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SECTION III
THEORY OF OPERATION
3.1
TRANSPORT COMMAND SIGNALS
TIll>
transp0rt command lines include H.UN FOHWARD,
I--AST FOHWARD, HUN HEVEHSE and FAST RE-
V EHSE. These control command inputs are supplied
from the interface to the Transport Electronics
Board.
On that module, the motion controls are
gated and applied to the ramp generator circuitry to
develop the proper ramp ups and ramp downs for
operating the drive motor.
A capstan tachometer
feeds back a regulating voltage and current for con-
trolling motor speed.
3.2 A AND B HOLE PHOTOSENSORS
These vertically mounted photo sensors detect the A
and B holes on the tape, which indicate beginning of
tape, end of tape and load point in accordance with
the proposed ANSI standard.
This information is
directed back to the interface in the form of Hole A
and Hole B status signals.
Note that the photocell outputs must be decoded and
stored in order to accurately determine position on
tape.
For example, when the cartridge is initially
illserted a rewind operation could be performed. A
fast reverse command could be issued until both the
A and B holes are detected simultaneously in re-
verse. When they are, beginning of tape has been
reached and fast reverse should be dropped. If it is
desired to advance to load point, run forward should
be issued until the A hole is detected in the for-
ward direction, the end of tape warning position has
been reached. Continued forward tape motion should
only be allowed until both the A and B outputs are
simultaneously detected, signifying "End of Tape."
3-1
3.3 SELECT A AND SELECT B SIGNALS
The selected read or write track is determined
by
the state of these interface signals.
For example,
SELECT A high and SELECT B high choose track
four for reading or writing. WHITE ENABLE false
places the machine in the READ DATA mode; WHITE
ENABLE true enables tape writing.
The SAFE SWITCH is activated when the WHITE
ENABLE plug on the 3M tape cartridge is in the SAFE
position.
With the SAFE switch on, the write eir-
cuitryon the Transport Electronics Board is disabled
to prevent accidental tape erasure.
3.4
WRITE DATA
The polarity of this signal is such that a high (false)
input state generates the ANSI standard erase polarity
for gap generation.
3.5 READ DATA
A
"0"
transition in lapse is defined as a voltage trans-
ition from low to high at the interface. A" 1" transi-
tion is defined as a voltage transition from high to
low at the interface. READ DATA OUTPUT is forced
high (false) during t he read gap.
3.6 CARTRIDGE RELEASE SOLENOID
This solenoid is actuated on command from the inter-
f3:ce.
releasing the 3M cartridge whenever CAH-
TRIDGE RELEASE goes true.
To protect the car'-
tridge release solenoid, the input command is inter-
nally timed out inthe event that CAHTRIDGE HELE..\SE
is not turned off after the cartridge has been ejected.
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