27C256-17 - 256Kbit UV EPROM 170ns 32Kx8 | Microchip Technology
MPN: 27C256-17 ✗ End of Life| Qty | Unit Price | Extended |
|---|---|---|
| 1 | $0 | $0.00 |
| 10 | $0 | $0.00 |
| 100 | $0 | $0.00 |
| 500 | $0 | $0.00 |
| 1,000 | $0 | $0.00 |
Drop-in alternatives for 27C256-17 — same package, pin-to-pin compatible. Different-package parts requiring PCB rework are excluded.
Quick Comparison Tool — Select alternative parts for side-by-side comparison:
27C256-12/L
✅ Drop-In✓ In Stock
$3.9 / Unit
View Datasheet →27C256A-12
✅ Drop-In✓ In Stock
$2.52 / Unit
View Datasheet →27C256L-12TMB
✅ Drop-In✓ In Stock
$8.25 / Unit
View Datasheet →27C256T-15/SO
✅ Drop-In✓ In Stock
$1.9 / Unit
View Datasheet →27C256-120N
✅ Drop-In✓ In Stock
$1.75 / Unit
View Datasheet →M27C256
✅ Drop-In📋 Reference alternative (not in catalog)
CY27C256
✅ Drop-In📋 Reference alternative (not in catalog)
TC57256AD
✅ Drop-In📋 Reference alternative (not in catalog)
27C256-17 Maximum Ratings & Electrical Characteristics
| Memory Type | EPROM - UV (windowed) |
| Memory Size | 256 Kbit |
| Organization | 32K x 8 |
| Access Time | 170 ns |
| Interface | Parallel |
| Supply Voltage | 4.5 V to 5.5 V |
| Technology | CMOS |
| Temperature Grade | Commercial |
| Package | CDIP-28 (windowed CERDIP, 0.6 inch) |
| Number of Terminals | 28 |
| Mounting Type | Through Hole |
| Address Lines | A0 - A14 (15) |
| Data Lines | 8-bit bidirectional |
| Control Signals | /CE, /OE, VPP |
| Erase Method | Ultraviolet light (quartz window) |
27C256-17 Pin Configuration
| Pin 1 | VPP — Programming supply voltage (used during programming) |
| Pin 2 | A12 — Address input |
| Pin 3 | A7 — Address input |
| Pin 4 | A6 — Address input |
| Pin 5 | A5 — Address input |
| Pin 6 | A4 — Address input |
| Pin 7 | A3 — Address input |
| Pin 8 | A2 — Address input |
| Pin 9 | A1 — Address input |
| Pin 10 | A0 — Address input |
| Pin 11 | O0 — Data output/input 0 |
| Pin 12 | O1 — Data output/input 1 |
| Pin 13 | O2 — Data output/input 2 |
| Pin 14 | GND — Ground |
| Pin 15 | O3 — Data output/input 3 |
| Pin 16 | O4 — Data output/input 4 |
| Pin 17 | O5 — Data output/input 5 |
| Pin 18 | O6 — Data output/input 6 |
| Pin 19 | O7 — Data output/input 7 |
| Pin 20 | /CE — Chip Enable (active low) |
| Pin 21 | A10 — Address input |
| Pin 22 | /OE — Output Enable (active low) |
| Pin 23 | A11 — Address input |
| Pin 24 | A9 — Address input |
| Pin 25 | A8 — Address input |
| Pin 26 | A13 — Address input |
| Pin 27 | A14 — Address input |
| Pin 28 | VCC — Power supply (5 V) |
Safe Operating Area (SOA) & Thermal Characteristics
No official SOA curve available for this digital IC. Always operate within absolute maximum ratings specified in the datasheet. Ensure adequate cooling and derate as needed.
Typical Applications
27C256-17 is suitable for 6 applications: Legacy Industrial Controller Firmware, Retro Computer and Arcade Board Repair, Classic Automotive ECU Memory, Firmware Development and Prototyping, Telecom and Test Equipment Maintenance, Music and Studio Hardware Restoration.
Legacy Industrial Controller Firmware
Many PLCs, motor drives and CNC controllers from the 1980s-1990s store their executive firmware in a 32K x 8 EPROM socket. The 27C256-17's 170 ns access time meets the timing of Z80, 8051, 68000 and similar CPUs in these systems, while the windowed CERDIP package lets maintenance engineers erase and rewrite the firmware as patches are issued. Because the part operates from a standard 4.5V-5.5V TTL rail with /CE and /OE control, it drops directly into the original socket with no board modification. XAIPART recommends keeping an erased, programmed spare set for each firmware revision to minimize production downtime during repairs.
Recommended
Retro Computer and Arcade Board Repair
Vintage PCs, consoles, pinball machines and arcade PCBs rely on 27C256-family EPROMs for BIOS, character generators and game code. Repair technicians favor the windowed 27C256-17 because images can be burned, tested, and re-burned as board faults are diagnosed, unlike OTP devices that are single-use. The 170 ns access time comfortably satisfies the bus timing of 6502, Z80 and 6809 based platforms of that era. Always cover the quartz window with an opaque label after final programming, since ambient fluorescent light can slowly degrade stored data. For boards needing field updates, the AT28C256 EEPROM is a near drop-in modernization path with the pin-1 caveat noted in the FAQ.
