DM74AS646

DM74AS646 DM74AS648 Octal Bus Transceiver and Register October 1986 Revised March 2000 DM74AS646 DM74AS648 Octal Bus Transceiver and Register General Description This device incorporates an octal bus transceiver and an octal D-type register configured to enable multiplexed transmission of data from bus to bus or internal register to bus. This bus transceiver features totem-pole 3-STATE outputs designed specifically for driving highly-capacitive or relatively low-impedance loads. The high-impedance third state and increased high-logic-level drive provide this device with the capability of being connected directly to and driving the bus lines in a bus-organized system without need for interface or pull-up components. It is particularly attractive for implementing buffer registers, I/O ports, bidirectional bus drivers, and working registers. The registers in the DM74AS646, DM74AS648 are edgetriggered D-type flip-flops. On the positive transition of the clock (CAB or CBA), the input bus data is stored. The SAB and SBA control pins are provided to select whether real-time data or stored data is transferred. A LOW input level selects real-time data, and a HIGH level selects stored data. The select controls have a “make before break” configuration to eliminate a glitch which would normally occur in a typical multiplexer during the transition between stored and real-time data. The enable G and direction control pins provide four modes of operation; real-time data transfer from ...