Cold Load Read - HP 9895A Service Manual

Cold Load Read
Type:
Purpose:
Description:
HP 9895A Disc Memory Command Set
A-39
Disc read.
The cold load read is a command to read from unit 0,
cylinder 0 at a specified head and sector address. This
command consists of a seek to cylinder 0 followed by a
read operation starting at the specified head and sector.
Consecutive sectors are read until the bus controller stops
the read.
Following reception of the Read command, parallel poll is
disabled, the DSJ cleared and the first status bit checked. If
first status is set, the format of the disc is determined and
the first status bit cleared. If the unit can be accessed, the
9895A performs a seek to cylinder 0, reads the specified
sector into the internal buffer, then asserts parallel poll
waiting for the send data secondary. After the send data
secondary is received, parallel poll is disabled and the buf-
fered sector is sent to the bus controller. When the sector
has been sent, the controller fills the buffer with the next
sector from the disc and then sends it to the bus controller.
This process is repeated until one of the terminating condi-
tions occurs:
1. If the unit becomes unavailable or a sector cannot
be found when the buffer is empty, a byte tagged
with EOI is sent.
2. If a CRC error or D bit is encountered, the sector is
sent followed by a byte tagged with an EOI.
3. If at any time during the operation the device notices
that it has been untalked or that the bus controller
has sent a byte, the transfer will be stopped.
Following any of the above terminations, status is updated
and parallel poll response is re-enabled. If there was an
error in reading data from the disc, the target address is left
pointing to the sector in which the error occurred. Other-
wise, the target sector points to the sector following the last
sector read from the disc.
The cold load read uses unbuffered HP-IB protocol, al-
though all sector transfers take place through the buffer.
Thus, there is a pause in data flow to the HP-IB each time
the buffer is re-filled from the disc. This pause occurs at the
beginning of the read and after every sector is transferred.
Depending on when the read is started and the staggering
of the sectors (see the Format command), this inter-sector
pause may be up to 160 milliseconds long.