TABLE VI Read-Modify-Write Function

DB OUTPUT

MEMORY

EDAC FUNCTION

CONTROL

BYTEn

OEBn

LATCH

CHECK IO

CB

ERROR FLAG

CYCLE

S1

S0

LEDBO

CONTROL

ERR MERR

Read

Read

Flag

H

L

Input

H

X

Input

H

Enabled

Input

Input

Read

Latch input data

H

H

data

H

L

check word

H

Enabled

check bits

latched

latched

Latch corrected

Output

Hi-Z

H

Read

data word into

H

H

data

HH

Output

Enabled

output latch

word

Syndrome

L

latched

bits

Input

Modify appropriate

modified

H

Modify

byte or bytes

LL

BYTE0

H

Output

LH

H

write

generate new

Ouput

check word

check word

unchanged

L

BYTE0

Read-Modify-Write (Byte Control)

Operations

The DP8402A and DP8403 devices are capable of byte-

write operations The 39-bit word from memory must first be

latched into the DB and CB input latches This is easily ac-

EDAC will then make any corrections if necessary to the

data word and place it at the input of the output data latch

This data word must then be latched into the output data

latch by taking LEDBO from a low to a high

Byte control can now be employed on the data word

through the OEB0 through OEB3 controls OEB0 controls

DB0 – DB7 (byte 0) OEB1 controls DB8 – DB15 (byte 1)

OEB2 controls DB16 – DB23 (byte 2) and OEB3 controls

DB24 – DB31 (byte 3) Placing a high on the byte control will

disable the output and the user can modify the byte If a low

is placed on the byte control then the original byte is al-

lowed to pass onto the data bus unchanged If the original

data word is altered through byte control a new check word

must be generated before it is written back into memory

This is easily accomplished by taking control S1 and S0 low

Table VI lists the read-modify-write functions

Diagnostic Operations

The DP8402A thru DP8405 are capable of diagnostics that

allow the user to determine whether the EDAC or the mem-

ory is failing The diagnostic function tables will help the

user to see the possibilities for diagnostic control

latched into the input latch while the data input remains

transparent This lets the user apply various data words

against a fixed known checkword If the user applies a diag-

nostic data word with an error in any bit location the ERR

flag should be low If a diagnostic data word with two errors

in any bit location is applied the MERR flag should be low

After the checkword is latched into the input latch it can be

verified by taking OECB low This outputs the latched

checkword With the DP8402A and DP8403 the diagnostic

data word can be latched into the output data latch and

verified It should be noted that the DP8404 and DP8405 do

not have this pass-through capability because they do not

contain an output data latch By changing from the diagnos-

diagnostic data word Also the syndrome bits can be pro-

duced to verify that the EDAC pinpoints the error location

Table VII DP8402A and DP8403 and Table VIII DP8404 and

DP8405 list the diagnostic functions

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