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74ACT899
Pin Descriptions Functional Description
The ACT899 has three principal modes of operation which
are outlined below. These modes apply to both the A-to-B
and B-to-A directions.
• Bus A (B) communicates to Bus B (A), parity is gener-
ated and passed on to the B (A) Bus as BPAR (APAR). If
LEB (LEA) is HIGH and the Mode Select (SEL
) is LOW,
the parity generated from B[0:7] (A[0:7]) can be checked
and monitored by ERRB
(ERRA).
• Bus A (B) communicates to Bus B (A) in a feed-through
mode if SEL
is HIGH. Parity is still generated and
checked as ERRA
and ERRB in the feed-through mode
(can be used as an interrupt to signal a data/parity bit
error to the CPU).
• Independent Latch Enables (LEA and LEB) allow other
permutations of generating/checking (see Function
Table).
Function Table
H = HIGH Voltage Level
L = LOW Voltage Level
X = Immaterial
Note 1: O/E
= ODD/EVEN
Pin Names Description
A
0
–A
7
A Bus Data Inputs/Data Outputs
B
0
–B
7
B Bus Data Inputs/Data Outputs
APAR, BPAR A and B Bus Parity Inputs
ODD/EVEN
ODD/EVEN Parity Select,
Active LOW for EVEN Parity
GBA
, GAB Output Enables for A or B Bus,
Active LOW
SEL
Select Pin for Feed-Through or Generate
Mode, LOW for Generate Mode
LEA, LEB Latch Enables for A and B Latches,
HIGH for Transparent Mode
ERRA
, ERRB Error Signals for Checking Generated
Parity with Parity In, LOW if Error Occurs
Inputs Operation
GAB
GBA SEL LEA LEB
H H X X X Busses A and B are 3-STATE.
H L L L H Generates parity from B[0:7] based on O/E
(Note 1). Generated parity → APAR.
Generated parity checked against BPAR and output as ERRB
.
H L L H H Generates parity from B[0:7] based on O/E
. Generated parity → APAR. Generated
parity checked against BPAR and output as ERRB
. Generated parity also fed back
through the A latch for generate/check as ERRA
.
H L L X L Generates parity from B latch data based on O/E
. Generated parity → APAR.
Generated parity checked against latched BPAR and output as ERRB
.
HLHXHBPAR/B[0:7] → APAR/A0:7] Feed-through mode. Generated parity checked
against BPAR and output as ERRB
.
HLHHHBPAR/B[0:7] → APAR/A[0:7]
Feed-through mode. Generated parity checked against BPAR and output as ERRB
.
Generated parity also fed back through the A latch for generate/check as ERRA
.
L H L H L Generates parity for A[0:7] based on O/E
. Generated parity → BPAR. Generated
parity checked against APAR and output as ERRA
.
L H L H H Generates parity from A[0:7] based on O/E
. Generated parity → BPAR. Generated
parity checked against APAR and output as ERRA
. Generated parity also fed back
through the B latch for generate/check as ERRB
.
L H L L X Generates parity from A latch data based on O/E
. Generated parity → BPAR. Gen-
erated parity checked against latched APAR and output as ERRA
.
LHHHLAPAR/A[0:7] → BPAR/B[0:7]
Feed-through mode. Generated parity checked against APAR and output as ERRA
.
LHHHHAPAR/A[0:7] → BPAR/B[0:7]
Feed-through mode. Generated parity checked against APAR and output as ERRA
.
Generated parity also fed back through the B latch for generate/check as ERRB
.
