DM74AS646WMX

Overview

The DM74AS646WMX is an octal bus transceiver and register utilizing Advanced Schottky TTL technology. It integrates an octal D-type register with edge-triggered storage for real-time or stored data selection via SAB/SBA pins. The device features totem-pole 3-STATE outputs capable of driving highly capacitive loads directly without pull-up components. Operating conditions include a supply voltage range of 4.5 V to 5.5 V and a free air operating temperature from 0 °C to +70 °C. Maximum clock frequency reaches 90 MHz, with propagation delays ranging from 17 ns to 29 ns depending on the signal path.

Feature Summary

• Integrates an octal bus transceiver with an internal octal D-type register for data storage and multiplexing.
• Features totem-pole 3-STATE outputs designed to drive low-impedance loads directly without interface components.
• Includes make-before-break select controls (SAB/SBA) to eliminate glitches during transitions between real-time and stored data.
• Functionally and pin-for-pin compatible with LS TTL counterparts.

Applications

• Implementing buffer registers in microprocessor systems.
• Configuring I/O ports for bidirectional data transfer.
• Acting as working registers within bus-organized systems.
• Driving bus lines in high-capacitance environments.

Procurement Notes

Verify the specific suffix 'WMX' against current manufacturer documentation to confirm package type and tape-and-reel availability. Confirm RoHS compliance status as historical datasheets may list non-compliant variants. Check for obsolescence notices or lifecycle changes from onsemi or Fairchild Semiconductor. Ensure the ordering code matches the required 24-lead SOIC package specification.

Selection Notes

Select this component when a combined bus transceiver and storage register is required for bidirectional data handling. The AS series offers higher speed than standard LS logic but maintains pin compatibility. Consider the 3-STATE output drive capability for direct bus interfacing. Evaluate the 90 MHz maximum clock frequency against system timing requirements. Verify that the 4.5 V to 5.5 V supply range aligns with the target power architecture.

Part Context

This part belongs to the DM74AS646/DM74AS648 family of Octal Bus Transceivers and Registers. The DM74AS646 provides true outputs, while the DM74AS648 provides inverted outputs. Both share identical control logic and pinouts. The 'M' suffix denotes the 24-Lead Small Outline Integrated Circuit (SOIC) package. The 'X' suffix typically indicates tape-and-reel packaging.

Replacement Considerations

Public confirmed substitute part numbers: DM74AS648WMX (inverted output variant).

FAQ

How does the device handle transitions between real-time and stored data?

The SAB and SBA control pins utilize a make-before-break configuration to eliminate glitches that typically occur during transitions between stored and real-time data modes.

What is the function of the G and DIR pins?

The Enable (G) and Direction (DIR) pins provide four modes of operation: real-time transfer from bus A to B, real-time transfer from bus B to A, real-time data transfer to internal storage, or internal store data transfer to the buses.