Table of Contents
Advertisement
A-32
HP 9895A Disc
Memory Command Set
Buffered Read
Type:
Purpose:
Description:
Disc Read Commands
Disc read.
Data is transferred through an internal buffer in 9895A
before being sent to the HP-IB. This allows HP-IB data
transfers to be asynchronous with the disc, and to vary
from an arbitrarily low rate to about 190K bytes per sec-
ond. The maximum number of bytes to be transferred in a
buffered read is 256
(1
sector); the read request must be
repeated for each additional sector transferred.
Following reception of the Read command, parallel poll
response is disabled and the status of the specified unit is
checked. If the unit can be accessed, then the current cy-
linder number is read and compared with the target cylin-
der address.
If
they differ, a seek to the target cylinder is
performed. This may occur if the actuator has slipped or if
an auto-increment to the next cylinder is required.
If an HP format disc is being used, then the target sector's
256 bytes are read into the controller's buffer.
If
the read
completes successfully, then the target address is in-
cremented by one sector.
If
the sector is not found, a CRC
error is indicated, or a D bit is encountered, the target
address is not incremented.
If an IBM disc is present, then the target sector's 128 bytes
are read into the controller's buffer. If the read completes
successfully, then the target address is incremented by
one.
If
the target sector is not found, a CRC error is indi-
cated, or the D bit is encountered, then the target address
is not incremented.
After the data has been buffered into the controller, the
parallel poll response is re-enabled, indicating that the de-
vice has data ready to transmit. The bus controller should
request the data by issuing the "Send Data" secondary.
Upon receiving the secondary, the 9895A again disables
parallel poll.
lf the read was terminated before data was loaded into the
buffer (i.e., any error except CRC or D bit on), the device
will respond by sending an EOI tagged byte and enabling
parallel poll response.
If
there was no error, the sector's worth of data is made
available. The bus controller can take any number of bytes
up to a sector's length. If more than one sector is re-
quested, the 9895A will send a byte tagged with an EOI.
The number of bytes taken has no effect on the updating of
the target address.
Advertisement
Table of Contents
