SN74LS85D - 4-Bit Magnitude Comparator | Texas Instruments
MPN： SN74LS85D ✓ 啟用中SN74LS85D 概覽
A magnitude comparator is a combinational logic IC that determines the relative order of two unsigned digital words. Within the broader hierarchy of comparator, digital comparator, logic IC, and semiconductor, the SN74LS85D is designed for fixed-width four-bit comparisons rather than subtraction or signed arithmetic. Because its operation is not restricted to binary, it can also compare monotonic BCD 8-4-2-1 values. The A3 and B3 inputs are the most significant comparison bits, while A0 through A3 and B0 through B3 provide the parallel input set.
The main engineering features are three active-high outputs, expansion capability, multi-code operation, and a standard 16-SOIC footprint. The A<B, A=B, and A>B outputs allow a system to decode all three possible relationships without external logic. Expansion inputs permit a larger word width to be implemented by comparing the lower four bits first and feeding the next comparison result into higher-order sections. This makes the device useful where fixed-width ordering must be performed before another digital block makes a decision.
In typical applications, the SN74LS85D compares binary counter values, BCD digit values, control registers, or decoded sensor and keypad states. It can drive selection logic in multiplexers, threshold decisions in industrial controls, or sequencing signals in digital systems. Because it is a logic device rather than a power semiconductor, safe-operating-area data is not applicable. Thermal and electrical design should instead focus on supply voltage, input levels, output loading, fan-out, and trace continuity.
A key design consideration is to treat unused inputs deliberately. Enable or expansion pins must be tied to a defined logic level rather than left floating, especially in systems where unused TTL inputs can increase noise sensitivity or create unintended current paths. Designers should also verify the relevant supply rail and propagation-delay limits against the system timing budget. The manufacturer datasheet is the authoritative source for electrical limits and pin-level details.
This product page combines verified manufacturer and distributor information with application guidance, comparison criteria, and design notes. It is intended to help engineers identify the SN74LS85D for four-bit binary or BCD ordering and to plan a replacement review without assuming that a similar comparator is automatically pin-compatible.
直接替換替代品，適用於 SN74LS85D — 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： SN74LS85D （相同外形與佔位） — 差異在於 Expansion, Logic Family, Mounting Type, Package, Supported Codes.
SN74LS85N
✅ 直接替換聯絡詢價
查看資料表 →SN74LS85DR
✅ 直接替換📋 參考替代品（不在目錄中）
SN74LS85NE4
✅ 直接替換📋 參考替代品（不在目錄中）
SN74LS85D
✅ 直接替換聯絡詢價
查看資料表 →SN74LS85D 最大額定值與電氣特性
| 功能 | 4-bit magnitude comparator |
| Comparison Width | 4 bits |
| Supported Codes | Straight binary and straight BCD 8-4-2-1 |
| Input Configuration | Two parallel 4-bit words, A and B |
| Outputs | A<B, A=B, and A>B active-high outputs |
| Expansion | Yes, expandable to wider word comparisons |
| Logic Family | 74LS low-power Schottky TTL |
| 封裝 | 16-pin SOIC (D) |
| 安裝類型 | Surface mount |
| Manufacturer | Texas Instruments |
| Operating Status | Active product listing |
| Input Pins | A0-A3 and B0-B3 |
| Most Significant Inputs | A3 and B3 |
| RoHS 狀態 | unknown |
| REACH Status | unknown |
SN74LS85D 16-pin soic (d) 腳位配置指南
SN74LS85D（16-pin soic (d) 封裝）的腳位配置。 此類比元件具有輸入、輸出與電源腳位。偏移歸零、補償與致能腳位配置請參閱製造商資料表。適用於訊號調理與放大器電路。
無詳細腳位定義資料： SN74LS85D.
請參閱 資料表 以查看完整腳位配置。
典型應用
SN74LS85D 適用於 6 應用s: Binary Counter Ordering, BCD Digit Comparison, Address and Control Decoding, Sorting and Threshold Control, Keypad and Input-State Validation, Multi-Section Digital Ordering.
Binary Counter Ordering
The SN74LS85D is useful when two four-bit binary counter or register values must be ordered before a control decision is made. Its dedicated A<B, A=B, and A>B outputs provide the relational result directly, avoiding a software comparison or a general-purpose processor in simple digital control paths. The device is especially appropriate when the comparison width is fixed at four bits and the design uses the 74LS logic family. System timing must still account for the propagation delay listed in the manufacturer datasheet, which was not included in the supplied search snippets. Inputs and outputs should be connected according to the verified pin configuration, and unused expansion or control pins must not be left floating.
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BCD Digit Comparison
SN74LS85D supports straight BCD 8-4-2-1 codes in addition to binary data, making it suitable for digit-oriented industrial and display-control logic. It can determine whether one decoded digit value is lower, equal, or higher than another while the surrounding controller handles keypad entry, display selection, or process sequencing. The verified manufacturer description explicitly confirms BCD support. Use a valid monotonic BCD code and define unused inputs to a stable logic level. Because the supplied data does not include propagation-delay or supply-limit values, timing and voltage-margin checks must be completed from the official Texas Instruments datasheet.
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Address and Control Decoding
A four-bit magnitude comparator can support simple address or register-range decoding when a design needs to know whether a value is below, equal to, or above a selected reference. The SN74LS85D provides all three decisions simultaneously, allowing control logic to generate enable, select, or sequencing signals without implementing a full comparator from gates. This is useful in legacy TTL-compatible industrial equipment and small controller boards. The design must preserve the input ordering and expansion rules shown in the manufacturer logic diagram. Confirm the actual SOIC land pattern and verify electrical timing before using it in high-speed address buses.
