PROM
Acronym of programmable read-only memory, a chip that can be programmed one or more times (depending on the type) and generally cannot be re-programmed without special equipment (if at all); once programmed and installed in a device, it functions as a read-only chip (as if it had been hard-wired to function that way).
PROM: one-time or limited-reprogram chip memory
A PROM is a semiconductor memory chip that holds data permanently after being programmed, without requiring continuous power to retain that data. In power generation, distribution, and control systems, PROMs store firmware, calibration tables, protection logic, and device configuration. Once programmed and installed, the chip functions as read-only: the processor reads from it but cannot alter it during normal operation. This immutability is the defining feature that separates PROMs from other memory types.
The name "programmable" refers to the manufacturing step, not field use. A PROM is blank when it leaves the chip fab. A specialized programmer device, using higher-than-normal voltages, writes data to it by permanently burning fuses or altering dopant patterns inside the silicon. The burning is irreversible in most PROM types. Once programmed and soldered into a relay, protection module, or generator control unit, that chip is fixed for the life of the equipment, or until deliberate removal and replacement.
Variants differ in reprogrammability. Standard PROMs can be programmed once only (OTP, one-time programmable). EPROMs (erasable PROMs) can be reprogrammed if removed from the circuit and exposed to ultraviolet light for minutes, then reprogrammed. EEPROMs (electrically erasable) can be erased and reprogrammed electrically, but still require special equipment and are slower to reprogram than RAM. Flash memory, which dominates modern designs, blurs the boundary by allowing in-circuit programming while retaining non-volatility. In older industrial control systems, PROMs and EPROMs remain common because the equipment itself does not change, and field updates are rare or zero.
Protective relays, automatic voltage regulators, and metering units rely on PROMs to store the logic sequences and thresholds that define how they respond to faults. A PROM holding trip curves for a motor protection relay cannot be accidentally overwritten by noise or transients. This reliability is critical in substations where a spurious memory change could cause blackouts or equipment damage. The non-volatile nature also means the device restarts with the correct behavior after a power loss, with no risk of memory corruption.
The trade-off is inflexibility. If a relay's settings must be changed, the chip must be physically removed, reprogrammed on a bench with a specialized programmer, and reinstalled. For distributed systems with thousands of nodes, this cost constrains field modifications. Modern replacements use flash memory or firmware downloaded over networks, but legacy systems still depend on PROMs. Technicians must identify the chip type, locate a programmer (not standard office equipment), and ensure data integrity before reinstalling to avoid cascading failures in the grid.