HP 7942 Service Manual page 19

CHANNEL INTERFACE
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QIJ
2-1.
INTR~ODUCTION
Interface to the disc/tape drive is accomplished
through Hewlett-Packard Interface Bus (HP-IB)
hardware and the CS/80 Instruction Set, a set of
commands formulated for mass storage devices.
The following paragraphs discuss the types of
CS/ 80 commands. Also provided is an overview of
HP-IB. For full details of CS/80, refer to the
CS/80 Instruction Set Programming Manual,
part
no. 5955-3442. For information on how the
CS/80 instructions apply to the tape drive refer to
the H P
9144A Service Manual,
Part II, Installation.
2-2. CS/,BO INSTRUCTION SET
The increase in capabilities of both host computers
and mass storage devices has emphasized the need
for efficient channel communication. The CS/80
Instruction Set increases the efficiency and speed
of channel operations between disc memories and
their associated host computers. Table 2-1, Device
Command Summary, provides a summary of all
CS/80 instructions. The CS/8 0 Instruction Set al-
lows a host computer to access special utilities
within the disc drive. Utilities are routines stored
in firmware which allow error rate tests to be per-
formed and the results of such tests to be examined
or logged. Utilities are listed in table 2-2, Disc
Drive Utilities. Refer to the
External Exerciser
Reference Manual,
part no. 5955- 3462, for full
details.
2-3. TRANSACTION STRUCTURE
A transaction is a logically complete operation be-
tween a system host computer and a peripheral
device (the disc drive) over a given channel
(HP-IB). Three phases may occur during each
transaction: command, execute, and report. A
transaction begins when a command is received by
the disc drive, and ends when a reporting message
indicating the status of the transaction is accepted
by the host. Figure 2 - 1 ill ustra tes the transaction
structure, and shows the relationship between the
disc drive operating states and the channel activity
relative to each phase.
A unit is a separately addressable entity within a
device (disc drive). A volume is a separately ad-
dressable portion of the storage media within a
given unit.
2-4. REAL TIME COMMANDS
Real time commands are optimized for execution
time. These commands are used most often in
host/ device transactions. One or more com pie men -
tary commands may precede a real time command
in order to modify the operation of that command.
Real time commands include: locate and read, lo-
cate and write, and cold load read.
2-5. COMPLEMENTARY COMMANDS
Complementary commands are used to set or up-
date programmable states in the disc drive. The
programmable states define characteristics such as:
set unit, set address, set block displacement, set
return addressing mode, set length, set burst mode,
set retry time, set release, set status mask, and set
Rotational Position Sensing (RPS) window size.
These commands may be included within Real
Time, General Purpose, or Diagnostic command
messages, or they may stand alone.
When a complementary command (or commands) is
embedded within another command, the para-
meters or conditions established by that com-
plementary command(s) are altered only for the
duration of the current command. A stand-alone
complementary command, however, sets the para-
meters or conditions until the same stand -alone
complementary command alters the set value or
until power-on occurs. Power-on resets all com-
plementary commands to their default values.
Therefore, at power-on, length is defaulted to
equal the entire volume. A stand-alone Set Length
command may give it a lise til value of 1 kbyte to be
used for an entire sequence of transactions, al-
though some special case commands could tem-
porarily override this value with an embedded
complementary command to set a "current" value
of 256 bytes (for 1 sector).
2-1