Heathkit H9 Manual 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.