Read Mode Sensing - IBM 5114 Maintenance Information Manual

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If the address mark is correct, the read operation
continues. If the address mark is not correct, a resync is
issued by the system. A resync command consists of
two separate commands. First the system sends the
RESET READ command over the 'bus out' line, diskette
control byte, bit 7. Then it sends the START READ
command over the 'bus out' line, diskette control byte,
bit 1. This causes the adapter to start looking for a sync
field again.
The presence of a 1 bit after the sync field starts a bit
and byte counter and the CRC generator.
The bit counter counts to 8 and is used to frame the
data into bytes. The data is blocked into bytes in the
read deserializer. As soon as 8 bits have been counted,
+t,ey are passed to the buffer. When the buffer receives
a
b'y~~
of data, the 'interrupt request 2' line is raised.
When thb
~ystem
receives this interrupt request, it takes
the data from ~a,e buffer over the 'bus in' line and resets
the interrupt reque..:.... This transfer of data to the buffer
and interrupt sequence is repeated every 8 bits. (32
microseconds in FM; 16
m:~roseconds
in M FM). This
sequence continues until reset
~~,
the system.
The data bits that are sent to the buffe. "'re also sent to
the CRC generator. This generator builds 2 ;.. ·'tes of
CRC information. The command to read the CRL
information comes from the system over the 'bus
OLh'
line, diskette control byte, bit 5. For a read operation,
this command causes the adapter to compare the CRC
generated during the read operation with the CRC read
from the diskette.
The read operation is terminated when the system
sends the RESET READ command over the 'bus out'
line, diskette control byte, bit 7.
READ MODE SENSING
During a read operation, the system is sensing the
condition of the adapter over the 'bus in' line by
sampling the diskette sense byte. For an explanation of
the different bits of the diskette sense byte. (See
Diskette Sense under Adapter Commands in this section.)
Operations 3·33