Heathkit H9 - Page 69

Refer to Pictorial 4-19
(Illustration
Booklet, Page 13J
for
TTL I/O connections. A TTL input signal is
coupled to
pins
9 and 10 of IC609C. The
output of
IC609C
drives
IC609D,
which drives the serial input
of the UART. The TTL
output signal comes directly
from the
output of
IC609B.
The
input
labeled
"break"
at
pin
2
of
IC609A
gen-
erates a continuous space at the serial
output
as long
as the break key on the keyboard
is
pressed.
The
"off
line" input at
pin
5 of
IC609B
causes a continuous
mark
to be
generated
at both the serial output and the
serial input. This
effectively
inhibits
the
Terminal
from
either transmitting
or
receiving.
IC602
successively divides the
4800
baud
rate
clock
by
two to
produce
the 2400, 1200, 600, and
gO0
baud
rate
clocks.
IC601, in
conjunction with IC607A and
IC608A,
divides the 1200 baud rate
clock
by 11 to
generate
the 110 baud rate clock.
One of
the baud rate
clocks
(600
-
9600) can
be
jumper-connected
to
the
"pteset
baud
out" from
the I/O
circuit board.
This
output and
the 300 baud
rate
output are
connected
to
Baud rate
switch,
SW2,
on the back panel
of the Ter-
minal. This
switch
selects
either 300
baud
or the
preset
baud rate.
IC607B,
IC607C,
and ICGO7D,
in
conjunction
with
the keyboard
baud
rate
key drive
the
clock
pulse
inputs
of the
UART with
either 110 baud
or the,baud rate
selected
by
the Baud
Rate
switch.
Integrated
circuits
IC613,
IC614,
IC61,T,
IC623,
IC624C,
and
IC624D
form
a special
character
detector
that
monitors
the ASCII
bus to
detect the
presence
of
certain
special
characters.
IC613
and IC61ZA
detect a
rubout
and
a control
character,
respectively.
The re-
maining IC's
detect
a Back
Space, Line
Feed,
Bell,
Space,
and
Carriage Return.
These
outputs are
used
by
the TPU for
cursor movements
and
sepcial writing
operations.