Read-only memory (ROM) is similar in design to static or dynamic RAM circuits, except that the “latching” mechanism is made for one-time (or limited) operation.
The simplest type of ROM is that which uses tiny “fuses” which can be selectively blown or left alone to represent the two binary states.
Obviously, once one of the little fuses is blown, it cannot be made whole again, so the writing of such ROM circuits is one-time only. Because it can be written (programmed) once, these circuits are sometimes referred to as PROMs (Programmable Read-Only Memory).
However, not all writing methods are as permanent as blown fuses. If a transistor latch can be made which is resettable only with significant effort, a memory device that’s something of a cross between a RAM and a ROM can be built.
Such a device is given a rather oxymoronic name: the EPROM (Erasable Programmable Read-Only Memory). EPROMs come in two basic varieties: Electrically-erasable (EEPROM) and Ultraviolet-erasable (UV/EPROM).
Both types of EPROMs use capacitive charge MOSFET devices to latch on or off. UV/EPROMs are “cleared” by long-term exposure to ultraviolet light. They are easy to identify: they have a transparent glass window which exposes the silicon chip material to light.
Once programmed, you must cover that glass window with tape to prevent ambient light from degrading the data over time. EPROMs are often programmed using higher signal voltages than what is used during “read-only” mode.
Initially, AC motors were constructed like DC motors. Numerous problems were encountered due to changing…
Learn about the AC Instrumentation Transducers like Potentiometer, LVDT, RVDT, Synchro, and Capacitive Transducers.
AC bridge circuit unknown impedance is balanced by a standard impedance of similar type on…
Power Quality is the general term given to represent an AC power system freedom from harmonic…
Hall effect - Voltage is proportional to current and strength of the perpendicular magnetic field.…
Learn about the Frequency and Phase Measurement from our free online electronics and electrical engineering…