2000 Fairchild Semiconductor Corporation
DS009172
www.fairchildsemi.com
October 1986
Revised February 2000
D
DM74ALS646
Octal 3-STATE 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 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.
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.
Features
I
Switching specifications at 50 pF
I
Switching specifications guaranteed over full tempera-
ture and V
CC
range
I
Advanced oxide-isolated, ion-implanted Schottky TTL
process
I
3-STATE buffer outputs drive bus lines directly
I
Multiplexed real-time and stored data
I
Independent registers for A and B buses
Ordering Code
Devices also available in Tape and Reel. Specify by appending the suffix letter “X” to the ordering code.
Order Number
DM74ALS646WM
DM74ALS646NT
Package Number
M24B
N24C
Package Description
24-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-013, 0.300 Wide
24-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-100, 0.300 Wide