Texas Instruments SN74LS640-1N

Overview

SN74LS640-1N from Texas Instruments is an octal bus transceiver with 3-state outputs, designed for bidirectional data flow control in TTL-level parallel bus systems. It features 8-bit non-inverting data paths, independent output-enable (OE) and direction (DIR) controls, and operates over 0°C to 70°C. It is used in legacy industrial control backplanes and microprocessor system address/data bus isolation.

For engineers reviewing the SN74LS640-1N datasheet, SN74LS640-1N pinout, SN74LS640-1N application, or SN74LS640-1N equivalent, key selection criteria include its 3-state output drive capability, DIR/OE logic polarity, propagation delay (t_PLH/t_PHL ≤ 25 ns), DC electrical specs (I_OH/I_OL), and PDIP-20 package compatibility with through-hole PCB layouts.

Technical Context

The SN74LS640-1N implements dual 4-bit or single 8-bit bidirectional data transfer using complementary DIR and OE inputs. Its internal architecture includes eight identical transceiver cells, each with separate input buffers, output drivers, and 3-state control logic. Output enable is active-low, and direction control determines signal flow from A-to-B (DIR = HIGH) or B-to-A (DIR = LOW).

It conforms to standard LS-TTL voltage thresholds (V_IH = 2.0 V min, V_IL = 0.8 V max) and drives ±8 mA at V_OH = 2.7 V and V_OL = 0.5 V. Propagation delays are specified at V_CC = 5 V, T_A = 25°C, with t_PD ≤ 25 ns (A↔B path) and t_EN/t_DIS ≤ 25 ns (output enable timing).

Key Specifications

Pinout & Package

SN74LS640-1N is housed in a 20-pin plastic dual in-line package (PDIP-N), 0.300-inch wide body, with 0.100-inch lead pitch and through-hole mounting. Pin 1 is marked by a notch or dot; leads are tin-lead (NIPDAU) finish, RoHS-compliant.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal bus transceiver applications.

Compared with SN74LS245N and SN74LS641N, the SN74LS640-1N uniquely integrates bidirectional flow control and 3-state outputs in one PDIP-20 package, eliminating external logic and enabling direct replacement in legacy LS-TTL bus architectures without layout changes.

Availability

SN74LS640-1N is available at Aetrix Electronics and suitable for industrial control backplanes, legacy microprocessor system upgrades, and test equipment signal routing requiring stable component supply and long-term obsolescence management.

Supply support for SN74LS640-1N 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 and embedded processing technologies with broad industrial, automotive, and consumer product portfolios.

The SN74LS640-1N belongs to TI's legacy 74LS logic family, engineered for reliability and interoperability in commercial-grade digital systems where backward compatibility with 1970s–1990s TTL designs remains critical.

FAQ

What is the function of the DIR pin on the SN74LS640-1N?

The DIR (Direction) pin on the SN74LS640-1N is an active-HIGH control input that determines data flow direction: when DIR = HIGH, data passes from Port A to Port B; when DIR = LOW, data flows from Port B to Port A. This pin enables dynamic bus arbitration in systems like microprocessor address/data multiplexing. The SN74LS640-1N uses this pin in conjunction with OE to achieve glitch-free bidirectional communication without external logic.

Can SN74LS640-1N replace SN74LS245N in an existing design?

No, SN74LS640-1N cannot directly replace SN74LS245N because they differ fundamentally in direction control architecture: SN74LS245N is unidirectional and requires external logic to manage flow, while SN74LS640-1N integrates bidirectional control via its DIR pin. Substituting SN74LS640-1N would require redesigning direction logic and verifying timing margins. The SN74LS640-1N is not a drop-in replacement for SN74LS245N.

What is the maximum clock rate supported by SN74LS640-1N in a data bus application?

The SN74LS640-1N does not operate on a clock signal-it is asynchronous. Its usable bus cycle rate depends on propagation delay (≤25 ns) and system setup/hold timing. In practice, it supports reliable operation in 20-MHz maximum bus systems (50 ns minimum cycle time), assuming proper PCB layout, termination, and load capacitance ≤ 50 pF per line. The SN74LS640-1N datasheet specifies no clock; timing is governed by edge-triggered control signals (OE, DIR).

Does SN74LS640-1N include internal pull-up or pull-down resistors on its I/O pins?

No, the SN74LS640-1N does not include internal bus-hold or pull-up/pull-down resistors on any A or B port pins. All I/O lines must be externally terminated when floating to prevent undefined logic states and noise susceptibility. This is consistent with standard 74LS logic design practice. Designers must add discrete resistors (typically 4.7 kΩ to V_CC or GND) where needed-especially on unused or intermittently driven ports of the SN74LS640-1N.

Is SN74LS640-1N RoHS compliant and what is its lead finish?

Yes, SN74LS640-1N is RoHS compliant (per TI Package Option Addendum). Its lead finish is NiPdAu (Nickel-Palladium-Gold), commonly referred to as NIPDAU, which provides solderability and corrosion resistance for lead-free reflow processes. The SN74LS640-1N meets JEDEC J-STD-020 moisture sensitivity level (MSL) requirements for PDIP packages - rated as "N/A for Pkg Type", indicating no floor life limitation for through-hole assembly.