Data Record Architecture
MOTOR
MOTOR
OFF
ON
1.
Leader 256 bytes (of ones)
2. Sync byte
ASCII
Sync Char
(X'16')
3.
Sync byte
(X
'16')
4.
Data
Blocks 256 bytes
5. CRC -- 2 bytes - for each data block
Error Recovery
Error recovery
is
handled by software. A cyclic redundancy
check (CRC) is used to detect errors. The polynomial used
is:
G(X)
- X16
«X12
« X5 X 1
Which
is
the polynomial used by the
SDLC
interface. Essen-
tially,
as
bits are written/read from tape, they are passed through
the CRC-register in software. After a block
of
data
is
written, the
complemented value
of
the calculated CRC-register
is
written on
tape. On reading the cassette data, the CRC bytes are read and
compared to the generated
CRC
value.
If
the read CRC does not
equal the generated one, the processor's carry flag is set and
status (AH)
is
set to X'O
l'
to indicate a CRC error has occurred.
Also, the routine
is
exited on CRC error.
3-10
To Next Page
Loading...
Need help?
General
