Texas Instruments SN74F257D

Overview

SN74F257D from Texas Instruments is a quad 2-line-to-1-line data selector/multiplexer with 3-state outputs, designed for bus interface in high-performance TTL systems. It features 4-bit multiplexing, independent output enable (OE) control, and operates over 0°C to 70°C with V_CC = 4.5–5.5 V. It directly interfaces with system buses in industrial control backplanes and legacy computing data paths.

For engineers reviewing the SN74F257D datasheet, SN74F257D pinout, SN74F257D application, or SN74F257D equivalent, key selection criteria include its 3-state bus-driving capability, propagation delay under 7 ns (t_PHL/t_PLH), output drive strength of ±24 mA, and SOIC-16 package compatibility with legacy F-series TTL layouts.

Technical Context

The SN74F257D implements four independent 2:1 multiplexers sharing a common A/B select line and a global 3-state output enable (OE). Each Y output reflects either the corresponding A or B input based on the logic level of A/B, while OE controls all four outputs simultaneously into high-impedance state.

Its bipolar F-series TTL architecture delivers fast switching (t_PHL ≤ 6.5 ns, t_PLH ≤ 7 ns at V_CC = 5 V, C_L = 50 pF), supports bus loading via 3-state outputs, and maintains compatibility with standard 5-V TTL logic thresholds (V_IL = 0.8 V, V_VIH = 2.0 V).

Key Specifications

Pinout & Package

SN74F257D is housed in a 16-pin SOIC (Small-Outline Integrated Circuit) package per TI drawing D, with 1.27 mm pitch, 10.0–10.4 mm body width, and 2.00 mm max height. Pin 1 is marked by a beveled corner or notch on the top surface.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar quad 2:1 multiplexer applications.

Compared with SN74LS257N and SN74HC257DR, the SN74F257D delivers optimal balance of speed, drive strength, and TTL compatibility in legacy 5-V bus environments - making it the preferred choice where timing-critical multiplexing and direct bus interfacing are required without redesign.

Availability

SN74F257D is available at Aetrix Electronics and suitable for industrial control backplanes, legacy computer repair, test equipment refurbishment, and embedded system boot recovery circuits requiring stable component supply across long-lifecycle programs.

Supply support for SN74F257D includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a U.S.-based semiconductor company founded in 1930, specializing in analog, embedded processing, and logic solutions with broad industrial and automotive reach.

The SN74F257D belongs to TI's 74F family of fast TTL logic devices, engineered for high-speed data routing and bus interface in 1980s–1990s computing, instrumentation, and control systems.

FAQ

What is the function of the A/B pin on the SN74F257D?

The A/B pin on the SN74F257D is a global select input that determines which of the two 4-bit data sources (A inputs or B inputs) is routed to the corresponding Y outputs. When A/B is low, all four Y outputs reflect the A inputs; when high, they reflect the B inputs. This synchronous selection enables coordinated data path switching across all four channels without individual control lines, simplifying timing in bus-oriented systems using the SN74F257D.

Does the SN74F257D support 3.3-V logic levels?

No, the SN74F257D is a 5-V-only TTL device with input thresholds defined for V_IL ≤ 0.8 V and V_IH ≥ 2.0 V at V_CC = 4.5–5.5 V. It does not guarantee correct operation with 3.3-V CMOS or LVTTL logic levels due to insufficient noise margin and potential input stage damage if driven above V_CC. For 3.3-V systems, consider level-shifting or selecting a 74LVC or 74AHC series equivalent instead of the SN74F257D.

Can the SN74F257D replace the SN74LS257 in an existing design?

Yes, the SN74F257D is pin-compatible and functionally identical to the SN74LS257, with identical pinout, truth table, and logic behavior. However, the SN74F257D offers faster propagation delays (≤7 ns vs ≤15 ns) and stronger output drive (24 mA vs 8 mA), so it can directly replace the SN74LS257 in most cases - especially where timing margins are tight or bus loading is higher. Verify power supply decoupling and fanout limits remain within spec when upgrading to the SN74F257D.

What is the maximum output current rating for the SN74F257D?

The SN74F257D is rated for a maximum low-level output current (I_OL) of 24 mA per Y output, as specified under recommended operating conditions (V_CC = 4.5 V, T_A = 0°C to 70°C). This allows driving up to 10 standard TTL loads or terminating short transmission lines. Exceeding this limit risks output voltage degradation or thermal stress. The SN74F257D must not be operated beyond this rating in sustained DC conditions.

Is the SN74F257D still in active production?

The base part number SN74F257D is marked "Obsolete" by Texas Instruments, but the tape-and-reel variant SN74F257DR.A remains Active and in production with SOIC-16 packaging, RoHS compliance, and Level-1 moisture sensitivity rating. Aetrix Electronics stocks and distributes the SN74F257DR.A as the functional and form-fit equivalent for new builds and legacy system support - ensuring continued access to the SN74F257D functionality.