AT&T 3B2/300 Technical Reference Manual page 475

FUNCTIONAL D E S C R I P T I O N - - - - - - - - - - - - - - - - - - - - - - - - - - -
Disk Interface
The Disk Interface consists of the following circuits.
Hard Disk Controller
The hard disk controller (NEC µPD7261) connects to the lower half of the demultiplexed
80186 Microprocessor data bus. Under the control of the 80186 Microprocessor, the hard
disk controller generates the ST-506 signals to control the associated disk drives.
Data Separator
The data separator processes (separates) the serial stream of bits read from the disk drive
into clock and data signals that are applied to the µPD7261.
Write Precompensation
The bits recorded on the inner tracks of a disk are more densely packed than the outer
tracks. Write precompensation is necessary to ensure that the data bits are stored at the
right place on the disk. The controller sends two signals to enable either an early or a late
write to the disk. These write enables are actually too late to correctly adjust the data.
The precompensation circuit is a 10 tap, 100-nanosecond delay line with the 50 percent
tap delaying the data enough to have valid early and late enable. The 80, 90, and 100
percent taps provide the early, normal, and late data. The write precompensation is
turned on or off by the Reduced Write Current (RWC) signal from the hard disk controller.
Driver /Receiver
The driver and receiver circuits interface the Control Bus, Data 0, and the Data 1 with the
associated disk drives.
CM 195K Equipped Device Table Data
The following table shows the Equipped Device Table (EDT) data for an XDC card. A value of 0
means NO; a value of 1 means YES. The indirect device (indir_dev) is only applicable to systems
equipped with Small Computer System Interface (SCSI). The request queue size (rq_size) and the
completion queue size (cq_size) are XDC software defined values. As such, the rq_size and cq_size
values may differ between versions of XDC software.
EDT ITEM DATA
IDENTIFICATION CODE (ID_code) Ox 0204
DEVICE NAME (dev_narne) XDC
REQUEST QUEUE SIZE (rq_size) Ox 14
COMPLETION QUEUE SIZE (cq_size) Ox 28
BOOT DEVICE (booLdev) 0
WORD SIZE (word_size) 1
BOARD SIZE (brcLsize) 0
SMART BOARD (srnrLbrd)
1
CONSOLE CAPABILITY (cons_cap)
0
CONSOLE FILE (cons_file) 0
INDIRECT DEVICE (indir_dev) 0
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