Texas Instruments SN74F657NT

Overview

SN74F657NT from Texas Instruments is an octal transceiver with integrated 8-bit parity generator/checker and 3-state outputs, designed for bidirectional data bus interfacing in TTL-level systems. It supports A↔B data flow direction control via T/R input, provides selectable odd/even parity via ODD/EVEN input, and delivers 64 mA sink capability at B-port outputs - enabling robust bus driving in industrial control backplanes.

For engineers reviewing the SN74F657NT datasheet, SN74F657NT pinout, SN74F657NT application, or SN74F657NT equivalent, key selection criteria include its dual-function parity logic (generator during transmit, checker during receive), high-impedance N-P-N inputs (70 µA loading), and guaranteed 0°C to 70°C operation with 5 V ±10% supply tolerance.

Technical Context

The SN74F657NT implements a synchronous bidirectional transceiver architecture where T/R controls signal direction and OE enables/disables both A and B ports simultaneously into high-impedance state. Its parity circuit operates in two distinct modes: as a generator outputting PARITY during A→B transmission, and as a checker asserting ERR when B-port data + PARITY input violates selected odd/even parity rule.

Electrical behavior is defined by TTL-compatible thresholds (VIL = 0.8 V, VIH = 2 V), with specified IOL = 64 mA (B-port) and IOL = 24 mA (A-port), and propagation delays ranging from 2.5 ns (OE→output) to 22.5 ns (B→ERR path). All timing parameters are validated under CL = 50 pF, R1/R2 = 500 Ω load conditions.

Key Specifications

Pinout & Package

SN74F657NT is packaged in a 24-pin plastic DIP (Dual In-line Package) with 300-mil width, pin-compatible with industry-standard through-hole mounting and wave-solder processes.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal transceiver-with-parity applications.

Compared with SN74F657NT, SN74F280BN requires external transceivers to achieve full bus interface capability, while SN74LS657N trades off speed and drive strength for reduced power - making SN74F657NT the optimal choice for new designs demanding integrated parity-aware bidirectional bus control at TTL-logic speeds.

Availability

SN74F657NT is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy system bus extensions, and test equipment data acquisition requiring stable component supply and long-term obsolescence management.

Supply support for SN74F657NT 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 global semiconductor leader founded in 1930, specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The SN74F657NT belongs to TI's 74F Fast TTL logic family, engineered for high-speed, low-power bus interface applications where timing predictability and noise immunity are critical in commercial-temperature environments.

FAQ

What is the function of the ODD/EVEN pin on the SN74F657NT?

The ODD/EVEN pin on the SN74F657NT selects the parity scheme used by the internal generator/checker circuit. When low, it configures the device for even parity; when high, for odd parity. During A→B transmission, this setting determines the logic state of the PARITY output. During B→A reception, it defines the expected parity condition that the ERR output validates against incoming B-port data and the PARITY input.

Can the SN74F657NT operate with a 3.3 V supply?

No, the SN74F657NT is a 74F-family TTL device specified only for 4.5 V to 5.5 V operation. Its input thresholds (VIH = 2 V min, VIL = 0.8 V max) and output drive characteristics are optimized for 5 V logic levels. Using 3.3 V will result in undefined operation, insufficient noise margin, and potential failure to meet timing or drive specifications - SN74F657NT requires strict adherence to its 5 V ±10% supply range.

How does the ERR output behave during SN74F657NT receive mode?

In receive mode (T/R = low), the ERR output on the SN74F657NT goes high only when the number of high bits on the B port matches the parity expectation set by ODD/EVEN and the PARITY input. For example, with ODD/EVEN = low (even parity), ERR = high if B-port has an even number of highs and PARITY = low, or an odd number of highs and PARITY = high - indicating correct parity. Any mismatch asserts ERR = low, signaling a detected error in the received data stream.

Is the SN74F657NT pin-compatible with the SN74F652NT?

No, the SN74F657NT is not pin-compatible with the SN74F652NT. While both are octal transceivers with parity functions, the SN74F652NT uses a different pinout: its ERR output is on pin 19 (same as SN74F657NT), but PARITY is on pin 12 (vs. pin 13 on SN74F657NT), and ODD/EVEN is on pin 18 (same location). Critical differences include T/R on pin 24 (SN74F657NT) versus pin 1 (SN74F652NT), making direct substitution impossible without PCB revision.

What is the maximum recommended capacitive load for SN74F657NT outputs?

The SN74F657NT switching characteristics are characterized with CL = 50 pF, and its output drivers are rated for reliable operation up to 50 pF total load capacitance per pin. Exceeding this value increases propagation delay nonlinearly and may cause marginal timing in high-speed applications. For loads >50 pF, TI recommends adding series termination resistors or using buffer amplifiers - the SN74F657NT itself does not support higher-capacitance bus segments without derating timing margins.