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12606B
Section IV
SECTION IV
THEORY OF OPERATION
4-1.
INTRODUCTION.
4-2.
This section explains the circuit theory of the data
channel interface card and the command channel interface
card. Operations of the disc memory and computer are
described only to the extent required for explaining the
functioning of the two interface cards.
4-3.
For brevity, the names of equipment items
mentioned in this section have been shortened as follows:
a.
The HP 12606-6001 Data Channel Interface card is
referred to as the "data card".
b. The HP 12606-6002 Command Channel Interface
card is referred to as the "command card".
c. The HP 2770A/-001/-002/-003 or 2771Aj-001j-
002/-003 Disc Memory is referred to as the "disc memory"
or "disc".
d. The HP 2772A or 2772A-02 Disc Memory Power
Supply is referred to as the "disc power supply".
e. The
HP
2114B, 2115A, 2116A, or 2116B
Computer is referred to as the "computer".
f.
The HP 12578A, 12578A-01, or 12607 A Direct
Memory Access is referred to as "DMA" or the "DMA
system".
4-4.
OVERALL FUNCTIONAL DESCRIPTION.
4-5.
The data card and command card, under control
of the computer, perform the following functions:
a. Determine when the first disc sector of a read or
write operation reaches the disc read/write head.
b. When writing on the disc, receive 16-bit parallel
words from the computer, generate a parity bit (odd parity
is used), and forward the resulting 17 -bit word to the disc
iil'Seriai fashion.
c. When reading from the disc, receive 17 -bit serial
words from the disc, check parity, and forward the 16 data
bits in parallel to the computer.
d. Prevent writing on protected tracks.
e. Forward a disc status word to the computer.
4-6.
Interface circuits for controlling the transfer of
data to and from the disc are situated principally on the
command card. Additional control circuits are located on
the data card, together with circuits for handling the data
transferred.
4-7.
The DMA system transfers to or from the
computer all words written on the disc or read from the
disc. DMA and the disc memory conduct these operations
without the performance of computer instructions, other
than those required to initiate the operation.
4-8.
DETAILED THEORY.
4-9.
REFERENCE INFORMATION.
4-10.
The following paragraphs present general informa-
tion which is required for understanding the detailed theory
discussion that follows.
4-11.
BINARY VOLTAGE LEVELS. The binary signal
levels on both interface cards are approximately +3.5 volts
and +0.2 volts. The levels may vary from these approximate
amounts, depending on the type of integrated circuit
providing the signal. The input and output voltage levels for
each type of integrated circuit are specified in section V of
this manual.
4-12.
LOGIC CIRCUITS. The logic circuits on both
interface cards principally employ positive logic. That is to
say, all inputs to an "and" or "nand" gate must be +3.5
volts for coincidence to occur. Similarly, if any input to an
"or" or "nor" gate is +3.5 volts, the output is +3.5 volts for
an "or" gate or +0.2 volts for a "nor" gate. The output
from the "set" side of a flip-flop is approximately +3.5
volts when the flip-flop is set, and +0.2 volts when the
flip-flop is reset. As an exception to the use of positive
logic, diodes CR1 through CR10 on the data card, together
with the circuits to which they connect on the command
card, form a negative "and" gate. Also, "nand" gates
MC84B, MC123A, and MC124B on the data card, and
"nand" gates MC14A, MC24A, and MC44A on the
command card, are used as negative-logic "nor" gates.
4-13.
In accordance with established usage for positive-
true logic circuits, the term "true" in this manual refers to a
nominal signal level of +3.5 volts, and "false" refers to a
nominal level of +0.2 volts.
4-14.
ABBREVIATIONS. Signal-name abbreviations are
listed in tables 5-3 and 5-5, together with the meanings of
the abbreviated designations. For the meanings of abbrevi-
ations and letter symbols which are not signal names, refer
to tables 4-2, 4-3, and 6-2.
4-15.
SIGNAL NAMES. Signals which enter or leave the
two interface cards are named in one of the following ways:
a. As a condition which either exists or does not exist.
b. As a command or order, expressed in the imperative
grammatical mode.
4-1
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