Viper 60s Theory Of Operation And Maintenance Manual page 148

THEORY OF OPERATION
Table 6·3. Head Assembly Functions
I
FUNCTION·
Read/Write Head Positioning
Tape Track Sensing
Molor Interface
DESCRIPTION
The Viper drive requires that the head be moved across the
width of the tape for writing and reading various data tracks on
the tape. To accomplish this, the read/write head is attached to
a nut which rides on a threaded leadscrew shaft. This shaft is
turned by a stepper motor which rotates in 15° increments.
Each 15° increment advances the head, by 0.001 inch across
the tape.
The stepper motor provides precision data track positioning
force, while additional head guidance is performed by ~ow
friction plastic bushings sliding on steel guide pins.
The stepper motor and associated control circuits move the
head assembly the required number of electronically calculated
steps to a designated track. Precise positioning of the head
assembly is essential to assure media interchangeability from
one drive to another.
The Viper models 125S and 150S use edge of tape sensing to
precisely locate the position for Track 0, and then use Track 0
as a reference point to step to the other track locations.
Because Viper model 60S does not require this level of track-
placement precision, Track 0 is located by positioning from the
stepper end stop.
Two stepper motor driver circuits control the unipolar stepper
motor. The drivers have lines connected to the stepper motor
in a full stepping mode of operation. This mode of operation
requires that two stepper lines be in a low state when stepping
and off when not stepping.
The motor interface circuit consists of two drivers. The four
stepper lines are connected to the microprocessor and are
under microprocessor and EPROM program control.
6-17