INTERNAL CHIP ORGANIZATION

• A memory chip is an electronic component in which a program, data or both can be stored.
• The program / data are stored in the memory chip as a series of numbers - zeros and ones (=bits). 
• A bit can be either a zero or a one. 
• Sixteen bits are a "word", eight bits are a "byte" and four bits are a "nibble".

ORGANIZATION OF A MEMORY CHIP

• For the 8 bit memory chips (the most common type) the bits are put together in a byte (= 8 bits) and stored under an "address". The bytes can be accessed at this address and then the eight bits of the accessed address are output on its eight data ports.

EPROM memory chips (27 / 27C...)

• EPROM stands for Erasable Programmable Read Only Memory. "Erasable" means that the data on it can be removed.
• "Programmable" means that a program or data can be programmed (burned) into this chip.
• "Read Only Memory" means that this type of memory can be read out but not programmed in the target device.
• EPROMs can also be programmed repeatedly without being erased as long as the bits are only changed from one to zero or remain on zero.
• Since the quartz glass window required for erasure with UV-C light is a big part of the production costs for the chip. Without the window, the chip cannot be erased using UV-C light.
• The EPROMs with windows are also called UV EPROMs; the ones without windows are called OTP (=One Time Programmable) EPROMs.
• After programming an EPROM that is erasable with UV-C light, the glass window should be closed with a sticker so no sunlight can enter. Sunlight also contains components of UV-C light and can eventually erase data from the EPROM.
• In the name of an EPROM, the "C" after the 27 indicates that it is a CMOS EPROM (CMOS=Complementary Metal Oxide Semiconductor).

EEPROM memory chips (28C...)

• The name EEPROM stands for Electrically Erasable Programmable Read Only Memory.
• These are constructed like EPROMS, but allow the erasing of individual bytes or the entire memory space electrically without UV light.
• Since individual bytes can be erased without erasing everything, these individual bytes can be overwritten, in effect.
• The burning process clearly takes longer than with an EPROM - up to several milliseconds per byte.
• To make up this disadvantage, EEPROMS with a function for the programming (called blocks). In this process, 64, 128 or 256 bytes at once are loaded into the memory chip and programmed simultaneously as a block. This clearly shortens programming times.

FLASH EPROMS (28F..., 29C..., 29F...)

• These chips can be erased electrically - completely or by the block.
• The Flash EPROMS, however, cannot always be used as a replacement for a normal EPROM.
• A 28F256 with 32 pins is therefore not pin compatible to a 27C256 with 28 pins and the same memory capacity.

Serial EEPROMS (24C..., 25C..., 93C...)

• Serial means that data output and address naming takes place bit by bit (=serially). 
• This means that only one bit at a time can be accessed, and the accessed address must be communicated bit by bit as well, but it has the major advantage that the serial EEPROM comes with a small 8-pin housing. 

RAM (52..., 62...)

• The name RAM stands for "Random Access Memory"
• These memory devices can be written to very quickly and each byte can be overwritten just as quickly and easily, i.e. it does not need to be erased first. 
• The disadvantage of this technology is that the chips lose their memory space when the power supply is cut off.

NVRAM (48Z..., DS12..., XS22...)

• The name NVRAM stands for Non Volatile Random Access Memory.
• The major advantages of the NVRAM chips
• Very high speed and easy overwriting of existing data.
• Retain their data when power is cut off.
• This can be achieved in two ways:
1. The first group removes the disadvantage of the original RAMS with a built-in battery that protects the memory space from losing its data when the power is cut off. According to the manufacturer, the battery lasts for ten years according to type.
2. The second group has an equally large EEPROM and when the power is cut off, it stores all data from the RAM on the EEPROM. When the power is restored, data are EEPROM copied back into the RAM. The advantages of fast RAM access and easy overwriting remain.

Erasure of EPROMs with ultraviolet light

• Erasure takes place by exposure to intensive ultraviolet light in the area of 254 nm wavelength.
• These only allow the light to be turned on after the housing is closed.
• Erasure takes 5 to 25 minutes, varying with light intensity and other conditions.

MEMORY CHIP NAMES AND HOW TO FIND REPLACEMENT CHIPS

• The name of a memory chip contains 
• The abbreviation for the manufacturer
• The technology
• The memory size
• The fastest permitted accessing speed
• The temperature range
• The form of housing as well as further internal manufacturer's data.
• A replacement type should utilize the same technology (EPROM/ EEPROM/ FLASH/ etc.), have the same size of memory and the same or a shorter access time and if applicable, the same or a better temperature range.