Table of Contents
Advertisement
7970B/7970C
Theory of Operation
SECTION II
THEORY OF OPERATION
2-1.
INTRODUCTION.
2·2.
This section provides an overall functional descrip·
tion of the tape transport and description of the transport
circuits. Block diagrams are included to aid in understand-
ing the operation of the transport circuits.
2-3.
OVERALL FUNCTIONAL DESCRIPTION.
2-4.
The tape transport controls the movement of mag-
netic tape, provides power for read and write data circuits,
and supplies status signals to the interface. Off-line com-
mands (High-Speed, Reverse, Forward, Rewind, and Off-
Line) are generated by the interface. The off-line and on-
line commands are processed by the control and status cir-
cuits and result in controlling signals for the capstan servo,
reel servo, and data circuits. Status signals for the interface
and front panel indicators are also provided by the control
and status circuits. Figure 2-1 is a functional block diagram
of the tape transport circuits.
2-5.
At initial power-on, the capstan motor circuit is
open, and the reel servo motors are shorted. When the LOAD
switch is pressed, the capstan motor and reel motor circuits
are completed. When tape tension is established, the tension
arms swing away from the tension limit switches. With tape
tensioned, the capstan and reel motor returns are maintained.
When the LOAD switch is released, the control and status
circuits initiate a load point search. During load point search,
the reel servo circuit operates with voltage feedback and the
capstan servo pulls tape forward at 20 ips.
2-6.
When the load point tab is detected by the photo-
sense assembly, the control and status circuits terminate the
load point search, and tape motion stops. The control and
status circuits provide a load point status to the interface,
and the front panel LOAD indicator illuminates.
2-7.
Pressing the ON-LINE switch establishes interface
control of the tape unit. Except for RESET, the front panel
controls have no control of the tape unit. Pressing RESET
releases the interface control and allows front panel control
of the tape unit.
2-8.
When the tape unit is under interface control and
data is being processed, the tape unit pulls tape at synchro-
nous speed as required. The reel servo circuits operate with
voltage feedback and tension is maintained. When the end-
of-tape tab is detected, the control and status circuits pro-
vide the interface with an end-of-tape (EOT) status.
Part 2
2-9.
A Rewind command generated by the interface
will initiate the rewind sequence. The control and status
circuits place the capstan servo circuits in a high-speed re-
verse mode and switch the reel servo feedback circuits. The
capstan rewinds tape at 160 ips and the reel servo operates
with current feedback. When the trailing edge of the
beginning-of-tape (BOT) tab is detected, the control and
status circuits terminate the rewind mode, and after a one-
second delay, establish a load point search mode.
2-10.
Pressing RESET releases the tape unit from inter-
face control. Pressing REWIND in this condition rewinds
the tape on the supply reel. When the tape is run-off of the
take-up reel, the tension arms contact the tension arm limit
switches, the capstan motor circuit is broken and the reel
motors are shorted. This provides dynamic braking of the
reel motors.
2-11.
CIRCUIT DESCRIPTIONS.
2-12.
The following paragraphs describe the power sup-
ply circuits, control and status circuits, capstan servo circuits
and reel servo circuits of the tape transport. Block Diagrams
of the circuits are provided as an aid in understanding the
operation of the circuits. Refer to section VII for detailed
schematic diagrams of the tape transport.
2-13.
POWER SUPPLY AND POWER DISTRIBUTION
CIRCUITS.
2-14.
The power supply of the tape transport consists of
a power transformer, three center-tapped bridge rectifiers,
filter capacitors, and bleeder resistors. (See figure 7 -2.) The
primary circuit of the power supply transformer includes a
switch that allows selection of 115 or 230 Vac operation, a
power on-off switch, and a line filter/power connector_
When 115 Vac power is selected, the two primary windings
of the power transformer are in parallel and fuse F1 pro-
vides overload protection. When 230 Vac power is selected,
the two primary windings of the power transformer are in
series, and fuse F2 is placed in series with the primary wind-
ing to provide overload protection.
2-15.
The power distribution circuits are on the power
distribution PCA. The assembly contains secondary fuses
and connectors for power distribution. The assembly also
contains a printed-circuit connector for the power regulator
printed-circuit assembly. Silicon diodes in the ±20 volt cir-
cuits and the ±12 volt distribution circuits provide circuit
protection.
2-1
Advertisement
Table of Contents
