October 1986
Revised June 2001
DM74ALS646
Octal 3-STATE Bus Transceiver and Register
General Description
Features
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.
■ Switching specifications at 50 pF
■ Switching specifications guaranteed over full tempera-
ture and VCC range
■ Advanced oxide-isolated, ion-implanted Schottky TTL
This bus transceiver features totem-pole 3-STATE outputs
designed specifically for driving highly-capacitive or rela-
tively low-impedance loads. The high-impedance state and
increased high-logic level drive provides this device with
the capability of being connected directly to and driving the
bus lines in a bus-organized system without the need for
interface or pull-up components. They are particularly
attractive for implementing buffer registers, I/O ports, bidi-
rectional bus drivers, and working registers.
process
■ 3-STATE buffer outputs drive bus lines directly
■ Multiplexed real-time and stored data
■ Independent registers for A and B buses
The registers in the DM74ALS646 are edge-triggered D-
type flip-flops. On the positive transition of the clock (CAB
or CBA), the input bus data is stored into the appropriate
register. The CAB input controls the transfer of data into
the A register and the CBA input controls the B register.
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 nor-
mally occur in a typical multiplexer during the transition
between store and real-time data.
The enable G and direction control pins provide four modes
of operation: real-time data transfer from bus A to B, real-
time data transfer from bus B to A, real-time bus A and/or B
data transfer to internal storage, or internally stored data
transfer to bus A or B.
When the enable G pin is LOW, the direction pin selects
which bus receives data. When the enable G pin is HIGH,
both buses become disabled yet their input function is still
enabled.
Ordering Code
Order Number Package Number
Package Description
DM74ALS646WM
DM74ALS646NT
M24B
N24C
24-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-013, 0.300" Wide
24-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300" Wide
Devices also available in Tape and Reel. Specify by appending the suffix letter “X” to the ordering code.
© 2001 Fairchild Semiconductor Corporation
DS009172
www.fairchildsemi.com