MC74F538N - 1-of-8 Decoder, 3-State Outputs | onsemi | DIP-20
部品番号: MC74F538N ✗ 生産終了MC74F538N 概要
A decoder/demultiplexer is a combinational logic device that converts a binary-coded input into a one-hot output selection, forming part of the broader logic IC hierarchy: decoder -> combinational logic -> SSI/MSI logic -> digital semiconductors. Decoders are fundamental building blocks for memory address decoding, peripheral chip-select generation, and data routing in digital systems.
Key features of the MC74F538N include the three-state (3-STATE) outputs that allow multiple devices to share a common bus, the polarity control input that lets a single design serve both active-LOW and active-HIGH chip-select schemes, and the F/FAST (Fairchild Advanced Schottky TTL) process family that delivers significantly faster propagation delays than standard 74-series TTL logic.
Architecturally, the device belongs to the FAST series of Schottky-clamped TTL circuits, which use an advanced oxide-isolated process to reduce propagation delay and increase toggle speeds while retaining TTL-compatible input thresholds. The 3-state output structure places each output in a high-impedance state when the output control is disabled, so the MC74F538N can be paralleled with other bus drivers without contention.
Typical applications include memory address decoding in embedded controllers, chip-select generation for peripheral ICs, data routing and bus switching in industrial control equipment, and line decoding in legacy telecommunications and instrumentation platforms that use 5V TTL logic levels.
When designing with this part, keep unused address inputs tied to defined logic levels and ensure the 3-state output control is asserted correctly during bus arbitration to avoid bus contention. Decoupling capacitors of 0.1uF per supply pin are recommended for FAST-series logic, which can draw significant transient supply current during output transitions.
This page adds unique value beyond the manufacturer datasheet by combining distributor sourcing information, drop-in alternative analysis, practical design notes, and lifecycle guidance in a single reference.
ドロップイン代替品: MC74F538N — 同一パッケージでピン互換。PCBの作り直しが必要な異なるパッケージの部品は除外されています。
74F538PC
✅ ドロップイン ⚠️ 仕様未検証📋 参考代替品（カタログ未掲載）
74F538SC
✅ ドロップイン ⚠️ 仕様未検証📋 参考代替品（カタログ未掲載）
MC74F538P
✅ ドロップイン ⚠️ 仕様未検証📋 参考代替品（カタログ未掲載）
74HC538N
✅ ドロップイン ⚠️ 仕様未検証📋 参考代替品（カタログ未掲載）
MC74F538N 最大定格＆電気的特性
| 機能 | 1-of-8 Decoder/Demultiplexer |
| Logic Family | F (FAST / Fairchild Advanced Schottky TTL) |
| Number of Address Inputs | 3 (A0-A2) |
| 出力数 | 8 |
| 出力タイプ | 3-STATE |
| Polarity Control Input | Yes (P input selects active LOW or active HIGH outputs) |
| Input Logic Levels | TTL compatible |
| パッケージ | PDIP-20 (N suffix) |
| 実装タイプ | Through Hole |
| ライフサイクル状態 | Obsolete (available via authorized excess distributors) |
| Manufacturer | Motorola / onsemi |
MC74F538N pdip-20 (n suffix) ピン配置ガイド
MC74F538N (pdip-20 (n suffix) パッケージ) のピン配置。 このデジタルICには、GPIO、通信インターフェース (UART、SPI、I2C)、および電源ピンが含まれています。代替ピン機能と設定オプションについては、メーカーのデータシートを参照してください。組み込みシステム設計とPCBレイアウトに不可欠です。
詳細なピン配置データはありません: MC74F538N.
詳細は データシート をご参照ください。
主な用途
MC74F538N の用途: 6 件s: Memory Address Decoding, Peripheral Chip-Select Generation, Data Routing and Bus Switching, Industrial Control and Relay Sequencing, Legacy Instrumentation and Test Equipment Repair, LED and Indicator Panel Decoding.
Memory Address Decoding
In legacy microprocessor and microcontroller systems, the MC74F538N decodes high address bits to generate chip-select signals for memory banks. The three address inputs (A0-A2) map directly onto address lines, and one of eight mutually exclusive outputs selects a single SRAM, EPROM, or memory-mapped peripheral. The FAST-series propagation delay keeps the decode path short enough for moderate-speed 5V buses, so the decoder does not become the system timing bottleneck. Because outputs are 3-state, the select lines can also be tri-stated during bus turnaround or DMA cycles via the output control. The polarity input (P) lets the same PCB serve memory devices with either active-LOW or active-HIGH chip selects without re-routing. Use the manufacturer datasheet timing tables to confirm decode delay versus memory access-time budget in the target system.
おすすめ
Peripheral Chip-Select Generation
I/O-mapped and memory-mapped peripheral ICs each need a dedicated chip-select, and the MC74F538N provides eight of them from a single 20-pin DIP. The decoder's TTL-compatible inputs accept microprocessor address and control lines directly, and the polarity control input (P) adapts the outputs to peripherals that require either active-LOW or active-HIGH selects - a significant convenience when mixing devices from different vendors on one board. The 3-state outputs allow the select bus to be released during processor hold states, preventing false peripheral activation. FAST-series drive is adequate for typical PCB select-line loads of one to three LSTTL loads per line. Verify the output drive and enable/disable timing figures against the peripheral's setup and hold requirements, and decouple the decoder's VCC pin with a 0.1uF ceramic capacitor for switching transients.
