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Action IV (Bus Out Check)
No Device End in Unit Status: Reissue the command.
Device End in Unit Status:
If
error occurs while writing, reposition tape and
reissue command. For all other commands, reissue the command.
If
error
persists after six attempts, provide operator message, post completion with error
condition, and exit to operating system. See "Terminal Action
Ia"
and
"Supplementary Information."
Action V (Data Check On Read or Read Back\Nard)
1. Determine if the block should be classified as a noise block (noise block is
less than the minimum block length of 12 bytes).
If
the block is valid data
(noise bit on [sense byte 1, bit O] or block length meets or exceeds minimum
length requirements), attempt another read operation.
If
the block is a noise
record, go to step 4.
To determine block validity:
a. When not data chaining, assume block is valid data if the
ccw
count
less the
csw
residual count is 12 or greater:
b. When data chaining, assume block is a noise record if the count in the
first
ccw
is less than 12 and the chain broke before the second
ccw.
c. When data chaining, assume the block is valid data if the sum of the
ccw
counts (up to and including the failing
ccw)
minus the
csw
residual count is 12 or greater.
2.
Set the correct mode (if seven-track), and reposition tape.
3.
Set the correct mode (if seven-track), and send the track-in-error
information (sense byte 2) to the tape control with a
TIE
command.
Note: For program simplicity, the mode set and
TIE
commands may be
issued whether required or not.
4.
Reissue the read or read backward command.
Note: Transfer in Channel
(TIC)
is the only command that may be executed
between steps 3 and 4, as other commands to the tape control may destroy
the track-in-error and mode set information. When attempting to correct a
nine-track block, use only the track-in-error information from that block.
5. Repeat steps 1 through 4 until the block is read successfully or at least 41
attempts are made. Perform a tape cleaner sequence after every fourth
attempt.
6.
If
the error persists, determine whether Read Opposite Recovery
(ROR)
is
possible. Read Opposite Recovery is not possible if:
a. Data chaining is being performed.
b. Operating in seven-track, data-convert mode (sense byte 1, bit 3 on).
c. Suppress data transfer bit is set in the failing read
ccw.
If
ROR
is not possible, provide an operator message, post completion with
error condition, and exit to the operating system.
If
ROR
is possible, proceed
to step 9 (tape is correctly positioned for first
ROR
attempt). See "Terminal
Action
Ia"
and "Supplementary Information."
7.
Determine if the block should be classified as a noise block. See Action V,
step 1.
8.
Set the correct mode (if seven-track), and reposition tape.
Section 5: Error Recovery Procedures
53
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