Recommended
Classic Automotive ECU Memory
Chipped ECUs in classic and performance vehicles (Bosch Motronic and similar systems) use 27C256-family EPROMs to hold fuel and ignition maps. Tuners erase and reprogram the windowed 27C256-17 repeatedly while iterating maps on a dynamometer, making the UV package more economical than disposable OTP chips. The device's 5V operation and 170 ns access time match the original 1980s-1990s ECU designs exactly. The chip should be seated firmly in its socket and its window covered, as engine-bay heat and sunlight pose both thermal and UV-erasure risks. Many tuning shops stock 27C256 in multiple speed grades for cross-vehicle compatibility.
Recommended
Firmware Development and Prototyping
During embedded firmware development on 8-bit and 16-bit targets, the windowed 27C256-17 enables an iterative burn-test-erase cycle: program in an EPROM programmer, validate in-circuit, then bulk-erase under UV in about 20-30 minutes and repeat. This workflow predates in-system flash programming but remains relevant for legacy toolchains, masked-ROM emulation benches, and education on memory fundamentals. The CMOS process keeps current draw low enough for battery-backed development rigs. Teams should standardize on one programmer algorithm from the Microchip datasheet and log erasure cycles, since EPROM cells have finite endurance over repeated UV erases.
Recommended
Telecom and Test Equipment Maintenance
Older oscilloscopes, logic analyzers, network multiplexers and PBX systems contain 27C256 sockets holding self-test code and configuration firmware. Field-service organizations use the windowed 27C256-17 to restore corrupted firmware during board-level repair, copying known-good images from reference units. The parallel 32K x 8 organization with standard /CE and /OE timing means replacement requires no schematic changes, and the 170 ns access time matches the original design center. Spare programmed devices should be stored in UV-opaque, ESD-safe tubes with the lot and image revision recorded, so that a 15-year-old diagnostic image can be reproduced verbatim years later.
Recommended
Music and Studio Hardware Restoration
Vintage synthesizers, drum machines, samplers and effects units from the 1980s store operating systems and wavetable data in 27C256 EPROMs. Restoration specialists use the windowed 27C256-17 to replace corroded or failed ROMs and to install community firmware updates (such as alternate OS versions for classic samplers), erasing and rewriting as revisions evolve. The device's 5V TTL interface and 170 ns access time reproduce the original timing environment, preserving MIDI and audio-clock behavior. Because audio hardware often runs continuously, ensure the socket contacts are cleaned and the window label applied before final installation to prevent data fade under stage lighting.
Recommended
Recommended Products Summary
Engineering reference data for 27C256-17 — comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | 27C256-12/L | 27C256A-12 | M27C256 | CY27C256 | TC57256AD |
|---|---|---|---|---|---|---|
| Package | CDIP-28 (windowed, 0.6 inch) | CDIP-28 (windowed) | DIP-28 | DIP-28 (windowed or OTP by suffix) | DIP-28 | DIP-28 |
| Brand | Microchip Technology | Microchip Technology | Microchip Technology | STMicroelectronics | Cypress Semiconductor | Toshiba |
| Organization | 32K x 8 | 32K x 8 | 32K x 8 | 32K x 8 | 32K x 8 | 32K x 8 |
| Access Time | 170 ns | 120 ns | 120 ns | [DATA_NEEDED] | [DATA_NEEDED] | [DATA_NEEDED] |
| 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 | 4.5 V to 5.5 V | 4.5 V to 5.5 V |
| Technology | CMOS | CMOS | CMOS | CMOS | CMOS | CMOS |
| UV Window | Yes (CERDIP window) | Yes (windowed variant) | [DATA_NEEDED] | By suffix (windowed or OTP) | [DATA_NEEDED] | [DATA_NEEDED] |
| Lifecycle Status | Obsolete (Rochester excess stock) | Obsolete / excess stock | Obsolete / excess stock | Obsolete / limited stock | Obsolete | Obsolete |
Key Differentiators
- Windowed CERDIP package enables unlimited erase/reprogram cycles (vs 27C256A-12)
- 170 ns access time fits mid-range legacy CPU timing (vs 27C256-12/L)
- Multi-source pin compatibility across manufacturers (vs M27C256)
Design Notes
Pin 1 of the 27C256 is VPP (programming voltage), while EEPROM replacements such as the AT28C256 use pin 1 as address A14. In many legacy sockets pin 1 is strapped to VCC or a pull-up, which makes an AT28C256 behave as though its first 16 KB is mirrored into the second half. Before substituting a non-EPROM device, verify what the socket does with pin 1 and, if needed, duplicate the firmware image into both halves or use a purpose-built 27C-to-flash adapter.
The CDIP-28 is a through-hole, 0.6 inch (15.24 mm) row-spacing device. Confirm your board footprint is 600-mil, not 300-mil - forcing a 600-mil EPROM into a 300-mil socket is a classic repair error. When reflow or wave processes are not used (hand soldering typical for legacy repair), no special thermal profile is needed. Always cover the quartz window with an opaque, adhesive-backed label after programming; repeated exposure to fluorescent light or sunlight will gradually erase cells even in indoor storage.
For reliable reads at 170 ns, keep address-line ringing under control: the device presents a primarily capacitive TTL load, so series damping resistors of 22-33 ohm on long bus lines (over ~15 cm) reduce overshoot at the EPROM inputs. Decouple VCC-GND (pins 28-14) with a 100 nF ceramic capacitor placed within 10 mm of the package. Ensure /CE and /OE are not left floating during CPU tri-state periods; a weak pull-up (10 kohm) on both controls prevents spurious bus contention when the host processor resets.
Compliance Information
Legacy EOL part; distributor listings for the Rochester-sourced 27C256-17/J do not state RoHS/REACH status. Ceramic CERDIP hermetic packages may fall under RoHS exemptions for lead in ceramic/seal materials - confirm with supplier for regulated-market use.
