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 generated

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 er-

ror to the CPU).

•

Independent Latch Enables (LEA and LEB) allow other

permutations of generating/checking (see Function Table

below).

Function Table

Inputs

Operation

GAB

GBA

SEL

LEA

LEB

H

H

X

X

X

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.

H

L

H

X

H

BPAR/B[0:7] → APAR/A0:7] Feed-through mode. Generated parity

checked against BPAR and output as ERRB.

H

L

H

H

H

BPAR/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

HLHL

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. Generated parity checked against latched APAR and output as

ERRA.

L

H

H

H

L

APAR/A[0:7] → BPAR/B[0:7]

Feed-through mode. Generated parity checked against APAR and

output as ERRA.

L

HHHH

APAR/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.

H = HIGH Voltage Level

L = LOW Voltage Level

X = Immaterial

Note 1: O/E = ODD/EVEN

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