Many old devices use socketed EPROMs for ROM and BIOS chips. Erasing these can be annoying and time consuming as they require a 20-30 minute UV erase cycle.

Replacing these chips with EEPROMs or Flash memory provides a convenient way to experiment with ROM and BIOS changes via reduced programming times. Fortunately, in many cases, equivalent parts exist and can still be sourced as New Old Stock or Used devices through parts brokers, Amazon, eBay, and Aliexpress.

27 Series

The 27 series of Parallel EPROMs use 5v (TTL). In many cases a larger memory part can replace a lower memory part by repeating the ROM. While some address lines will potentially float, this ensures they will always resolve to the same memory.

Access speed matters — in some cases replacing an EPROM with a slower EEPROM will cause issues. This is not always the case — sometimes manufactures used faster chips than were required in the circuit. If you want to use caution, always replace an EPROM with an EEPROM or Flash memory with faster (lower) access times than the chip you are replacing.

These EPROMs were made in a variety of technologies over the years, including MOS, CMOS, and NMOS. In most applications EEPROMs or Flash can substitute for any of the three.

These ROMs correspond to the JEDEC pinout standards – generally replacements that follow this standard are compatible.

27x040

Atmel Datasheet

• Packages: DIP32, PLCC32
• Size: 4096kbits (512k * 8) = 512KiB

DIP32 Pinout

      ┌─────◡─────┐
  Vpp │ ●1     32 │ Vcc
  A16 │  2     31 │ A18
  A15 │  3     30 │ A17
  A12 │  4     29 │ A14
   A7 │  5     28 │ A13
   A6 │  6     27 │ A8
   A5 │  7     26 │ A9
   A4 │  8     25 │ A11
   A3 │  9     24 │ OE
   A2 │ 10     23 │ A10
   A1 │ 11     22 │ CE
   A0 │ 12     21 │ O7
   O0 │ 13     20 │ O6
   O1 │ 14     19 │ O5
   O2 │ 15     18 │ O4
  GND │ 16     17 │ O3
      └───────────┘

Example Original Parts

• AMD Am27C040
• Atmel/Microchip AT27C040
• Hitachi HN27C4001G
• Philips/Signetics/ST/NXP 27C040
• Texas Instruments TMS27C040
• Toshiba TC574000D

Replacement Parts

• Winbond W27E040
  • Type: EEPROM
  • Speeds: 90ns/120ns
  • Compatible Packages: DIP32, PLCC32

27C020

Atmel Datasheet

• Packages: DIP32, PLCC32
• Size: 2048kbits (256k * 8) = 256KiB

DIP32 Pinout

      ┌─────◡─────┐
  Vpp │ ●1     32 │ Vcc
  A16 │  2     31 │ PGM
  A15 │  3     30 │ A17
  A12 │  4     29 │ A14
   A7 │  5     28 │ A13
   A6 │  6     27 │ A8
   A5 │  7     26 │ A9
   A4 │  8     25 │ A11
   A3 │  9     24 │ OE
   A2 │ 10     23 │ A10
   A1 │ 11     22 │ CE
   A0 │ 12     21 │ O7
   O0 │ 13     20 │ O6
   O1 │ 14     19 │ O5
   O2 │ 15     18 │ O4
  GND │ 16     17 │ O3
      └───────────┘

Example Original Parts

• AMD Am27C020
• Atmel/Microchip AT27C020
• Holtek HT27C020
• Texas Instruments TMS27C020

Replacement Parts

• Greenliant GLS27SF020
  • Type: Flash
  • Speeds: 70ns
  • Compatible Packages: DIP32, PLCC32
• Microchip SST39SF020
  • Type: Flash
  • Speeds: 55ns/70ns
  • Note: Drop in replacement for read-only applications. For writing, pin 1 should be NC unlike the original 27C020.
  • Compatible Packages: DIP32, PLCC32
• SST SST27SF020
  • Type: Flash
  • Speeds: 70ns
  • Compatible Packages: DIP32, PLCC32
• Winbond W27C020
  • Type: EEPROM
  • Speeds: 70ns/90ns/120ns
  • Compatible Packages: DIP32, PLCC32

27C010

Atmel Datasheet

• Packages: DIP32, PLCC32
• Size: 1024kbits (128k * 8) = 128KiB

DIP32 Pinout

      ┌─────◡─────┐
  Vpp │ ●1     32 │ Vcc
  A16 │  2     31 │ PGM
  A15 │  3     30 │ NC
  A12 │  4     29 │ A14
   A7 │  5     28 │ A13
   A6 │  6     27 │ A8
   A5 │  7     26 │ A9
   A4 │  8     25 │ A11
   A3 │  9     24 │ OE
   A2 │ 10     23 │ A10
   A1 │ 11     22 │ CE
   A0 │ 12     21 │ O7
   O0 │ 13     20 │ O6
   O1 │ 14     19 │ O5
   O2 │ 15     18 │ O4
  GND │ 16     17 │ O3
      └───────────┘

Example Original Parts

• Atmel/Microchip AT27C010
• Fairchild/National Semiconductor NM27C010
• Holtek HT27C010
• Maxwell 27C010
• Toshiba TC571000D

Replacement Parts

• Greenliant GLS27SF010
  • Type: Flash
  • Speeds: 70ns
  • Compatible Packages: DIP32, PLCC32
• Microchip SST39SF010
  • Type: Flash
  • Speeds: 55ns/70ns
  • Note: Drop in replacement for read-only applications. For writing, pin 1 should be NC unlike the original 27C010.
  • Compatible Packages: DIP32, PLCC32
• SST SST27SF010
  • Type: Flash
  • Speeds: 70ns
  • Compatible Packages: DIP32, PLCC32
• Winbond W27C010
  • Type: EEPROM
  • Speeds: 70ns
  • Compatible Packages: DIP32, PLCC32

