27C256-12/L - 256Kbit 120ns CMOS EPROM PLCC-32 | Microchip
MPN： 27C256-12/L ✗ 停產27C256-12/L 概覽
An EPROM (Erasable Programmable Read-Only Memory) is a non-volatile semiconductor memory that is programmed electrically and erased by exposure to ultraviolet light through a quartz window. Within the memory hierarchy, it belongs to the broader category of read-only memory, sitting between masked ROM and electrically erasable EEPROM/Flash devices. EPROMs such as the 27C256 became the de facto industry standard for firmware storage in embedded systems.
Key features include CMOS technology for low power consumption, with 20 mA typical active current and 100 uA standby current, and 90 ns access time available in the faster speed grades of the family. The asynchronous architecture requires no clock, and auto-ID support aids automated programming equipment. Factory programming is also available from Microchip.
The device uses a standard 27-series pinout compatible with the industry-standard 27256 JEDEC footprint, making it interchangeable with 27C256 devices from multiple manufacturers. The J-bend PLCC package provides reliable surface mounting, improved board-level vibration resistance, and a smaller footprint than DIP equivalents.
Typical applications include legacy firmware storage in industrial controllers, embedded boot code in telecommunications equipment, automotive ECUs, and replacement ROMs for vintage computer and test-equipment repair.
A key design consideration is that this part is discontinued by Microchip; for new designs, electrically erasable alternatives such as the AT28C256 EEPROM or 29C-series Flash should be evaluated, noting pin 1 and pin 27 functional differences between EPROM and EEPROM pinouts.
This page synthesizes distributor availability data, drop-in alternatives, and practical design notes not found in the manufacturer datasheet.
直接替換替代品，適用於 27C256-12/L — 相同封裝，腳位對腳位相容。需重新設計 PCB 的不同封裝零件不列入。
此系列的變體
與下列產品可直接替換的同系列型號： 27C256-12/L （相同外形與佔位） — 差異在於 Package, Memory Type, Mounting Type, Organization, Erase Method.
27C256A-12
✅ 直接替換聯絡詢價
查看資料表 →27C256L-12TMB
✅ 直接替換聯絡詢價
查看資料表 →27C256L-20R
✅ 直接替換聯絡詢價
查看資料表 →27C256T-15/SO
✅ 直接替換聯絡詢價
查看資料表 →M27C256
✅ 直接替換📋 參考替代品（不在目錄中）
CY27C256
✅ 直接替換📋 參考替代品（不在目錄中）
TC57256AD
✅ 直接替換📋 參考替代品（不在目錄中）
27C256-12/L 規格
| 記憶體容量 | 256 Kbit (32K x 8) |
| 組織 | 32K words x 8 bits |
| Maximum Access Time | 120 ns |
| Supply Voltage Range | 4.5 V to 5.5 V |
| 記憶體類型 | EPROM (UV erasable) |
| Technology | CMOS |
| Active Current | 20 mA |
| Standby Current | 100 uA |
| 介面類型 | Asynchronous (parallel) |
| 封裝 | PLCC-32 (PQCC32), J-bend |
| 安裝類型 | Surface Mount |
| Terminal Form | J-bend |
| Auto ID | Yes (aids automated programming) |
| Factory Programming | Available |
| Family Faster Speed Grade | 90 ns access time available |
| 腳位數 | 32 |
| 生命週期狀態 | Obsolete / discontinued (last-time-buy via Rochester Electronics) |
27C256-12/L plcc-32 (pqcc32), j-bend 腳位配置指南
27C256-12/L（plcc-32 (pqcc32), j-bend 封裝）的腳位配置。 腳位編號、功能與連接圖定義於製造商資料手冊中。請參閱該手冊以取得確切的封裝尺寸與焊接指引。
無詳細腳位定義資料： 27C256-12/L.
請參閱 資料表 以查看完整腳位配置。
典型應用
27C256-12/L 適用於 6 應用s: Legacy Embedded Firmware Storage, Industrial PLC and Motor Control Boards, Telecommunications Equipment ROM, Vintage Computer and Test Equipment Repair, Automotive ECU Calibrated ROM, Character Generator and Lookup Tables.
Legacy Embedded Firmware Storage
The 27C256-12/L stores boot code and application firmware in 8-bit and 16-bit microcontroller systems originally designed around 27-series EPROMs. Its 120 ns access time meets typical bus timing for controllers like the 8051, 68HC11, and Z180 families without wait states, while the asynchronous parallel interface requires no configuration. The 32K x 8 organization matches 15-address-line firmware images up to 32 KB. Because it is non-volatile and electrically read-only in-circuit, it resists corruption from noise and power cycling, making it dependable in long-lifecycle industrial deployments. The trade-off is that updates require UV erase or device replacement rather than in-system reprogramming.
推薦
Industrial PLC and Motor Control Boards
Programmable logic controller CPU boards and motor drive control boards from the 1980s and 1990s commonly socket 27C256 EPROMs for ladder-logic interpreters and drive firmware. The 27C256-12/L fits these sockets directly via the industry-standard 27256 pinout, and its PLCC-32 J-bend package survives vibration on machinery better than DIP devices. CMOS technology limits active current to 20 mA and standby to 100 uA, easing power supply loading on densely populated control boards. For repair depots maintaining installed bases, stocking a pin-compatible 120 ns or faster EPROM avoids timing regressions when replacing aged or window-erased devices.
