Datasheet
SCBS123F − AUGUST 1992 − REVISED APRIL 2004
2
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
description (continued)
Transceiver function is controlled by output-enable (OE) and direction (DIR) inputs. When OE is low, the
transceiver is active and operates in the A-to-B direction when DIR is high or in the B-to-A direction when DIR
is low. When OE
is high, both the A and B outputs are in the high-impedance state, effectively isolating both
buses.
Data flow is controlled by clock (CLKAB and CLKBA) and select (SAB and SBA) inputs. Data on the A bus is
clocked into the associated registers on the low-to-high transition of CLKAB. When SAB is low, real-time A data
is selected for presentation to the B bus (transparent mode). When SAB is high, stored A data is selected for
presentation to the B bus (registered mode). The function of the CLKBA and SBA inputs mirrors that of CLKAB
and SAB, respectively. Figure 1 shows the four fundamental bus-management functions that can be performed
with the ’ABT8646.
In the test mode, the normal operation of the SCOPE bus transceivers and registers is inhibited, and the test
circuitry is enabled to observe and control the I/O boundary of the device. When enabled, the test circuitry
performs boundary-scan test operations as described in IEEE Standard 1149.1-1990.
Four dedicated test pins control the operation of the test circuitry: test data input (TDI), test data output (TDO),
test mode select (TMS), and test clock (TCK). Additionally, the test circuitry performs other testing functions
such as parallel-signature analysis (PSA) on data inputs and pseudorandom pattern generation (PRPG) from
data outputs. All testing and scan operations are synchronized to the TAP interface.
The SN54ABT8646 is characterized for operation over the full military temperature range of −55°C to 125°C.
The SN74ABT8646 is characterized for operation from −40°C to 85°C.
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 disabled 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 disabled 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 are always enabled; i.e., data at
the bus pins is stored on every low-to-high transition of the clock inputs.