27C512

Atmel Datasheet

• Packages: DIP28, PLCC32, TSOP28
• Size: 512kbits (64k * 8) = 64KiB

DIP28 Pinout

      ┌─────◡─────┐
  A15 │ ●1     28 │ Vcc
  A12 │  2     27 │ A14
   A7 │  3     26 │ A13
   A6 │  4     25 │ A8
   A5 │  5     24 │ A9
   A4 │  6     23 │ A11
   A3 │  7     22 │ OE/Vpp
   A2 │  8     21 │ A10
   A1 │  9     20 │ CE
   A0 │ 10     19 │ O7
   O0 │ 11     18 │ O6
   O1 │ 12     17 │ O5
   O2 │ 13     16 │ O4
  GND │ 14     15 │ O3
      └───────────┘

Example Original Parts

• Atmel/Microchip AT27C512
• Holtek HT27C512
• Intel 27512 / 27C512
• Macronix MX27C512
• Maxwell 27C512
• Mitsubishi M5M27C512AK
• ST M27C512
• Toshiba TC57512AD

Replacement Parts

• Greenliant GLS27SF512
  • Type: Flash
  • Speeds: 70ns
  • Compatible Packages: DIP28, PLCC32
• SST SST27SF512
  • Type: Flash
  • Speeds: 70ns
  • Compatible Packages: DIP28, PLCC32
• Winbond W27C512
  • Type: EEPROM
  • Speeds: 45ns/70ns/90ns/120ns
  • Compatible Packages: DIP28, PLCC32

27C256

Atmel Datasheet

• Packages: DIP28, PLCC32, TSOP28
• Size: 256kbits (32k * 8) = 32KiB

DIP28 Pinout

      ┌─────◡─────┐
  Vpp │ ●1     28 │ Vcc
  A12 │  2     27 │ A14
   A7 │  3     26 │ A13
   A6 │  4     25 │ A8
   A5 │  5     24 │ A9
   A4 │  6     23 │ A11
   A3 │  7     22 │ OE
   A2 │  8     21 │ A10
   A1 │  9     20 │ CE
   A0 │ 10     19 │ O7
   O0 │ 11     18 │ O6
   O1 │ 12     17 │ O5
   O2 │ 13     16 │ O4
  GND │ 14     15 │ O3
      └───────────┘

Example Original Parts

• Atmel/Microchip AT27C256
• AMD Am27C256
• Fairchild FM27C256
• Fairchild/National Semiconductor NM27C256
• Intel 27256 / 27C256
• Macronix MX27C256
• Maxwell 27C256
• Mitsubishi M5L27256K
• Philips/Signetics/NXP 27C256
• Cypress CY27C256
• ST M27C256 / WS57C256
• Toshiba TC57256AD

Replacement Parts

• Atmel/Microchip AT29C256
  • Type: Flash
  • Speeds: 70ns
  • Compatible Packages: DIP28, PLCC32
• SST SST27SF256
  • Type: Flash
  • Speeds: 70/90ns
  • Compatible Packages: DIP28, PLCC32
• Winbond W27E257
  • Type: EEPROM
  • Speeds: 100ns/120ns/150ns
  • Compatible Packages: DIP28, PLCC32

27C128

Atmel Datasheet

• Packages: DIP28, PLCC32
• Size: 128kbits (16k * 8) = 16KiB

DIP28 Pinout

      ┌─────◡─────┐
  Vpp │ ●1     28 │ Vcc
  A12 │  2     27 │ PGM
   A7 │  3     26 │ A13
   A6 │  4     25 │ A8
   A5 │  5     24 │ A9
   A4 │  6     23 │ A11
   A3 │  7     22 │ OE
   A2 │  8     21 │ A10
   A1 │  9     20 │ CE
   A0 │ 10     19 │ O7
   O0 │ 11     18 │ O6
   O1 │ 12     17 │ O5
   O2 │ 13     16 │ O4
  GND │ 14     15 │ O3
      └───────────┘

Example Original Parts

• AMD Am27C128
• Atmel AT27C128
• Fairchild/National Semiconductor NMC27CP128
• Intel 27128 / 27C128
• Microchip 27C128
• Mitsubishi M5L27128K
• ST M27C128 / M27128A
• Texas Instruments TMS27128 / TMS27C128

Replacement Parts

• None

27C64

Atmel Datasheet

• Packages: DIP28, PLCC32, TSOP28
• Size: 64kbits (8k * 8) = 8KiB

DIP28 Pinout

      ┌─────◡─────┐
  Vpp │ ●1     28 │ Vcc
  A12 │  2     27 │ PGM
   A7 │  3     26 │ NC
   A6 │  4     25 │ A8
   A5 │  5     24 │ A9
   A4 │  6     23 │ A11
   A3 │  7     22 │ OE
   A2 │  8     21 │ A10
   A1 │  9     20 │ CE
   A0 │ 10     19 │ O7
   O0 │ 11     18 │ O6
   O1 │ 12     17 │ O5
   O2 │ 13     16 │ O4
  GND │ 14     15 │ O3
      └───────────┘

Example Original Parts

• Atmel/Microchip AT27C64
• Fairchild/National Semiconductor NMC27C64
• Fujitsu MBM2764
• Intel 2764A / 27C64 / M27C64
• Mitsubishi M5L2764K
• Texas Instruments TMS2764

Replacement Parts

• Atmel/Microchip AT28C64
  • Type: EEPROM
  • Speeds: 120ns
  • Compatible Packages: DIP28, PLCC32, TSOP28
• Catalyst/Onsemi CAT28C64
  • Type: EEPROM
  • Speeds: 90ns/120ns/150ns
  • Compatible Packages: DIP28, PLCC32, TSOP28