推薦
Telecommunications Equipment ROM
Channel cards, line cards, and protocol converters in legacy telecom racks used 27C256 devices for bootloader and lookup-table storage. The 27C256-12/L supplies deterministic access timing with no software initialization, supporting power-on self-test routines that must execute immediately at reset. Its wide 4.5V to 5.5V tolerance accommodates distributed 5V backplane supplies with realistic regulation drift. The 100 uA standby current suits redundant card slots that keep multiple ROM-equipped processors warm. Field upgrades follow the classic UV-erase-and-reprogram workflow or module swap, which service organizations already documented for 27-series infrastructure.
推薦
Vintage Computer and Test Equipment Repair
Restoration of vintage computers, EPROM-based engine calibrators, and instruments like the HP 8920A signalling board relies on 27C256-class devices. Community repair documentation explicitly lists the 27C256 footprint as the target for drop-in substitutions, including ST M27C256 and Toshiba TC57256AD equivalents. The 27C256-12/L's PLCC-32 package matches the sockets common in late-1980s equipment, and its 120 ns grade is compatible with virtually all clock speeds used in that era. Restoration technicians should verify whether the target socket expects a windowed ceramic package; the plastic -12/L may be windowless and effectively one-time programmable.
推薦
Automotive ECU Calibrated ROM
Engine control units of the 1980s and early 1990s read calibration tables (fuel and ignition maps) from 27-series EPROMs, and TunerCat's EPROM documentation confirms 27C256 usage in calibration applications. The 27C256-12/L provides the standard 32K x 8 map capacity and immediate access at crank with no boot latency. Its PLCC package tolerates under-hood electronics-box vibration. Tuners reprogram images via UV erase or, more commonly today, install Flash-based adapters such as RE-PROM modules that emulate 27-series sockets while allowing field reflashing. When replacing originals, match or exceed the 120 ns speed grade to preserve timing margins across the full temperature range.
推薦
Character Generator and Lookup Tables
Fixed lookup tables - font/character generators for dot-matrix and CRT displays, sine tables for DDS-style analog synthesis, and CRC or scrambling tables - map naturally into a 32K x 8 EPROM. The 27C256-12/L's asynchronous 120 ns access supports video-rate addressing for medium-resolution character generation, and its contents cannot be corrupted by bus contention or software faults. CMOS standby current of 100 uA allows tables to remain powered in always-on display equipment. Because table images are rarely changed, the UV-EPROM update limitation is acceptable, and the industry-standard pinout permits second-sourcing across Microchip, ST, and Cypress equivalents without redesign.
推薦
推薦產品摘要
工程參考資料，適用於 27C256-12/L — 比較、設計指引及合規資訊。
選型指南
與替代品比較
| 參數 | 本產品 | 27C256A-12 | 27C256L-12TMB | M27C256 | CY27C256 |
|---|---|---|---|---|---|
| Package | PLCC-32 (PQCC32, J-bend) | PLCC-32 - same footprint | PLCC-32 - same footprint | PLCC-32 option available | PLCC-32 option available |
| Brand | Microchip Technology | Microchip Technology | Microchip Technology | STMicroelectronics | Cypress Semiconductor |
| Organization | 32K x 8 | 32K x 8 | 32K x 8 | 32K x 8 | 32K x 8 |
| Supply Voltage | 4.5 V to 5.5 V | 4.5 V to 5.5 V | 4.5 V to 5.5 V | 4.5 V to 5.5 V (5V grade) | 4.5 V to 5.5 V (5V grade) |
主要差異化特點
- Low-power CMOS L-grade (對比27C256T-15/SO)
- Family A-version availability (對比27C256A-12)
- Multi-vendor second sourcing (對比M27C256)
- True EPROM behavior vs EEPROM substitutes (對比AT28C256)
設計說明
Design the 5V rail for 20 mA active current per device plus 100 uA standby, per the Microchip 27C256 datasheet feature list. Estimated: a board with 4 EPROMs reading continuously draws roughly 80 mA just for memory, so budget the regulator accordingly. Place a 0.1 uF ceramic decoupling capacitor within 5 mm of each VCC pin, plus one 10 uF bulk capacitor per bank, to supply the transient current demanded during address-to-data access transitions.
Do not substitute an AT28C256 EEPROM into a 27C256 socket without verifying pin wiring: pin 1 on a 27C256 socket is typically VPP (tied to +5V in many boards), while pin 1 on the AT28C256 is address line A14; pin 27 also differs (/PGM vs /WE). Boards with pin 1 hard-wired high will only address the lower half of an EEPROM, causing duplicated 16 KB images. This failure mode is widely documented in EPROM replacement guides and repair forums.
For PLCC-32 sockets in repair work, prefer high-quality open-frame or solenoid-type PLCC sockets with gold-plated contacts; low-cost closed-frame sockets exhibit contact creep and intermittent reads over thermal cycling. Keep address-line stubs short: at 120 ns access times the timing margin is generous for most 8-bit buses, but long unterminated stubs above 15 cm on multi-drop address buses can cause reflections that reduce margin at cold temperatures.
The plastic PLCC-32 package of the -12/L variant may be windowless, making it one-time programmable in practice - there is no quartz window for UV erase. Before committing firmware, burn a verification image and confirm function on the programmer and in-target. For development iterations, use a windowed ceramic 27C256 in the same footprint or an electrically erasable adapter, reserving the plastic -12/L for released production firmware.
合規資訊
Part predates RoHS and is discontinued; compliance documentation not stated in available distributor data. The /L suffix denotes low-power CMOS grade, not lead-free. Confirm with supplying distributor for regulated-market legacy repair use.
