only memory which can be written to once (write once, read many memory/WORM), and thereafter becomes fixed in value.
PROM: blank ROM that you burn data into once
A programmable read-only memory (PROM) is a semiconductor chip that arrives from the factory blank and empty. Unlike a standard ROM, which is manufactured with its data already baked into the silicon at the foundry, a PROM lets you write your own pattern of 1s and 0s exactly once. After that write operation, the data is permanent and cannot be altered or erased. This write-once, read-many (WORM) behavior made PROMs valuable for prototyping and small production runs before committing to the expense of masked ROM fabrication.
The programming process itself is a destructive one at the bit level. Early PROMs used fusible links, tiny wires at the intersection of address and data lines that literally melted when exposed to overvoltage pulses, thereby breaking the connection and encoding a 0 (or 1, depending on design). A PROM programmer, a dedicated benchtop tool, supplied these high-voltage pulses in a controlled sequence. The process typically took seconds to minutes per chip, depending on capacity. Reprogrammable variants like EPROM (erasable PROM), which used floating-gate transistors and required ultraviolet light to reset, appeared in the 1970s and gradually displaced fuse-based PROMs for applications where design changes were frequent.
Capacity ranged from a few hundred bytes in early devices to 32 KB or more by the 1980s, when the technology was at its most widely used. A typical fused PROM might contain 1024 or 2048 bits arranged in a matrix of addresses and data lines. The actual silicon die was small, but the ceramic or plastic package surrounding it was far larger because bonding pads and leads had to provide access to each address line and each data line separately, plus power and ground. Heat was never the problem it is with newer memory; the voltage pulses were brief and the total energy per write operation modest.
Why PROMs matter in industrial contexts
In industrial equipment manufactured from the 1970s through the 1990s, PROMs stored small control programs, lookup tables for analog-to-digital conversions, and calibration constants. A simple CNC machine or measurement instrument might contain one or more PROMs alongside a microprocessor. When a bug was discovered in the field or a calibration needed updating, technicians had to physically remove and replace the chip, then reprogram a blank one in the service shop. This made PROM-based designs less flexible than later designs using EEPROM or flash memory, but also more robust against corruption: once programmed, the data could not be accidentally overwritten by a software fault.
Scarcity of replacement PROMs and programmers is now a real issue for maintaining vintage industrial hardware. Fuse-based PROM programming equipment is obsolete and rarely serviced. If a control board fails and the PROM is unreadable or the source code lost, repair becomes difficult. Some engineers working on legacy systems now copy PROM contents to microcontroller flash memory or design adapter boards to bridge old PROM sockets to modern memory modules, a workaround that extends equipment life when original parts are unavailable.