Datasheet

SN54HCT646, SN74HCT646
OCTAL BUS TRANSCEIVERS AND REGISTERS
WITH 3-STATE OUTPUTS
SCLS178C – MARCH 1984 – REVISED MARCH 2003
2
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
description/ordering information (continued)
The select-control (SAB and SBA) inputs can multiplex stored and real-time (transparent mode) data. DIR
determines which bus receives data when OE
is active (low). In the isolation mode (OE high), A data can be
stored in one register and/or B data can be stored in the other register.
When an output function is disabled, the input function still is enabled and can be used to store data. Only one
of the two buses, A or B, can be driven at a time.
To ensure the high-impedance state during power up or power down, OE should be tied to V
CC
through a pullup
resistor; the minimum value of the resistor is determined by the current-sinking capability of the driver.
FUNCTION TABLE
INPUTS
DATA I/O
OPERATION OR FUNCTION
OE
DIR CLKAB CLKBA SAB SBA A1–A8 B1–B8
OPERATION
OR
FUNCTION
X X ↑ X X X Input Unspecified
†
Store A, B unspecified
†
X XX ↑ X X Unspecified
†
Input Store B, A unspecified
†
H X ↑ ↑ X X Input Input Store A and B data
H X H or L H or L X X Input disabled Input disabled Isolation, hold storage
L L X X X L Output Input Real-time B data to A bus
L L X H or L X H Output Input Stored B data to A bus
L H X X L X Input Output Real-time A data to B bus
L H H or L X H X Input Output Stored A data to B bus
†
The data-output functions can be enabled or disabled by various signals at OE
and DIR. Data-input functions always are enabled; i.e., data at
the bus terminals is stored on every low-to-high transition of the clock inputs.