www.fairchildsemi.com 2
74ABT652
Truth Table
H = HIGH Voltage Level
L = LOW Voltage Level
X = Immaterial
= LOW to HIGH Clock Transition
Note 1: The data output functions may be enabled or disabled by various signals at OEAB or OEBA
inputs. Data input functions are always enabled, i.e.,
data at the bus pins will be stored on every LOW to HIGH transition on the clock inputs.
Functional Description
In the transceiver mode, data present at the HIGH imped-
ance port may be stored in either the A or B register or
both.
The select (SAB, SBA) controls can multiplex stored and
real-time.
The examples in Figure 1 demonstrate the four fundamen-
tal bus-management functions that can be performed with
the ABT652.
Data on the A or B data bus, or both, can be stored in the
internal D flip-flop by LOW to HIGH transitions at the
appropriate Clock Inputs (CPAB, CPBA) regardless of the
Select or Output Enable Inputs. When SAB and SBA are in
the real time transfer mode, it is also possible to store data
without using the internal D flip-flops by simultaneously
enabling OEAB and OEBA
. In this configuration each Out-
put reinforces its Input. Thus when all other data sources to
the two sets of bus lines are in a HIGH impedance state,
each set of bus lines will remain at its last state.
Logic Diagram
Please note that this diagram is provided only for the understanding of logic operations and should not be used to estimate propagation delays.
Inputs Inputs/Outputs (Note 1) Operating Mode
OEAB
OEBA
CPAB CPBA SAB SBA
A
0
thru A
7
B
0
thru B
7
L H H or L H or L X X Input Input Isolation
L H
X X Store A and B Data
X H
H or L X X Input Not Specified Store A, Hold B
H H
X X Input Output Store A in Both Registers
L X H or L
X X Not Specified Input Hold A, Store B
L L
X X Output Input Store B in Both Registers
L L X X X L Output Input Real-Time B Data to A Bus
L L X H or L X H Store B Data to A Bus
H H X X L X Input Output Real-Time A Data to B Bus
H H H or L X H X Stored A Data to B Bus
H L H or L H or L H H Output Output Stored A Data to B Bus and
Stored B Data to A Bus
