Data Input - Radio Shack TRS-80 Service Manual

2.4
DATA
INPUT
(Figure 2-3)
Inputs to the
printer consist
of seven standard
parallel data
lines
(DATA 1-DATA
7)
and an
active
low
data strobe
(DSTA
(N))
The
seven data
lines represent the
7-bit
USASCII
code
(refer to the operator's manual).
The
data strobe
is
used
to
synchronize the input
data with the printer
electronics.
In
response
to
received
data,
the
printer
generates an
active low acknowledge
(ACK(N)) pulse
to
acknowledge
reception
of a character.
If
the received character
caused
the
printer to perform
some
function such
as paper
movement
character
printing,
etc.,
the
printer responds with
a
BUSY
signal
and does
not acknowledge
until termination
of
the busy
condition.
2.4.1
DATA
INPUT TIMING
In
general, the data transfer
sequence
consists
of
the input device placing the appropriate
code on
the data
lines to
the
printer
and then
generating
a
data strobe
pulse.
The
printer
responds with an
acknowledge
pulse,
or
if
the received data
caused
a
busy
condition,
the
printer
first
activates
the
busy
line for
the duration
of
the
busy
condition
and
then responds
with an
acknowledge
pulse.
Refer
to
Appendix
A
for the timing involved
in
transferring data, which does
not cause
a
busy
condition,
and
the timing
involved
in
receiving
any
character,
which causes
a
busy
condition
in
the
printer.
2.4.2
FUNCTION
DECODER
In addition to monitoring
the input data
for the
first
printable character, the
function
decoder monitors
the data
for the four
special
control
codes used
in
the
printer. Line Feed/Carriage
Return and
Top
of Form. The
data
lines are
decoded and
if
one
of these control
codes
is
detected, the following action occurs:
FUNCTION
Line Feed/Carriage
Return
Top
of
Form
INPUT
DEVICE
,
DATA 1-DATA
7
DSTA
ACK
r
PENOTSEL
CONTROL CODES
(DECIMAL)
10or
13
11 or
12
|
ELECTRONICS BOARD
I
HJ
10
12
DATA 1-DATA
;
y
w
PUT
GATIMG
PRINTER ACTION
Advances
the paper one
line
and
returns the
print
head
to
its
home
position.
Advances
the paper to top
of the next page
(as
defined
by BASIC. See LEVEL
II,
BASIC
Reference Manual
page
10/3).
PRINTABLE DATA
DATA
LINES )
FUNCTION
DECODER
LOGIC
L_.
CONTROL
CODES
sz.
MEMORY
(RAMI
1
.J
Figure 2-3.
DATA
INPUT
2.5
CHARACTER
PRINTING
(Figure 2-4)
When
the
memory
is
full
(CARRY
(N)) or
a carriage return
code
(DCCR(N))
is
detected
in
the input
data, the print
command
(CIP)
is
generated causing the
print
head
to
move
left to
right
across the page. The
address
counter
is
set
to count up
then
clocked
to
select via the eight address
lines,
the appropriate character
location
in
the
RAM.
The
read/write (R/W(N))
input
to the
RAM
is
held high during the
print cycle. Conversely, the R/W(N)
input
is
pulsed low
during the data
input cycle
The
/-bit
character code, specifying the character,
is
sent
to the
ROM.
As
the
print
head
moves
across the page, the timing inputs
generated by
ME7
and ME8.
are used
by the
ROM
to register
the
column
of dots
in
the printed character.
The ROM,
in
response
to
the character
code and
timing
signals, generates seven
logic
signals (CG1-CG7) which
are
amplified
and
sent
to
the seven
solenoids
in
the
print head.
There
the
signals activate the solenoids
driving
the
print
wires
which form
the appropriate
dot pattern
for
the addressed
character and
column.
The
standard character
line
length
is
132
characters. The end
user has
the option
of
varying the
line
length by
adding
jumper
wires and making
cuts
on
the printed
circuit board. Refer
to
Section 3.17
Electronic Board Assembly
for the pro
cedure on changing
the
line length.
2-3