Radio Shack TRS-80 Service Manual page 104

PART 2
THEORY OF OPERATION
INTRODUCTION
The Disk Drive consists of the mechanical and electrical
components necessary to record and read digital data on a
diskette. DC power at ±12V and +5V (provided by the
user for the external drives) is required for operation,
ORGANIZATION OF THE DISK DRIVE
All electrical subasosemlJlies in the Disk Drive are constructed
with leads which terminate in 4 or 5 pin connectors, ena-
bling the individual assemblies to
be
removed.
The millJl1t!lic head is connected to the PC board via a cable
terminating in a S·pin female connector and its associated
male socket which is localed in close proximity to the read/
write data electronics.
Interface signals and power are provided via connectors at
the rear of the Drive. A detailed description of these signals
is presented in Part
3
of this manual.
FUNCTIONAL BLOCK DIAGRAM DESCRIPTION
Figure 4 is a functional block diagram of the Disk Drive and
should be referred to in conjunction with the following
discussion;
The Disk Drive consists of the following functional groups:
*
Index Pulse Shapl!f
*
Write Protect Sensor
*
Track 00 Sensor
*
Spindle Drive Control
*
Carriage Position Control
*
Write/Erase Control
*
Read Amplifier and Digitizer
INDEX
An index pulse is provided to the user system via the
INDEX PULSE interface line. The index circuitry consists
of an Indel< LED,lndCI< Phototransistor and a Pulse Shaping
Network. As the indel< hole in thc disk passes the Indel<
LEDlPhototransistor combination, light from the LED
strikes the Index Phototransistor causing it to conduct. The
signal from the Indel< Phototransistor is passed to the Pulse
Shaping Network which produces a pulse for each hole de·
tected. This pulse is- presented to the user on the INDEX
PULSE interface line,
WRITE PROTECT
A Write Protect signal is provided to the user system via the
WR ITE PROTECT interface line. The write protect circuitry
consists of a Write Protect Sensor and circuitry to route the
signal produced.
When a write protected diskette is inserted in the drive, the
sensor is activated and the logic disables the write elect·
ronics and supplies the status signal to the Interface.
TRACK 90 SWITCH
The level on the TRACK
00
interface line is a function of
the position of the magnetic head assembly. When the head
is positioned at Track 00 and the stepper motor is
;u
the
home position, a true level is generated and sent to the
user.
SPINDLE DRIVE
The Spindle Drive system consists of a spindle assembly
driven by a DC motor·tachometer combination through a
dri"e belt.
Associated with the spindle drive motor are the servo elec-
tronics required for control.
The control circuitry also includes a current limiter and an
interface control line. When the DRIVE MOTOR ENABLE
interface line is true, the drive motor is allowed to come up
to speed. When the current through the drive motor el<ceeds
1.3 A, the current limit circuitry disables the motor drive.
POSITIONER CONTROL
Thc HCild Positioning system utilizes a four-phase stepper
motor drive which changes one phase for each track ad·
vancement of thc ReadlWrite carriage. In addition
to
the
logic neccsSilry for motion control, a gilte is provided as an
element tor inhibiting positioncr motion during a write
opcriltion.
97