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Sorting and Threshold Control
The three active-high outputs of SN74LS85D make it appropriate for a small sorting or threshold-control block. A downstream state machine can use the equality output to detect a match, the less-than output to enable an upstream stage, and the greater-than output to advance a sequence. The device supports straight binary values, so it can be placed beside counters and registers in fixed-width automation logic. The main trade-off is that it compares only four bits per section; wider comparisons require cascading and additional propagation-delay margin. Confirm the electrical limits and unused-pin treatment in the manufacturer datasheet.
推薦
Keypad and Input-State Validation
SN74LS85D can compare a four-bit decoded keypad or input value against a reference code and report equality or ordering. The A=B output is particularly useful for recognizing a selected key or command, while A<B and A>B can support range checking and menu navigation. Because BCD 8-4-2-1 operation is explicitly supported, the device is compatible with decimal digit encoding when the input codes are valid. Keep the input source electrically compatible with the 74LS logic family and use defined logic levels for unused pins. The supplied data does not state input capacitance or switching limits, so those values require datasheet verification.
推薦
Multi-Section Digital Ordering
The SN74LS85D is expandable, allowing engineers to compare values wider than four bits by arranging lower-order and higher-order sections. Each section evaluates a four-bit group, and the comparison relationship is passed to the next section to preserve the ordering of the complete word. This architecture is useful for legacy control, educational instrumentation, and fixed-format digital datapaths. Cascading adds wiring and propagation-delay considerations, so the timing budget must be calculated from the official electrical data rather than assumed. Confirm every pin against the manufacturer datasheet and preserve the 16-pin SOIC footprint of the D variant.
推薦
推薦產品摘要
工程參考資料，適用於 SN74LS85D — 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 | SN74LS85N | SN74LS85DR | SN74LS85NE4 |
|---|---|---|---|---|
| Function | 4-bit magnitude comparator | 4-bit magnitude comparator | 4-bit magnitude comparator | 4-bit magnitude comparator |
| Brand | Texas Instruments | Texas Instruments | Texas Instruments | Texas Instruments |
| Package | 16-pin SOIC (D) | 16-pin DIP; not the same physical footprint | 16-pin SOIC (D) | 16-pin DIP; not the same physical footprint |
| Comparison Width | 4 bits | 4 bits | 4 bits | 4 bits |
| Input Words | A and B, four parallel bits each | A and B, four parallel bits each | A and B, four parallel bits each | A and B, four parallel bits each |
| Decision Outputs | A<B, A=B, A>B active-high | A<B, A=B, A>B active-high | A<B, A=B, A>B active-high | A<B, A=B, A>B active-high |
| Code Support | Straight binary and straight BCD 8-4-2-1 | Straight binary and straight BCD 8-4-2-1 | Straight binary and straight BCD 8-4-2-1 | Straight binary and straight BCD 8-4-2-1 |
| Expandability | Yes | Yes | Yes | Yes |
| Logic Family | 74LS Schottky TTL | 74LS Schottky TTL | 74LS Schottky TTL | 74LS Schottky TTL |
主要差異化特點
- Dedicated three-way decision outputs (對比SN74LS85N)
- D-package footprint for surface-mount designs (對比SN74LS85DR)
- Binary and BCD support in one comparator (對比SN74LS85NE4)
設計說明
Do not treat every SN74LS85 package suffix as mechanically interchangeable. The verified D variant is a 16-pin SOIC (D), while the N and NE4 references are associated with 16-pin DIP packages. Confirm the PCB footprint, pin count, and manufacturer pinout before substituting either part. The functional comparison is similar, but a different package requires different board artwork or an adapter.
Leave no unused comparator or expansion input floating. Assign every unused input to a defined logic level after checking the SN74LS85 logic diagram, because floating TTL inputs can increase current draw and make switching behavior unpredictable. For longer traces or noisy industrial wiring, keep the A and B buses short, use a common ground reference, and place any required termination or filtering near the source according to the system architecture.
Route the eight comparison inputs as a grouped parallel bus and keep the three decision outputs away from high-current switching nodes. The 16-pin SOIC footprint should be checked against the current package drawing rather than inferred from the D suffix alone. Preserve a short ground return and avoid placing sensitive comparator inputs beneath long power-routing loops. The verified data confirms the package type but does not provide a complete land-pattern drawing, so use the official TI package information for PCB release.
Validate the supply rail and input thresholds against the official SN74LS85 datasheet before connecting the device to a system bus. The supplied snippets confirm the 74LS family and comparison function but do not state the recommended supply voltage, input levels, or output loading. Ensure the downstream logic has compatible voltage levels and account for fan-out and propagation delay when the comparison result controls a timing-sensitive state machine.
For expanded comparisons, assign each four-bit slice consistently from least significant to most significant and follow the manufacturer's expansion wiring sequence. The additional section introduces cumulative propagation delay, so the clock or control path must leave enough timing margin. Use identical logic families and stable input levels in every stage; otherwise a functionally equivalent-looking board can fail during transitions even when steady-state comparison results appear correct.
合規資訊
The supplied verified web data identifies the component as a Texas Instruments 74LS logic IC but does not provide RoHS, REACH, lead-free, halogen-free, conflict-minerals, or automotive qualification statements. These fields remain unknown rather than assumed.