Introduction - Radio Shack TRS-80 Service Manual

SECTION
2
THEORY
OF OPERATION
2.1
INTRODUCTION
This section contains
a description of
each major
function performed
by the Line
Printer electronics.
This section
is
organ-
ized as
follows:
2.2 Block
Diagram
Discussion
2.3
Initializing
the Printer
2.4
Data
Input
2.5
Character
Printing
2.6 Carriage
and
Ribbon
Movement
2.7
Paper Motion
2.8
Power
Supplies
Each function
is
discussed
at a
subassembly
level,
supplemented
with block diagrams showing
signal
flow between major
subassemblies and
the
connector/pin
locations of each
signal.
Figure 2-1
is
a simplified
block diagram
of the
overall printer
electronics.
"1
1
1
ELECTRONICS BOARD
1
POWER
DRIVERS
1
1
>
MEMORY
(RAMI
f
>
CHARACTER
GENERATOR
(ROM)
K
\
INTERFACE
CONNECTOR
1
1
DATA 1-DATA
7
>
>
CG1-CG7
>
PRINT
HEAD
' L
V /*-
j
V
SOLbNUtUS
ADDRESS
TIMING
INPUT
CONTROL
LOGIC
LINES
DATA
INPUT (OATA l-DATA
71
—
^
LF
orCR
1
1
1
1
1
1
DATA STROBE
ADDRESS
COUNTER
STROUT
PRINT
CONTROL
LOGIC
HEAD MOTION
\
CIP.CIR
1
T
1
1
1
i
1
1
I
DSTA
y
PRINT
ACKNOWLEDGE.
BUSY
COMMAND
I
TIMING
INPUTS
LINE
FEED
SITY
PMSOL
1
T0
1
1
i
'
i
1
1
OCSILLATOR
5
DEN
L
CONTROL
PANEL
MARGIN
INDICATOR'S
Figure 2-1.
SIMPLIFIED BLOCK DIAGRAM, ELECTRONICS
NOTE
THE NOTATION
(N)
AFTER
A
SIGNAL
MNEMONIC
IS
USED TO
INDICATE THE "NOT" OR
INVERSE
FUNCTION OF THE
SIGNAL.
FOR EXAMPLE:
CIP
(N) IS
THE NOT
FUNCTION OF THE
CIP SIGNAL.
THE
(N)
NOTATION
IS
EQUIVALENT TO THE BAR APPEARING
OVER A
SIGNAL AS
SHOWN
ON
THE SCHEMATIC.
2.2
BLOCK DIAGRAM
DISCUSSION
(Figure 2-1)
The
unit
is
a
unidirectional
printer,
whose
print
head moves
to
the
right
when
printing
and
left
when
returning the
carriage.
The
seven
print
wires
in
the
print
head are
selectively activated
to
form
the dot matrix pattern
for
the specified character.
The
print
commands
to
the
seven
print
wires
are developed by the
character generator
which
consists
of a
read-only-
memory
(ROM). Attached
to
the carriage
assembly
are the
print
head and
ribbon
cartridge.
By
activating either
the forward
or
reverse carriage solenoid the carriage attaches
to
a
continuously
rotating fibre band.
The
forward solenoid
is
activated
during the
print cycle
and
the reverse solenoid
is
activated
to
return the
carriage.
Paper
is
advanced one
line
by
momentarily
activating the paper
drive
solenoid
in
the
line
feed
clutch
unit.
The
Line Printer electronics
is
contained on one
printed
circuit
board.
As shown
in
Figure
2-1, printable data goes from
left
to right across the
diagram
to the print head, with
control of
this
data
coming from
the input control
logic
and
the
print
control
logic.
When
the
printer
is
first
turned on, the
printer
logic
is
initialized
and
the
printer
is
not
print enabled.
To
receive
data,
the
printer
must then be enabled by
depressing the
PRINT
rocker switch on
the operator's
panel.
2-1