おすすめ
Data Routing and Bus Switching
The MC74F538N's 3-state outputs make it suitable as a one-of-eight data path selector in 5V TTL backplane and control systems. By gating the output control with a read/write or direction signal, the decoder routes a source signal to exactly one of eight destinations and releases all outputs when inactive, letting other drivers take over the line. The polarity input permits routing either active-LOW pulse trains (interrupt acknowledges, strobes) or active-HIGH level signals without external inverters. FAST-series switching speeds suit control-signal distribution in industrial controllers and test equipment, though this part is intended for control routing rather than high-speed parallel data buses. Account for enable/disable times from the datasheet when sequencing multiple decoders that share a common line, and keep PCB trace stubs short to limit reflections on distributed selects.
おすすめ
Industrial Control and Relay Sequencing
Programmable logic controllers and industrial sequencers often decode a binary state counter into one-of-eight actuation lines for relays, solenoid drivers, or indicator banks. The MC74F538N performs this decode directly from a 3-bit counter or register output, with the polarity input matching the sense of the downstream driver stage - active-LOW for common transistor-relay drivers, active-HIGH for LED indicators. The DIP-20 through-hole package suits the repaired-and-maintained nature of industrial boards, and FAST TTL thresholds give good noise margin at 5V in electrically noisy cabinets when combined with proper grounding and 0.1uF decoupling. The 3-state capability allows a supervisory controller to blank all actuation outputs during startup or fault states by disabling the output control. Validate that the selected output remains glitch-free across counter transitions using the datasheet timing parameters.
おすすめ
Legacy Instrumentation and Test Equipment Repair
Range selectors, channel multiplexers, and display drivers inside older test instruments frequently used 74F538-family decoders, and the MC74F538N is the canonical replacement part for failed devices in Motorola-sourced boards. Its identical FAST-series timing and DIP-20 footprint mean a failed part can be socketed and swapped without PCB changes or timing re-verification in most instruments. The polarity control allows the repair technician to substitute correctly even where an instrument used the P input to invert the decode sense, and the 3-state outputs preserve the original bus-sharing scheme among multiple decoder ICs. Because the part is obsolete, repair shops should secure several units per instrument type through excess-stock channels. Confirm the instrument's operating ambient stays within the MC-prefix temperature rating recorded in the manufacturer datasheet.
おすすめ
LED and Indicator Panel Decoding
Eight-way indicator panels - status lamps, alarm columns, or seven-segment-plus-decimal digit selects - map naturally onto the MC74F538N's eight outputs. A 3-bit binary source (counter, latch, or MCU port) addresses the decoder, and one lamp illuminates per address with no software overhead. The polarity input lets designers drive common-cathode or common-anode arrangements by choosing active-HIGH or active-LOW output sense, and the 3-state control blanks the entire panel during multiplexing dead-time to reduce ghosting and average current. FAST-series drive at 5V suits direct LED-with-resistor loads on most indicator lines; for high-current panels, buffer each output with a transistor or driver IC. Keep the decoder's decode-to-output delay in mind when multiplexing: the datasheet enable and disable times define the minimum blanking interval between digit switches.
おすすめ
おすすめ製品の概要
エンジニアリング参考データ: MC74F538N — 比較、設計ガイダンス、コンプライアンス情報。
選定ガイド
代替品との比較
| パラメータ | この製品 | 74F538PC | 74F538SC | MC74F538P | 74HC538N |
|---|---|---|---|---|---|
| Package | PDIP-20 (N suffix) | PDIP-20 - same | PDIP-20 - same | PDIP-20 - same | PDIP-20 - same |
| Brand | onsemi (Motorola legacy) | Fairchild / onsemi | Fairchild / onsemi | Motorola / onsemi | Philips / NXP Semiconductors |
| Logic Family | F (FAST Schottky TTL) | F (FAST) | F (FAST) | F (FAST) | HC (CMOS) |
| Function | 1-of-8 decoder, 3-state outputs, polarity input | Identical | Identical | Identical | Identical function, CMOS electricals |
| Output Type | 3-STATE | 3-STATE | 3-STATE | 3-STATE | 3-STATE (CMOS) |
主な差別化要因
- Polarity control input (P) for active-LOW or active-HIGH outputs （vs 74F138 (typical FAST 3-to-8 decoder))
- 3-STATE outputs enable bus sharing （vs 74HC538N)
- FAST-series speed in through-hole DIP （vs 74HC538N)
設計ノート
FAST-series Schottky TTL draws substantial transient supply current on every output transition. Decouple the MC74F538N with a 0.1uF ceramic capacitor placed within a few millimeters of the VCC pin, plus a bulk 10uF electrolytic per board region. Estimated: with eight 3-state outputs switching simultaneously, per-device dynamic current can reach the tens-of-milliampere range during transitions, so a shared ground plane with low inductance is important to prevent decode glitches caused by ground bounce.
Never leave the address inputs (A0-A2) or the polarity input (P) floating. TTL inputs left floating typically bias to an undefined region near the threshold, where noise causes random output selection or oscillation. Tie unused inputs to GND or VCC through 1k to 10k resistors. Similarly, when the 3-state output control is deasserted, ensure no other driver contends with the F538 outputs - FAST-series devices can source and sink enough current during bus contention to damage outputs.
For through-hole DIP-20 boards, use sockets when replacing the MC74F538N in legacy equipment so future maintenance does not require desoldering. Keep the decode output traces short and, for distributed select lines on backplanes, consider series resistors (22 to 33 ohms) at the decoder outputs to damp reflections. Estimated: a 15 cm unterminated trace behaves as a lumped load at FAST-class edge rates but can still ring; a small series resistor or a ground-adjacent return path resolves it.
コンプライアンス情報
This is a legacy Motorola/FAST-series device originally produced before RoHS; most remaining excess stock is tin-lead (non-RoHS) DIP material, but compliance must be confirmed per lot with the distributor.
